AN2647
Application note
Using the STR91xFA external memory
interface (EMI)
Introduction
The STR91xFA EMI bus is a very flexible bus and is user programmable. The bus can be
configured to interface to different types of memory devices, including SRAM, Flash
memory, ROM or PSRAM. Some of the programmable features of the EMI bus are:
● Multiplexed or non-multiplexed bus
● Bus width: 8 or 16 bit
● Address lines select: A0-15 or A0-A23
● Read and Write signal timing and wait state insertion
● Asynchronous or synchronous bus access
● Page or burst mode access
● Chip select signals (CS0-3)
● ALE polarity and pulse width
● Bus clock (BCLK) frequency
This application note covers the EMI asynchronous mode configuration for interfacing to
standard memory devices in Section 1: Interfacing with asynchronous memory.
It covers also EMI Synchronous mode which has a different bus timing and configuration in
Section 2: Interfacing with synchronous memory.
Software is available with this application note and can be downloaded separately online
from www.st.com.
December 2007 Rev 1 1/34
www.st.com
Contents AN2647
Contents
1 Interfacing with asynchronous memory . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Multiplexed or non-multiplexed EMI bus selection . . . . . . . . . . . . . . . . . . . 4
1.1.1 Multiplexed EMI bus configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Read bus cycle timing configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
EMI_ALE signal configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
EMI_RD signal configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Write bus cycle timing configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
EMI_WR signal configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Byte and word access in 16-bit multiplexed mode . . . . . . . . . . . . . . . . . . . . . . . . 10
Address shifting in 16-bit multiplexed mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
1.1.2 Non-multiplexed EMI bus configuration . . . . . . . . . . . . . . . . . . . . . . . . . 11
Read bus cycle timing configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
EMI_RD Signal Configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Write bus cycle timing configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
EMI_BWR signal configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Half word and word access in 8-bit non-multiplexed mode . . . . . . . . . . . . . . . . .13
Page mode timing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
1.2 Other EMI bus configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.1 Bus control register (BCR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.2.2 EMI port configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.2.3 BCLK clock frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.2.4 Memory bank read and writing timings . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.3 EMI bus configuration examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.4 Executing code from external SRAM vs. internal Flash . . . . . . . . . . . . . . 18
2 Interfacing with synchronous memory . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1 PSRAM overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1.1 Bus interface signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2 Connecting the EMI bus to a PSRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.1 PSRAM bus operating modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Asynchronous mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
EMI bus configuration for asynchronous mode . . . . . . . . . . . . . . . . . . . . . . . . . . 25
EMI read and write asynchronous bus cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
Synchronous burst mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
PSRAM BCR register configuration for synchronous burst mode . . . . . . . . . . . .28
EMI bus configuration for synchronous burst mode . . . . . . . . . . . . . . . . . . . . . . .28
2/34
AN2647 Contents
EMI burst read bus cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
EMI burst write bus cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
DMA for high speed data transfer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
3 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
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Interfacing with asynchronous memory AN2647
1 Interfacing with asynchronous memory
1.1 Multiplexed or non-multiplexed EMI bus selection
The multiplexed EMI bus is selected if you have a 16-bit system bus or using 16-bit memory
device. The multiplexed bus requires a 16-bit address latch, as most memory devices do not
accept multiplexed address/data input.
The 8-bit non-multiplexed bus is suitable for 8-bit memory or I/O devices. In this
configuration, the EMI memory banks are limited to 64 KB each, as there are only 16
address lines available.
In general the non-multiplexed bus has a shorter bus cycle that can be completed in one bus
clock period (t
multiplexed bus has a 16 bit data bus, and can provide high data transfer rate for memory
device like PSRAM that supports burst mode. The DMA Controller can be programmed to
perform burst data transfer between the internal SRAM and the EMI bus.
1.1.1 Multiplexed EMI bus configuration
The multiplexed bus has different bus signals and port assignments than the nonmultiplexed bus. As shown in Table 1, Ports 8 and 9 provide the multiplexed 16 bit address
and data bus (AD0-AD15), the optional higher address A16-A23 are assigned to Port 7.
Figure 1. & Figure 2. show a typical EMI multiplexed bus connecting to two 16-bit memory
devices: a 4 MB ISSI SRAM and a 4 MB SPANSION Flash memory. The SN74LVC16373A
is a high speed 16-bit address latch with maximum t
on Port 7 are address lines, the remaining four pins are for other I/O functions.
Table 1. Multiplexed bus signals
) while the multiplexed bus takes minimum of 3 BCLK clocks. The
BCLK
delay of 4.2 ns. Note only four pins
PD
Signal name Pin / Port assignment Signal description
AD0-AD7 Port 8 Multiplexed address/data bus AD0-AD7
AD8-AD15 Port 9 Multiplexed address/data bus AD8-AD15
A16-A23 Port 7 Address A16-A23, pin configurable
ALE EMI_ALE Address Latch signal. Polarity and width is programmable
Read EMI_RDn Read signal
Write Low
(Low Byte Select)
Write High
(High Byte Select)
Write Enable EMI_WEn
CS0-CS3
EMI_BWRn
(EMI_WRLn or EMI_LBn)
EMI_WRHn
(EMI_UBn)
Port 0(P0.4-P0.7) or
Port 5 (P5.4-P5.7) or
Port 7 (P7.4-P7.7)
Low Byte (D0-D7) Write signal or
Low Byte Select signal.
High Byte (D8-D15) Write signal or
High Byte Select signal
Write Enable signal, use together with the UB/LB byte
select signals in synchronous mode.
One chip select for each of the 4 Memory Banks.Can be
assigned to any of the 3 port.
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AN2647 Interfacing with asynchronous memory
Figure 1. 16-bit Flash multiplexed bus connection
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Interfacing with asynchronous memory AN2647
Figure 2. 16-bit SRAM multiplexed bus connection
Note: Please note the logic gates used are needed to tolerate the 8-bit data operations.
