Analog Devices EE191 Application Notes
Engineer To Engineer Note EE-191
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Technical Notes on using Analog Devices' DSP components and development tools
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Implementing a Glueless UART Using The SHARC® DSP SPORTs
Contributed by Dan Ledger May 6, 2003
Introduction
Using the synchronous serial ports (SPORTS) on
the SHARC® DSP, it is possible to implement a
full-duplex, asynchronous serial port to
communicate with UARTs, EIA-232 (RS-232)
devices and MIDI interfaces with minimal
software overhead (less than 1 MIP for fullduplex, 115,200 bps operation).
This application note presents both the software
and hardware to implement a full UART or RS232 interface which has been extensively tested
in hardware up to 115,200 bits per second.
Because the serial ports on SHARC DSP operate
well into the tens of megabits/second, the
asynchronous interface can operate well beyond
115,200 bps.
Introduction to Asynchronous Serial Communications
The primary difference between synchronous and
asynchronous serial communication is the
presence of a clock signal, and possibly a frame
sync signal. A synchronous serial port has both a
clock signal and an optional frame sync signal
while an asynchronous port does not.
In the absence of a clock signal, asynchronous
ports must communicate at a predetermined data
rate typically referred to as the bit rate,
specifying the bits per second (bps).
In the absence of a frame sync, word framing
information is embedded in the data stream. A
start-bit and stop-bit typically mark the
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beginning and end of a transmission. The word
length is predetermined between the receiver and
transmitter.
For more information on asynchronous serial
communications, visit this website:
http://www.freebsd.org/doc/en_US.ISO8859-1/articles/serial-uart/
The Asynchronous SPORT Transmitter
The UART transmitter on the SHARC DSP
serial port is quite simple. The transmit side of
the serial port must be configured with a clock
rate equal to the desired bit rate of the UART.
This is done by setting the transmit portion of the
clock divider register (DIVx) for that serial port.
The formula for calculating the divider value can
be found below or in the Serial Port chapter of
the appropriate SHARC Hardware Reference
Manual.
Divisor
=
2
×
FrequencyClockCoreDSP
1
−
ratebit
The clock is only used to synchronize the serial
port to desired bit rate but the actual clock signal
(TCLKx) does not connect to anything. The
frame sync signal is also left floating.
The configuration of the transmit side of the
SPORT and associated DSP software is
presented in detail in the Software Overview
portion of this document.
s
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The Asynchronous SPORT Receiver
The receive portion is slightly more complicated
than the transmit portion as the serial port needs
to determine where a new work is beginning
without the presence of frame sync signal. To
accomplish this, the transmit pin of the UART
connects to both the Data Receive pin (DRx) and
the Receive Frame Sync (RFSx) on the SHARC
DSP SPORT. The RFS pin is polled on the
active edge of the internal RCLKx signal by the
DSP. When RFS is asserted, the DSP begins
receiving a new word and will not check the RFS
line until all N (N is programmable in the
SPORT control register) bits have been received.
The SPORT will thus use the start bit as a frame
sync to kick of the reception. As the SPORT has
no way of guaranteeing any phase
synchronization with the incoming bit stream, it
is necessary to over-sample the incoming
asynchronous data stream. To accomplish this,
the receive clock on the SPORT must be set to
three times the desired bit rate. So, for example,
if the DSP is communicating with a UART
device at 9,600 bits/second, the transmit clock on
the SPORT needs to be set at 9,600 bits/second
and the receive clock on the SPORT needs to be
set at 3 * 9600, or 28,800 bits/second. Again,
this is accomplished by calculating the
appropriate divisors and programming the DIVx
register.
The timing diagram below demonstrates how the
SHARC DSP SPORT receives an 8-bit
transmission from a UART device. The arrow
indicates where the SPORT polls the RFS signal
indicating the beginning of a new word.
presented in detail in the Software Overview
portion of this document.
Hardware Overview: UART
The diagram below shows the connection
between a SHARC DSP serial port and the TX
(transmit) and RX (receive) pins of a basic
UART port on another device.
TCLKx
TFSx
DTx
DRx
RFSx
RCLKx
Figure 2:SHARC DSP to UART Interface
No additional logic is required in this interface;
however, the SHARC DSP with 3.3V I/O are not
5V tolerant. To connect a SHARC DSP device
with 3.3V I/O to a 5V UART is often fairly
simple. The DTx pin of the SPORT can often
connect directly to the RX pin of the UART if it
is a CMOS device as 3.3V is greater than the
minimum high level signal in 5V CMOS. In the
other direction (TX to DRx and RFSx), a series
resistor or diode can be used to drop the voltage
from 5V to 3.3V. The resistor value can be
calculated using the high and low level input
currents listed in the data sheet for both the
UART device and the SHARC DSP.
RX
TX
Hardware Overview: EIA-232 (RS-232) Device
Figure 1: Over-sampling the incoming asynchronous
data
The configuration of the receive side of the
SPORT and associated DSP software is
Implementing a Glueless UART Using The SHARC® DSP SPORTs (EE-191) Page 2 of 21
Connecting the DSP to an RS-232 port is very
similar to a basic UART interface except an RS232 interface will require an RS-232 interface
device to boost the RX and TX signals up to 9V
as well as provide the necessary RS-232
hardware handshake signals like CTS and RTS.
The ADM3202 from Analog Devices is an RS-
a
232 interface device which provides a 3.3V
interface to the DSP. It is shown in the
schematics below.
Two flag pins can be optionally connected to the
ADM3202 to manage the CTS and RTS control
signals.