After power up or system reset, the EMI bus is default to a very slow, asynchronous,
multiplexed bus. User need to configure the GPIO ports. The read and write timings listed
below to set up a memory bus that meets your system's requirement for optimal
performance.
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AN2647 Interfacing with asynchronous memory
Read bus cycle timing configuration
Figure 3. shows a typical Read Bus Cycle. All bus timings are referenced to the internal
BCLK clock signal. BCLK clock is only available on an external pin for various usage on the
144 pin BGA package.
Figure 3. Read bus cycle, 16-bit multiplexed bus
EMI_ALE signal configuration
The EMI_ALE signal is used to latch the address A0-15 by the external address latch, or as
a direct input to memory device that have an ALE pin. The EMI_ALE by default is one BCLK
period in length with active high polarity. It can be programmed to be 2 BCLK period in
asynchronous mode and the polarity can be active low. The ALE length and polarity are
defined in the SCU_SCR0 register.
Address A0-A15 are valid at the leading edge of EMI_ALE and are driven for another half
BCLK period after the trailing edge. The AD bus is tri-stated after the address phase is over.
A16-23 are not multiplexed and remain stable until the end of the bus cycle.
EMI_RD signal configuration
The EMI_RDn timing is controlled by the WSTRD value in the EMI_RCRx register (Read
Wait State Control) and the WSTOEN value in the EMI_OECRx register (Output Enable
Control).
● WSTOEN: Output Enable. WSTOEN specifies the delay between the assertion of the
chip select and the time EMI_RDn signal goes low. The delay is defined in terms of
t
.
BCLK
The minimum WSTOEN value is 2 in a multiplexed bus (for ALE with one BCLK period
width). EMI_RDn becomes active after 2 t
of the bus cycle.
● WSTRD: Read wait state. WSTRD specifies the pulse width, or the rising edge of
EMI_RDn. The pulse width is defined in terms of number of t
WSTOEN+1).
so as not to overlap the address phase
BCLK
and is = (WSTRD-
BLCK
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Interfacing with asynchronous memory AN2647
The choice of the WSTRD value depends on the access time of the memory device, slow
memory requires more wait states. Typically, the memory access time must meet the
following condition:
Where t
Memory Read access time < (t
is the EMI read address setup time, tRP is the EMI_RDn pulse width and t
RAS
RAS
+ tRP - t
RDS
)
is
RDS
the data setup time.
The EMI Bus stops driving address A16-A23 and the CSx signal at the rising edge of
EMI_RDn.
The read bus cycle in Figure 3. has the following configuration:
ALE Length = 1
WSTOEN = 2
WSTRD = 3
The above read signal configuration can read a memory device with access time of less
than (4*t
BCLK
- t
)ns, where t
RDS
is the data setup time as specified in the STR91xFA
RDS
data sheet.
Write bus cycle timing configuration
In asynchronous bus configuration, the default write signals are EMI_WRLn and
EMI_WRHn. The other set of write signal selection, EMI_WEN, EMI_LBn and EMI_UBn,
are available in BGA package in synchronous mode only. Figure 4. shows a typical Write
Bus Cycle. The write bus cycle timing differs from the read bus cycle in two ways:
– The write signals EMI_WRLn and EMI_WRHn (EMI_WRXn) align with the falling
edge of the BCLK
– The write signals are terminated half BCLK period before the CSx signal.
Figure 4. Write bus cycle, 16-bit multiplexed bus
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AN2647 Interfacing with asynchronous memory
EMI_WR signal configuration
The EMI_WRxn timings are controlled by the WSTWR value in the EMI_WCRxn register
(Write Wait State Control) and the WSTWEN value in the EMI_WECRx (Write Enable
Control) register.
● WSTWEN: Write Enable. WSTWEN specifies the delay period between the assertion
of the CSx chip select and the falling edge of EMI_WRxn. The delay is defined in terms
of BCLK clock periods which is (WSTWEN + 1/2) for asynchronous write cycles. The
minimum WSTWEN value is 1 in a multiplexed bus (for ALE with one BCLK period
width).With WSTWEN=1, EMI_WRxn becomes active only after the address phase is
over.
● WSTWR: Write wait state. WSTWR specifies the pulse width of EMI_WRx. The pulse
width is defined in terms of BLCK periods and is equal to (WSTWR-WSTWEN+1) for
asynchronous write cycles.
The choice of the WSTWEN and WSTWR values depend on the address and data setup
time and data hold time of the memory device. The STR9 provides at least 1.5 t
address setup time with WSTEN=1; and by default 0.5 t
of data and address hold time.
BCLK
The WSTWR or pulse width is then set accordingly to meet the data setup time of the
memory device
BCLK
of
The write bus cycle in Figure 4. provides address setup time of 2.5 *t
of 1*t
BCLK
and 0.5*t
of data hold time.
BCLK
, data setup time
BCLK
Byte and word access in 16-bit multiplexed mode
The previous sections describe the read and write timings for 16-bit or half word access.
The CPU can generate 8-bit or 32-bit access using the assembly instruction of STRB, LDRB
or STR, LDR. The 8-bit bus cycle is the same as the 16-bit bus cycles, except AD15-AD8
are not driven during the data phase of the cycle.
For a 32-bit access, the EMI generates two consecutive 16-bit bus cycles. The two bus
cycles are identical to the standard 16-bit cycle, the only difference is the EMI_CS chip
select signal stays low during the two write cycles. Figure 5. shows the bus timings when the
CPU executes a 32-bit STR instruction followed by a LDR instruction. Note the EMI_CS
signal stays low in the two write bus cycles, but are separated in the two read bus cycles.
The two half words that were read in the two LDR read cycles are combined into one word in
the EMI block before being transferred to the CPU.
Figure 5. Word (32-bits) write and read bus cycles in 16-bit multiplexed mode
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Interfacing with asynchronous memory AN2647
Address shifting in 16-bit multiplexed mode
The internal address (A31-A0) in the ARM core is "byte addressable address", so every
address points to a byte location. The internal address is right shifted by one when it is
driven to the external 16-bit EMI bus. The resulting EMI address is then pointing to a 16-bit
or half word location. There is no need to shift the address by one when connecting the EMI
bus to a 16-bit memory device. The EMI A0 address should be connected to the A0 pin of
the memory device.
Note: 1 The EMI multiplexed bus can also be configured as an 8-bit bus by setting the memory width
bits in the EMI_BCRx register to "00". In this configuration, Port 8 drives the AD7-AD0
address/data bus, while Port 9 drives A15-A8. Please note address A15-A8 must be latched
externally as in 16-bit mode.
2 A related software is provided for both SRAM and Flash interfaces already defined.
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AN2647 Interfacing with asynchronous memory
1.1.2 Non-multiplexed EMI bus configuration
The 8-bit non-multiplexed bus has a different set of bus signal, port assignment and bus
timing. Table 2 shows the non-multiplexed bus signal pin assignments.
The non-multiplexed bus has 16 address lines and this limits the memory bank size to no
more than 64 KB. Figure 6. shows a typical non-multiplexed bus connection to an 8-bit
Cypress SRAM.
Table 2. Non-multiplexed bus signals
Signal name Pin / Port assignment Signal description
D0-D7 Port 8 Data bus D0-D7
A0-A7 Port 7 Address A0-A7
A8-A15 Port 9 Address A8-A15
Read EMI_RDn Read signal
Write EMI_BWRn Write signal, same as EMI_WRLn
One chip select for each of the 4 Memory
Banks.Can be assigned to any of these two
ports.
CS0-CS3
Port 0(P0.4-P0.7) or
Port 5 (P5.4-P5.7) or
Figure 6. Non-multiplexed bus port connection
Read bus cycle timing configuration
Figure 7 shows a typical Read Bus Cycle. All bus timings are referenced to the internal
BCLK clock signal. BCLK clock is only available on an external pin for various usage on the
144 pin BGA package.
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Interfacing with asynchronous memory AN2647
Figure 7. Read bus cycle, non-multiplexed bus
EMI_RD Signal Configuration
The EMI_RD signal's timing is controlled by the WSTRD value in the EMI_RCRx register
(Read Wait State Control) and the WSTOEN value in the EMI_OECRx register (Output
Enable Control).
● WSTOEN: Read Enable. WSTOEN specifies the delay between the assertion of the
chip select and EMI_RDn goes low. The delay is defined in terms of BCLK clock
periods.The minimum WSTOEN value is 0 in a non-Multiplexed bus, EMI_RDn can be
active at the same time as the CSx signal.
● WSTRD: Read wait state. WSTRD specifies the pulse width, or the rising edge of
EMI_RDn. The pulse width is defined in terms of BLCK periods and is = (WSTRDWSTOEN+1)The choice of WSTRD value depends on the access time of the memory
device, slow memory requires more wait states. Typically, the memory access time
must meet the following condition:
Memory Read access time < (t
where t
is the read address setup time, tRP is the EMI_RDn pulse width and t
RAS
data setup time.
The read bus cycle in Figure 7. has the following configuration:
WSTOEN = 1
WSTRD = 1
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RAS
+ tRP - t
RDS
)
is the
RDS
AN2647 Interfacing with asynchronous memory
Write bus cycle timing configuration
Figure 8. shows a typical Write Bus Cycle. The write bus cycle timing differs from the read
bus cycle in two ways:
Figure 8. Write bus cycle, non-multiplexed bus
– The write signal (EMI_BWRn) aligns with the falling edge of the BCLK
– EMI_BWRn is terminated half t
before the CSx signal
BCLK
EMI_BWR signal configuration
The EMI_BWRn signal timing is controlled by the WSTWR value in the EMI_WCRxn
register (Write Wait State Control) and the WSTWEN value in the EMI_WECRx (Write
Enable Control) register.
● WSTWEN: Write Enable. WSTWEN specifies the delay period between the assertion
of the CSx chip select and the falling edge of EMI_BWRn. The delay is defined in terms
of BCLK clock periods and is (WSTWEN + 1/2) for asynchronous write cycles. The
minimum WSTWEN value can be 0 in a non-Multiplexed bus.
● WSTWR: Write wait state. WSTWR specifies the pulse width of EMI_BWRn. The pulse
width is defined in terms of BLCK period and is equal to (WSTWR-WSTWEN+1) for
asynchronous write cycles.
The choice of the WSTWEN and WSTWR values depend on the address setup time, data
setup time and data hold time of the memory device. The STR9 provides at least 1.5 t
of address setup time with WSTEN=1; and by default 0.5 t
of data and address hold
BCLK
BCLK
time. The WSTWR or pulse width is set accordingly to meet the memory's data setup time
requirement.
The WSTWEN and WSTWR are set to one in Figure 8., providing address setup time of
1.5*t
, data setup time of 1.5*t
BCLK
BCLK
and 0.5*t
of data hold time.
BCLK
Half word and word access in 8-bit non-multiplexed mode
In the 8-bit non-multiplexed mode, the CPU is able to read or write 16-bit or 32-bit data using
the assembly instruction of STRH, LDRH or STR, LDR. For 16-bit access, the EMI
generates two consecutive 8-bit bus cycles; while 32-bit access results in 4 consecutive 8-
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Interfacing with asynchronous memory AN2647
bit bus cycles. The bus cycle is identical to the standard 8-bit cycle except the EMI_CS chip
select signal stays low during the consecutive write cycles. Figure 9. shows the bus timings
when CPU executes a halfword write instruction (STRH) followed by a LDRH instruction.
Note the EMI_CS signal stays low throughout the two write bus cycles. The two bytes that
are read in the LDRH cycles are combined into one halfword in the EMI block before being
transferred to the CPU.
Figure 9. Halfword write and read bus cycles in 8-bit non-multiplexed mode
Page mode timing
The EMI bus in non-Multiplexed mode can be configured to support memory with Page
mode. When in Page mode data are transferred in burst of four, eight or sixteen bytes. The
Page mode bit and the data transfer size is specified in the EMI_BCRx register.