Figure 3 : SHARC DSP to RS-232 Interface
Implementing a Glueless UART Using The SHARC® DSP SPORTs (EE-191) Page 3 of 21
Software Overview
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The software required to manage the
asynchronous data moving in and out of the
SPORT is minimal. A full set of C-callable,
hand-coded assembly functions is available at the
end of this application note and included in the
associated .zip file. All code examples in this
section are included within the driver code.
On the transmit side, a start and stop bit must be
placed into the N bit word to be transmitted.
Thanks to the efficiency of the SHARC DSP
architecture, this can be performed in 6 cycles
per transmitted word.
R0 = dm(VALUE_TO_TRANSMIT);
R2 = b#01111111100;
R4 = b#10000000001;
R1 = lshift R0 by 2;
R1 = R1 and R2;
R1 = R1 OR R4;
Listing 1: UART Transmit Byte
The receive side is slightly more complex as the
SPORT is receiving over-sampled data. As the
data is over-sampled by a factor of 3, the serial
port must be programmed to receive 29 bit
words. The first two bits received will be the
second and third bits of the frame sync and the
remaining 27 bits will represent 9 bits of data (8
bit-word and 1 stop bit) over-sampled by a factor
of 3.
recovers an 8-bit word from the over-sampled
data in 16 cycles.
R0 = dm(RX3B); // Word from DR
R4 = fext R0 by 25:1; // bit 0
R1 = fext R0 by 22:1; // bit 1
R4 = R4 or fdep R1 by 1:1;
R1 = fext R0 by 19:1; // bit 2
R4 = R4 or fdep R1 by 2:1;
R1 = fext R0 by 16:1; // bit 3
R4 = R4 or fdep R1 by 3:1;
R1 = fext R0 by 13:1; // bit 4
R4 = R4 or fdep R1 by 4:1;
R1 = fext R0 by 10:1; // bit 5
R4 = R4 or fdep R1 by 5:1;
R1 = fext R0 by 7:1; // bit 6
R4 = R4 or fdep R1 by 6:1;
R1 = fext R0 by 4:1; // bit 7
R4 = R4 or fdep R1 by 7:1;
// r4 contains received word
Listing 2: UART Receive Byte
The software to support the UART functionality
of the SPORT is very light weight. In fact,
sustaining full-duplex transfers at 115200 bps
requires only 0.4% of the available MIPS on an
ADSP-21161N SHARC DSP clocked with
100MHz. The C-callable UART drivers
included with this application note include buffer
management functionality and context-save and
restore within interrupts requiring about 1% of
the available MIPS on an ADSP-21161N.
To extract the actual data from our over-sampled
word, the
fext function can be used to process
this data efficiently. The following function
Implementing a Glueless UART Using The SHARC® DSP SPORTs (EE-191) Page 4 of 21
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Driver Function Prototypes
The following functions have been written for the ADSP-21161N SHARC DSP, however, are
fundamentally compatible with all devices in the SHARC DSP family. By default, these functions are
designed to work on 8-bit asynchronous data but can easily be changed to support other data widths.
Note : This code uses label.end: notations to terminate assembly functions and thus may generate errors when assembled
under tools versions prior to VisualDSP++™ 3.0. Simply remove these declarations at the end of the assembly functions to
assemble under earlier releases of the tools.
Function Prototype
float Setup_UART( float DSP_Frequency, float BAUD )
Description
Setup_UART is used to configure SPORT1 and SPORT3 for UART emulation. DSP_Frequency
is the internal clock frequency (In Hz, not Mhz) of the DSP. For most ADSP-21161N
applications, this value will be 100000000.0. BAUD is the desired bit rate of the serial port. As
the clock rate of the serial port is derived from the core clock frequency using a clock divider, it is
not always possible to achieve an exact match to the baud rate; however, is most conditions the
error is low enough that it should not impact the functionality of the UART. Only at very high
operating frequencies (500kbits/second and up) will this pose a problem. This function returns a
floating point value representing the deviation (desired bit rate – actual bit rate).
In addition, Setup_UART contains self-modifying code which will overwrite the SPORT transmit
vector to point to a function called UartTx_Service. This function manages asynchronous buffer
transfers in the background so the processor is not held up when transmitting a block of data.
Example
#include “UART_driver.h”
float err;
err = Setup_UART(100000000.0,115200.0);
Implementing a Glueless UART Using The SHARC® DSP SPORTs (EE-191) Page 5 of 21
Function Prototype
char UartRx();
Description
UartRx returns the value received from the UART device. This function should be placed within a
SPORT receive interrupt service routine.
Example
#include <signal.h>
#include “UART_driver.h”
…
void UART_RX_ISR( int sig_int )
{
RX_Buffer[Rx_Indx++] = UartRx();
}
main()
{
interruptf(UART_RX_INTERRUPT, UART_RX_ISR);
}
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note : UART_RX_INTERRUPT and UART_TX_INTERRUPT are defined within UART_Driver.h and can
be used to replace the interrupt definitions in the stock signal.h as seen in the example above.
Implementing a Glueless UART Using The SHARC® DSP SPORTs (EE-191) Page 6 of 21
Function Prototype
void UartTx_Word( char word );
Description
a
UartTx_Word copies the value,
word into the UART transmit buffer. If the transmit buffer is
empty, this function enables the transmit interrupt. The transmit buffer, which is setup as a
software circular buffer, facilitates data transmission and is managed by the UartTx_Service
function. This function is automatically attached to the transmit interrupt during the UART_Setup
function.
Example
#include “UART_driver.h”
UartTx_Word(12); // clear terminal screen
Implementing a Glueless UART Using The SHARC® DSP SPORTs (EE-191) Page 7 of 21
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