The configuration of the EMI_RDn signal timing is the same as in asynchronous read
cycles. When page mode is enabled, the read cycle consists of an asynchronous read,
followed by multiple burst reads of one byte per BCLK clock. The EMI_RDn and chip select
signal will automatically be extended and stay active until the end of the last page read.
If the page mode access time of your memory device plus the EMI data setup time (t
more than one BCLK period, you need to insert burst read wait state in the EMI_BRDCRx
register.
1.2 Other EMI bus configurations
After the bus read/write bus timing configuration is determined, the next step is to set up the
Bus Control register and other GPIO port control registers to support the bus type that you
selected (Mux or Non_Mux).
1.2.1 Bus control register (BCR)
Each of the memory banks has its own Bus Control register that specifies the bus operating
mode. The bits that need to be configured for asynchronous memory read and write are
listed in Tab l e 3 . . Some of the register bits that are dedicated for synchronous bus mode are
not included in the table.
RDS
) is
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AN2647 Interfacing with asynchronous memory
Table 3. BCR register configuration bits
Bit Name Bit value
Bit 17 SYNCWRITEDEV
Bit [11:10] BRLEN[1:0]
Bit 9 SYNCREADDEV
Bit 8 BPM
Bit[5:4] MW[1:0]
Bit 3 WP
0: Asynchronous write device (default)
1: Synchronous device
Burst Read Transfer Length. These bits are written by
software to define the transfer length for burst or page mode
read cycle. Page mode is limited to 4 or 8 transfer.
00: 4-transfer burst read
01: 8-transfer burst read
10: 16-transfer burst read
11: Continuous (synchronous only)
Synchronous read access device. Access the device using
synchronous accesses for reads:
0: Asynchronous device (default).
1: Synchronous device.
Burst and Page Mode Read Selection This bit is set and
cleared by software to select/deselect Burst or Page Mode
read.
00: 8-bit
01: 16-bit
Memory width define the data bus and memory width of the
EMI bus.
00: 8-bit EMI bus
01: 16-bit EMI bus
Write protect - to protect/unprotect the bank from write
access.
0: Bank not write protected
1: Bank write protected
1.2.2 EMI port configuration
Ports 7, 8 and 9 at power up are default to GPIO ports; your software has to program the
ports to function as an EMI bus. Table 4. below shows all possible port function and
configuration. The registers that need to be configured include:
SCU_GPIOOUTx - Select the Alternative output function: ALT2 or ALT3
SCU_GPIOTYPEx - Select the port driver: push-pull or open drain
SCU_EMI -Select EMI mode
SCU_SCR0 - Select the multiplexed mode, ALE length and polarity.
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Interfacing with asynchronous memory AN2647
Table 4. EMI port configuration
Multiplexed Bus
Port Register
configuration
Port7 Port8 and 9
EMI bus signals:A16-
A23 or CS0-CS3
(Multiplexed bus)
A0-A7
(Non-multiplexed bus)
SCU_GPIOOUT7
[13:0]=to ALT 2 function
SCU_GPIOOUT7
[15:14]=to ALT 3 function
SCU_GPIOOUT7=0XEA
AA
(A16-A23 are pin
selectable, you only need
to configure the pins that
are required)
SCU_GPIOTYPE7
=0X00 (push-pull)
EMI_Multiplexed mode
bit in SCU_SCR0 [6]=0
EMI bus
signals:AD0-AD15
(Multiplexed bus)
D0-D7 & A8-A15
(Non-multiplexed
bus)
SCU_EMI Register
GPIOEMI bit =0
(Set ports 8 and 9 as
EMI AD bus)
SCU_GPIOTYPE8,9
=0X00 (push-pull)
Port0
(P0.4-P0.7)
EMI bus signals:
CS0-CS3
SCU_GPIOOUT0
[8:15]= to ALT 3
function.
SCU_GPIOOUT0=
0XFFxx
(CS0-3 are pin
selectable, you only
need to configure the
pins that are
required)
SCU_GPIOTYPE0
=0X0x (push-pull)
Port5
(P5.4-P5.7)
EMI bus signals:
CS0-CS3
SCU_GPIOOUT0
[8:15]= to ALT 3
function.
SCU_GPIOOUT0=
0XFFxx.
(CS0-3 are pin
selectable, you only
need to configure
the pins that are
required)
SCU_GPIOTYPE5
=0X0x (push-pull)
SCU_GPIOOUT7
[13:0]=to ALT 2 function
SCU_GPIOOUT7
[15:14]=to ALT 3 function
Non-Multiplexed
Bus Port Register
Configuration
SCU_GPIO=0XEAAA
SCU_EMI Register
SCU_GPIOTYPE7
=0X00 (push-pull)
EMI_Multiplexed mode
bit in SCU_SCR0 [6]=1
1.2.3 BCLK clock frequency
The EMI bus clock (BCLK) can be configured to have the same frequency or half the
frequency of the AHB bus clock (HCLK). The maximum BCLK frequency supported by the
EMI bus is specified in the STR91xFA Data Sheet.
The BCLK frequency is controlled by the EMIRATIO bit in the SCU_CLKCNTR register to
divide the HCLK by 1 or by ½.
The BCLK pin is available as an output in BGA package only. The BCLK clock is enabled by
setting bit 1 in the SCU_EMI register
GPIOEMI bit =1
(set ports 8 as data
bus and 9 as
address bus)
SCU_GPIOType8,9
=0X00 (push-pull)
Same as Multiplexed
bus
Same as
Multiplexed bus
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AN2647 Interfacing with asynchronous memory
1.2.4 Memory bank read and writing timings
The EMI supports 4 external memory banks, up to 64Mbyte in each bank. Depending on the
address in the current bus cycle, the EMI activates one of the 4 chip selects (CS0-CS3) to
select the targeted device.
Every memory bank has its own set of bus configuration registers and therefore is
configured to have its own set of bus timing as well. Usually high speed device like SRAM
resides in one bank while slow Flash memory and I/O device reside in another bank. Bus
timing can be switched dynamically from one bank to the other bank.
1.3 EMI bus configuration examples
Ta bl e 5 lists all the configuration registers and control bits that you need to configure the
EMI bus for asynchronous mode. The multiplexed mode example is configured for the 10ns,
16-bit SRAM (IS61LV25616AL) in Figure 2. The non-multiplexed mode example is
configured for the 20ns, 8-bit SRAM (C1019CV33) in Figure 6. The timings in Ta bl e 5 are
based on a 96 MHz BCLK clock.
Table 5. EMI Bus configuration examples for asynchronous memory devices
Configuration
register
Bit Name
Register bit Multiplexed 16-bit bus Non-Multiplexed 8-bit bus
Bit
value
Function
Bit
value
Function
17 SYNCWRITEDEV 0
9 SYNCREADDEV 0
EMI_BCRx
EMI_RCR 4:0 WSTRD 11
EMI_OECR 3:0 WSTOEN 10
EMI_WCR 4:0 WSTWR 11 Write Wait State 11 Write Wait State
EMI_WECR 3:0 WSTWEN 10
EMI_ICR 3:0 IDCY 11
EMI_BRDCR 4:0 WSTBRD 00
8 BPM 0
5:4 MW 01
3WP 0
Asynchronous
write device
Asynchronous
read device
Normal mode
(non-burst or page
mode)
16-bit data bus
width
Bank not write
protected
Read Wait State
10 ns access time
EMI_RDn
assertion delay
EMI_WRn
assertion delay
Idle Cycle = 3
BCLK
Burst read wait
state=0
0
0
0
00 8-bit data bus width
0
11
00
00
11 Idle Cycle=3 BCLK
00 Burst read wait state=0
Asynchronous write
device
Asynchronous read
device
Normal mode (non-
burst or page mode)
Bank not write
protected
Read Wait State
20 ns access time
EMI_RDn assertion
delay
EMI_WRn assertion
delay
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Interfacing with asynchronous memory AN2647
Table 5. EMI Bus configuration examples for asynchronous memory devices
Register bit Multiplexed 16-bit bus Non-Multiplexed 8-bit bus
Configuration
register
Bit Name
Bit
value
Function
Bit
value
Function
SCU_CLKCNTR 18:17 EMIRatio 00
8 EMI_ALE_LNGT 0
SCU_SRC0
7 EMI_ALE_POLR 1
6 EMI_MULTIPLEXED 0 Multiplexed mode 1 Non-Multiplexed mode
BCLK=HCLK
(96 MHz)
ALE length=1
t
BCLK
ALE polarity-active
high
01
0Don’t care
0Don’t care
BCLK=HCLK
(96 MHz)
1.4 Executing code from external SRAM vs. internal Flash
The EMI bus is designed for supporting external data storage. However, the CPU can also
fetch and execute instruction codes from external memory; the performance would be lower
when compared to execution from internal Flash. Please note the Prefetch Queue and
Branch Cache are dedicated for internal Flash memory and are not available for external
code fetch.
There is a four-word buffer on the AHB bus to facilitate the external code fetch. The buffer is
flushed whenever there is a non-sequential fetch, this results in a series of burst fetch
requests to the EMI bus to fill up the buffer. Using memory that has burst mode or page
mode would enhance the CPU performance.
The ARM instructions are 32-bits, to fetch an instruction the EMI in general generates 1
burst mode read request, of length 4, when the EMI bus is 8-bit wide and 2 read requests for
16-bit bus. Ta bl e 6 below lists the number of BCLK clocks needed to fetch an instruction
sequentially from external memory and internal Flash. The 8-bit non-multiplexed EMI bus is
configured for page mode or fast SRAM access, which takes one BCLK clock to read a byte.
The 16-bit multiplexed bus is configured for the shortest possible read cycle and takes 3
BCLK clocks to read a halfword. As shown in the table, the CPU takes more time to fetch an
instruction from external memory and results in lower performance.
Table 6. Number of BCLK clocks required to fetch a sequential instruction
Program code
External memory 8bits non-mux bus
(page mode)
External memory 16bit mux bus (SRAM)
Internal Flash
memory 32-bit bus
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Read bus cycle
length
1 BCLK Clock 4 2 (1BCLK*4)+2=6
3 BCLK Clock 2 2 (3BCLK+2)*2=10
1 BCLK Clock 1 0 1
#of bus cycle per
instruction
#of BCLK
between cycles
#of BCLK per
instruction fetch
AN2647 Interfacing with synchronous memory
2 Interfacing with synchronous memory
STR91xFA has up to 96 KB of internal SRAM which is enough for most applications. For
systems that need large amount of SRAM, an attractive alternative is to use PSRAM
(Pseudo Static RAM). PSRAM is widely used in mobile phone, PDA and other portable
electronic products, mainly due to its high density, high performance, low power and low
cost. It takes more work to interface and configure a PSRAM than the standard SRAM, but it
is worth the effort. Please note the STR91xFA with the 144-ball BGA package is the only
device that has the additional EMI signals to interface to a PSRAM.
This Part of application note describes how to configure the STR91xFA EMI bus to access a
PSRAM. Topics include:
● Connecting a PSRAM to the EMI bus
● Asynchronous mode configurations and timings
● Burst mode configurations and timings
● High Speed data transfer using the DMA Controller
Please refer to Section 1 to get familiar with the basic EMI bus timings and configurations.
There are many PSRAM suppliers to choose from, device with density ranging from 16 Mb
to 128Mb, and clock rate from 66 MHz to 133 MHz. The PSRAM used in this application
note is a Micron Technology 64 Mb Async/Page/Burst CellularRAM
MT45W4MW16BCGB-708 WT). It is organized as 4M x 16 with a clock rate of up to 80
MHz. This AN includes a brief overview of the PSRAM; it is recommended that the reader
refer to the Data Sheet for detail descriptions.
TM
1.5 Memory (part #
19/34
Interfacing with synchronous memory AN2647
2.1 PSRAM overview
The key functional block of the MT45W4M (MT45W4MW16BC), as shown in Figure 10 on
page 21, consists of a 4096K x 16 DRAM memory array, the Control Logic, the
Configuration registers and the Input/Output Buffer. The Bus Configuration Register (BCR)
controls the operating modes and timings of the MT45W4M.
2.1.1 Bus interface signals
The MT45W4M is usually connected to a non-multiplexed bus, with a 16-bit data bus and 23
address inputs. However, it can be configured to interface to a multiplexed bus like the
STR91xFA EMI bus. In addition to the standard SRAM control signals such as CE, OE, WE,
UB and LB, the PSRAM has four additional signals that are required for synchronous (burst)
mode operation and are described in Tab l e 7.
Table 7. PSRAM control signals
Signal
name
CLK Input
ADV# Input
CRE Input
Wait Output
CE# Input
OE# Input
WE# Input
Type Description
Clock: Synchronizes the memory to the system operating frequency during
synchronous operations. When configured for synchronous operation, the address
is latched on the first rising CLK edge when ADV# is active. CLK must be static
LOW during asynchronous access READ and WRITE operations
Address valid: Indicates that a valid address is present on the address inputs.
Addresses can be latched on the rising edge of ADV# during asynchronous READ
and WRITE operations.
Control register enable: When CRE is HIGH, WRITE operations load the RCR or
BCR registers, and READ operations access the RCR, BCR, or DIDR registers.
Wait: Provides data-valid feedback during burst READ and WRITE operations.
WAIT is used to arbitrate collisions between refresh and READ/WRITE operations.
WAIT is asserted at the end of a row. WAIT is asserted and should be ignored
during asynchronous mode.
Chip enable: Activates the device when LOW. When CE# is HIGH, the device is
disabled and goes into standby or deep power-down mode.
Output enable: Enables the output buffers when LOW. When OE# is HIGH, the
output buffers are disabled.
Write enable: Determines if a given cycle is a WRITE cycle. If WE# is LOW, the
cycle is a WRITE to either a configuration register or to the memory array.
LB# Input
UB# Input
20/34
Lower byte enable. DQ[7:0]
Upper byte enable. DQ[15:8]
AN2647 Interfacing with synchronous memory
Figure 10. MT45W4MW16BC functional block diagram
21/34
Interfacing with synchronous memory AN2647
2.2 Connecting the EMI bus to a PSRAM
The EMI bus operates in 16-bit multiplexed mode. The PSRAM itself has a non-multiplexed
bus interface with separate address and data bus inputs. However, the ADV# signal in the
PSRAM is able to latch the address internally and the address is not required to stay
throughout the bus cycle. The EMI's multiplexed bus takes advantage of this feature, it
drives the address and the ADV# in the address phase of the bus cycle. After the address is
latched by PSRAM, the EMI then drives data out in a write cycle or floats the bus in a read
cycle.
Figure 11. shows the EMI bus connection to a PSRAM. Port 7 drives the non-multiplexed
address A[21:16]. A[23:22] pins are not needed, the P5.1 and P6.0 pins are configured to
drive the chip select signals CS1 and the CRE signal respectively. CRE (Configuration
Register Enable) must be high when the EMI is accessing the configuration registers of the
PSRAM. The CS1 is generated automatically by the EMI when accessing the PSRAM, but
CRE is a software controlled GPIO signal and must be set before you can access the
PSRAM registers. Please note no logic gate is needed in the EMI/PSRAM interface.
Figure 11. Glue-less" bus interface: connecting the PSRAM to the EMI bus
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AN2647 Interfacing with synchronous memory
2.2.1 PSRAM bus operating modes
The MT45W4M supports multiple configurations in Asynchronous mode, Synchronous burst
mode, and Page mode. Page mode requires a non-multiplexed bus connection and is not
compatible with the EMI bus.
In this application note, only the PSRAM configurations that are compatible to the EMI bus
are discussed. The EMI bus supports both the asynchronous and synchronous mode; The
mode can be changed by software on the fly to match the PSRAM bus cycle. Typically,
Synchronous burst mode is recommended for high speed data transfer, while asynchronous
mode is used by the EMI bus to:
– Read or write a single halfword to the PSRAM
– Configure the PSRAM registers
Asynchronous mode
After power up the MT45W4M is default to asynchronous mode. There are variations of
asynchronous mode, the one that the EMI bus can support is the "Asynchronous access
Using ADV". The read bus timing is shown in Figure 12. Please note the address inputs are
latched by the rising edge of ADV# and become "don't care" after the latching. The PSRAM
WAIT# output signal is not used in asynchronous mode and is ignored by the EMI.
Figure 12. PSRAM asynchronous read using ADV (EMI_ALE)
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Interfacing with synchronous memory AN2647
Figure 12. and Figure 13. show typical PSRAM asynchronous read and write bus cycles
using ADV# to latch the address. The EMI bus is configured to match the PSRAM timing and
input signal waveform. The EMI_ALE is configured as active low with a 2 t
pulse width
BCLK
to meet the ADV# address setup and hold time. The EMI_UBn and EMI_LBn are enabled by
setting the Byte_En bit in the SCU_EMI register and the BLE bit in the EMI_BCR register.
The EMI_WEn and EMI_RDn are configured with enough wait state to meet the 70ns of
read and write access time of the MT45W4M.
Note: Setting the EMI_ALE_LNGT bit in SCU_SCR0 selects an EMI ALE length of Two clock
cycles in asynchronous mode but one and a half clock cycles in synchronous Mode.
Figure 13. PSRAM asynchronous write using ADV# (EMI_ALE)
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AN2647 Interfacing with synchronous memory
EMI bus configuration for asynchronous mode
Ta bl e 8 shows the EMI bus registers and control bits that are required for the STR91xFA to
access the PSRAM in asynchronous mode. The burst mode parameters are included in the
table but are not don't cares in asynchronous mode
The key parameters that you have to configure are:
1. BCLK clock frequency, which is set to 80 MHz to match the MT45W4M's maximum
clock rate. Note the BCLK pin is disabled in asynchronous mode.
2. Select the EMI_WEn, EMI_UBn, EMI_LBn signals for 16-bit write
3. Set EMI_RDn timing (WSTRD=5, WSTOEn=4), EMI_WEn timing (WSTWR=5,
WSTWEn=0). The read and write timings should meet the MT45W4M's 70 ns access
time.
4. Set EMI_ALE to 2 t
5. Configure Port5,6, 7, 8 and 9 as EMI bus port. Set Port 6 pin P6.0 as CRE output and
Port 5 pin P5.1 as CS1 chip select.
Table 8. EMI and port register configuration for PSRAM async mode (80 MHZ BCLK)
wide and active low.
BCLK
EMI configuration
register
19:18 BWLEN 00 Burst write transfer length
EMI_BCRx (maintain
reserved bits' default
value)
EMI_RCR 4:0 WSTRD 101
EMI_OECR 3:0 WSTOEN 100 EMI_RDn assertion delay = 4
EMI_WCR 4:0 WSTWR 101 Write Wait State = 5
EMI_WECR 3:0 WSTWEN 000 EMI_WRn assertion delay = 0
EMI_ICR 3:0 IDCY 011 Idle Cycle = 3 BCLK
EMI_BRDCR 4:0 WSTBRD 000 Burst read wait state= 0
11:10 BRLEN 00 Burst read transfer length
Register bit Multiplexed 16-bits asynchronous mode
Bit Name Bit value Function
17 SYNCWRITEDEV 0 Asynchronous write device
16 BMWrite 0 Burst mode write disabled
9 SYNCREADDEV 0 asynchronous read device
8 BPM 0 Normal mode (non burst or page mode)
5:4 MW 01 16-bit data bus width
3 WP 0 Bank not write protected
0 BLE 1 Byte Lane Enable
Read Wait State = 5
70 ns PSRAM access time
2 BYTE_EN 1 Enable EMI_UBn and LBn signals
SCU_EMI
SCU_CLKCNTR 18:17 EMIRatio 00 BCLK=HCLK (80 MHz)
1 BCLK_EN 1 Disable BCLK output
0 GPIOEMI 1 Set Port 8/9 as EMI bus
25/34
Interfacing with synchronous memory AN2647
Table 8. EMI and port register configuration for PSRAM async mode (80 MHZ BCLK)
EMI configuration
register
SCU_SCR0
SCU_GPIOTYPE [5:9] 7:0 Push-pull 0x00 Configure Port 5,6,7,8 and 9 as push-pull
SCU_GPIOOUT7 15:0 Output Function 0x0AAA Set Port 7 pins P7.5-P7.0 as A[21:16]
SCU_GPIOOUT6 15:0 Output Function 0x0001 P6.0 as GPIO out
SCU_GPIOOUT5 15:0 Output Function 0x0C00 Set P5.5 as CS1
Register bit Multiplexed 16-bits asynchronous mode
Bit Name Bit value Function
8 EMI_ALE_LNGT 1 ALE length=2 t
7 EMI_ALE_POLR 0 ALE polarity-active low
6 EMI_MULTIPLEXED 0 Multiplexed mode
BCLK
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AN2647 Interfacing with synchronous memory
EMI read and write asynchronous bus cycle
The EMI configurations in Ta bl e 8 will generate the write and read timing as shown in
Figure 14. The EMI_ALE signal, which indicates the start of a bus cycle, is connected to the
ADV# pin. The MT45W4M latches the address "33h" at the rising edge of EMI_ALE, and
then latches the write data "CDh" at the rising edge of EMI_WEn. Please note the
MT45W4M ignores the inputs on its address pins after the address latching. This allows the
EMI to take over the AD bus and drive the data "CDh".
Figure 14. EMI bus write and read bus cycles in asynchronous mode
Note: For your reference an asynchronous software example is delivered with this application
note.
27/34
Interfacing with synchronous memory AN2647
Synchronous burst mode
PSRAM BCR register configuration for synchronous burst mode
For a system that requires high speed data transfer, you have to configure the PSRAM and
the EMI bus to operate in synchronous burst mode. Before writing to the PSRAM BCR (Bus
Configuration Register) to set up burst mode, the CRE signal must be driven high by writing
a 1 to GPIO pin P6.0. After the configuration, CRE must be set to low to return to normal bus
operation. The BCR configuration bus cycle operates in asynchronous mode and is
described in the previous section.
Table 9. shows the BCR configuration bit values. When writing to BCR, the register gets the
write data value not from the data bus input, but rather from the address lines A[21:0]. The
BCR data must then be embedded in your EMI address. Please note the internal 32-bit
ARM address is a byte aligned address. Before it is driven to the EMI bus, the address lines
are right shifted by one and is converted to a half-word (16-bit) aligned address.
Table 9. PSRAM bus configuration register bit setting for burst mode
Register
bit
21:20 reserved 0
19:18 Select BCR 10 Select BCR as target register
Bit name
Bit
value
Function
17:16 reserved
15 Operating mode 0
14 Initial latency 1 Fixed Latency; first access time in a burst cycle
13:11 Latency counter 100 Set to code 4
10 Wait polarity 0 Active low
9reserved0
8 Wait configuration 1 Asserted one data cycle before delay (default)
7:6 reserved 00
5:4 Drive strength 01 ½ (default)
3 Burst Wrap 1 Burst no wrap (default)
2:0 Burst length 111 Continuous Burst (default)
Synchronous Burst access(default=1, asynchronous
access)
After the PSRAM BCR register is configured for burst mode, the EMI bus also need to be reconfigured from asynchronous to synchronous burst mode. An EMI burst cycle consists of
an asynchronous access to read or write the first halfword and then followed by multiple
burst cycles, where each data transfer takes only one BCLK clock to complete. The timings
of the first access in a burst cycle are the same as defined in asynchronous mode.
EMI bus configuration for synchronous burst mode
Ta bl e 1 0 shows the EMI bus registers and control bits for synchronous mode operation. The
configuration is similar to the asynchronous mode, with the following additional bits are set
in the EMI_BCR register:
28/34
AN2647 Interfacing with synchronous memory
1. Enable synchronous mode
2. Enable BCLK clock
3. Enable burst read and write
4. Burst read data transfer length
5. Burst write data transfer length
Table 10. EMI and port register config. for PSRAM sync mode (80 MHZ) BCLK)
Register bit Multiplexed 16-bits asynchronous mode
EMI configuration
register
Bit Name
Bit
value
Function
19:18
17
BWLEN
SYNCWRITEDE
V
Burst write transfer length: Continuous
11
Burst
1 synchronous write device
16 BMWrite 1 Burst mode write enabled
EMI_BCRx (maintain
reserved bits' default
value)
11:10 BRLEN 00
9 SYNCREADDEV 0 synchronous read device
Burst read transfer length: Continuous
Burst
8 BPM 0 Burst mode read
5:4 MW 01 16-bit data bus width
3 WP 0 Bank not write protected
0 BLE 1 Byte Lane Enable
EMI_RCR 4:0 WSTRD 101
Read Wait State = 5
70 ns PSRAM access time
EMI_OECR 3:0 WSTOEN 100 EMI_RDn assertion delay =4
EMI_WCR 4:0 WSTWR 101 Write Wait State =5
EMI_WECR 3:0 WSTWEN 000 EMI_WRn assertion delay =0
EMI_ICR 3:0 IDCY 011 Idle Cycle = 3 BCLK
EMI_BRDCR 4:0
WSTBRD
000 Burst read wait state=0
2 BYTE_EN 1 Enable EMI_UBn and LBn signals
SCU_EMI
1 BCLK_EN 0 Enable BCLK output
0 GPIOEMI 1 Set Port 8/9 as EMI bus
SCU_CLKCNTR 18:17 EMIRatio 00 BCLK=HCLK (80 MHz)
8 EMI_ALE_LNGT 1 ALE length=1.5 t
SCU_SCR0
SCU_GPIOTYPE [5:9] 7:0 Push-pull 0x00
7 EMI_ALE_POLR 0 ALE polarity-active low
EMI_MULTIPLE
6
XED
0 Multiplexed mode
Configure Port 5,6,7,8 and 9 as push-
pull
29/34
BCLK
Interfacing with synchronous memory AN2647
Table 10. EMI and port register config. for PSRAM sync mode (80 MHZ) BCLK)
Register bit Multiplexed 16-bits asynchronous mode
EMI configuration
register
SCU_GPIOOUT7 15:0 Output Function 0x0AAA Set Port 7 pins P7.5-P7.0 as A[21:16]
SCU_GPIOOUT6 15:0 Output Function 0x0001 P6.0 as GPIO out
SCU_GPIOOUT5 15:0 Output Function 0x0C00 Set P5.5 as CS1
Bit Name
Bit
value
Function
EMI burst read bus cycle
In synchronous mode, the PSRAM latches the address at the rising clock edge while ALE is
low. The write signal WEn must be high when the address is sampled to start a read bus
cycle. The STR91xFA always reads in 16-bit with the UB/LB byte select signals staying low
for the entire cycle. For the initial access of the read cycle, the PSRAM is set up for fixed
latency with a latency counter value of 4. The read wait state WSTRD in the EMI is set up to
5 to match the PSRAM timing. Burst read cycle starts after the first initial access and the
PSRAM is driving data out every BCLK clock until the end of the chip select signal CSx. The
timing in Figure 15. shows a burst read cycle initiated by STR91xFA while executing a Load
Multiple instruction.
Figure 15. EMI read bus cycle in burst mode
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AN2647 Interfacing with synchronous memory
EMI burst write bus cycle
In synchronous mode the write signal WEn is sampled at the same time the address is
latched. This requires the write assertion delay WSTWEN to be "0" so WEn will stay low at
the beginning of the bus cycle. STR91xFA always writes to PSRAM in 16-bit mode and the
two byte-select signals UB/LB are low for the entire write cycle.
The latency counter in the BCR register defines when the write data is sampled by PSRAM,
and this time has to match the EMI write timing so the first write data is latched at the right
clock edge. In this example the write wait state WSTWR is set to 5 and the latency counter
is set to 4. Refer to the MT45W4M data sheet for the timing details.
Figure 16. EMI write bus cycle in burst mode
Note: The EMI_BAA signal is not needed for the PSRAM interface.
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Interfacing with synchronous memory AN2647
DMA for high speed data transfer
STR91xFA has up to 96 KBytes of internal SRAM for data storage. DMA is the most efficient
way to transfer data between internal SRAM and the external PSRAM. The advantages of
using the DMA Controller are:
– Reduce the load on CPU
– Data is transferred in high speed burst mode
You can select one of the eight available DMA channel for data transfer. The transfer is of
the memory-to-memory type. Therefore, the DMA channel commences transfers without
DMA requests.
For burst Write, the source address is the internal SRAM and the destination address is the
external PSRAM.For more details user can refer to the PSRAM synchronous example
accompanied with the current application note which use DMA to generate burst writes in
the external PSRAM.
For burst read, the source address is the PSRAM and the destination address is the internal
SRAM. The continuous burst read cycle in Figure 8 supports a data transfer rate of one
BCLK clock period per halfword. Note the chip select CSx and EMI_RDn stay low until the
burst cycle is completed.
Figure 17. EMI continuous burst read during DMA
32/34
AN2647 Revision history
3 Revision history
Table 11. Document revision history
Date Revision Changes
13-Dec-2007 1 Initial release.
33/34
AN2647
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