ST AN1753 Application note

AN1753

APPLICATION NOTE

SOFTWARE UART USING

ST7 12-BIT AUTORELOAD TIMER

by Microcontroller Division Applications

INTRODUCTION

This application note des cribes a softw are im plementati on of a U niversal As ynchronous Re­ceiver/Transmitter (UART). This can be used on devices, like the ST7LIT E0, with no on- chip
SCI peripheral.

In this example, a software UART is implemented for the ST7FLITE0, using the 12-bit Autore­load timer and two I/O ports for asynchronous receive and transmit. The UART software pro­vides the following features:

• Half-duplex operation

• Asynchronous operation

• Flexible data formats (7 or 8 data bits, 1 or 2 stop bits)

• Baudrate: 2400 to 19200 baud
To test this interrupt-driven software UART, you can use the “Hyperterminal” application run-

ning on a Windows PC.
The program code is quite small (357 bytes) and can easily be adapted to specific application

requirements.

Rev. 1.0

AN1753/1103 1/22

1

Table of Contents

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 UART COMMUNICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 MAIN FEA TU RES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 BAUD RATE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3 FRAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 RS232 COMMUNICATION WITH A PC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 MAIN FEA TU RES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 PC CONFIGURATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 ST7FLITE0 CON FIGURATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.1 CLOCK SOURCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.2 INPUT INITIALIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.3 AUTO-RELOAD TIMER REGISTER CONFIGURATION . . . . . . . . . . . . . . . . . . 6

4 UART IMPLEM ENTATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.1 BAUD RATE DEFINITION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.2 MAJORITY VOTING SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.3 STATUS HANDLING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.4 TRANSM IT & RECEIVE IMPLEMENTATION . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5 HARDWARE SETUP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6 FUNCTIONAL SOFTWARE FLOW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6.1 MAIN SOFT W ARE ROUTINES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6.2 SOFTWAR E FLOW CHARTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

7 TEST PROCEDURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

8 SOFTWARE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

1 UART COMM UNICATION

The main features of a standard UART are summarized below.

1.1 MAIN FEATURES

The UART offers a flexible means of full-duplex data exchange
with external equipment requiring an industry standard NRZ asynchronous serial data format.
The UART allows a very wide range of baud r ates and different baud rates for transmission

and
reception.
In UART communication, a minimum of only two signals are needed, one for transmission and

other for reception. No clock signal is needed as it works in asynchronous mode. Each device
has to hav e a Transm it Data Ou tput pin (the PA3 pin is used in o ur example for the
ST7FLITE0) and a Receive Data Input pin (PA7 pin in our example). (Refer to Figure 1.)

Figure 1. ST7 UART Communication Set-Up

ST7 System

Terminal
Device

(Host)

ST7FLITE0

(TDO) PA3

(RDI) PA7

You must be very careful to identify the use of each pin. A simple method is to put the device
in transmission and check with an oscilloscope if a transmission frame is present or not.

1.2 BAUD RATE

Transmission and r eception can be driven by their own baud rate generator. However be
aware that for correct communication, the receiver must have a reception baud rate strictly
equal to the transmission baud rate of the transmitter. If not, the communication will be cor-

rupted. A s long as this condition is met, a wide range of baud rates is possible.

RDI

TDO

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

1.3 FRAME

Any transmission is Least Significant Bit first. A data word is us ually 8 bi ts long. A data frame
begins with a «star t bit», which i s a ‘0‘ bit and ends wit h a «stop bit», wh ich is a ‘1‘ bit. See
Figure 2.

Figure 2. Frames

8-bit Wo rd l ength

Next Data Frame

Data Frame

Start

Bit

Bit0 Bit1

Bit2

Bit3 Bit4

Bit5 Bit6

Bit7

Stop

Bit

Next
Start

Bit

In some cases, a 9th bit can be used, as a parity bit or as a second stop bit.

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

2 RS232 COMMUNICATION WITH A PC

2.1 MAIN FEATURES

Electrical and protocol characteristics of R S232 are different from those used by the UART. In
RS232 communication, high level is typically +7V and low level is typically -7V, while the ST7
I/Os work at CMO S levels (0, +5V).

Furthermore, the polarities are different. A ‘1‘ bit coming from the UART corr esponds to a ‘0‘
bit

in RS232, and a ‘0‘ bit to a ‘1‘ bit. It is true for all bits including the START and STOP bits. So
it is necessary to implement a conversion between the PC and the ST7. In the appl ication, a
MAX232 is used for this purpose.

2.2 PC CONFIGURATION

The PC will be used as a terminal interface. “Hyperteminal”, the terminal application software
is used as a interfacing software to test the functionality [.zip file attached]. The test environ-

ment is Windows 2000.
Under Windows , open the Hy pertermin al applicatio n. To configur e it, go to the port s ettings

and set the parameters to your application requirements. The options in this window must be
the same as the ones defined for your ST7FLITE0, communication device, except the port.

After selecting the right serial commun ication port, select the same baud rate as t he one set
for the ST7. As the PC accepts only one baud rate, transmission and reception baud rates will

have the same value. Data word can be 8/7 bits, but y ou can choos e to use 1 or 2 s top bits.
«Flow control» can be either Xon/Xoff or none. The PC is then correctly configured.

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

3 ST7FLITE0 CONFIGURATION

3.1 CLOCK SOURCE

This application is implemented using ST7FLITE0 device with an 8 MHz internal clock. PLL *
8 is used to generate this 8 MHz clock.

3.2 INP UT INITIALIZATION

Two pins of the ST7FLITE0 are used:

- PA3: Pin of PortA

- PA7: Pin of PortA with interrupt
Pin PA3 is normal Input/Output port pin with no alternate function, used for transm ission.
During initialization it is configured as an output.
Pin PA7 is normal Input/Output port pin with no alternate function, used for data receive.
During initialization it is configured as an input. While in receive mode, at start, same pin is

used with interrupt enabled (“ei1”) to sense “START” bit. So this pin is configured with “pull up
interrupt input” by setting PADDR to 0 & PAOR to 1. And to set interrupt sensitivity “Falling

edge only” we set IS11=1 & IS10=0 in the EICR register.
Refer to the device datasheet for detailed description of the I/O and interrupt control registers .

3.3 AUTO-RELOAD TIMER REGISTER CONFIGURATION

The AT timer is based on free running 12-bit up counter with 12-bit auto reload register (ATR).
Apart from this it also includes other functionality like, PWM s ignal generator & Output Com­pare Function & etc.

We are using “Output Compare” functionality for this application. To use this function, the OE
bit must be 0, otherwise the compare is done with the shadow register instead of the DCRx
register. Software must then write a 12-bit value in the DCR0H and DCR0L registers. W hen
the 12-bit upcount er (CNTR) re aches the value s tored in th e DCR0H and DCR0L reg isters,
the CMPF 0 bit in th e PWM0 CSR register is set and an interrup t req uest is gen erated if the
CMPIE bit is set.

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

The registers that are used in application note are:

TIMER CONTROL STAT US RE GISTER (ATCSR):

CMPIEOVFIEOVFCK0CK1000

- CK1 & CK0: They select the clock frequency of the counter.
For fcounter = fcpu, set CK1=1 & CK0= 0,

- CMPIE: It allows to mask the interrupt generation when CMPF bit is set.

When it is 0: CMPF interrupt disabled

1: CMPF interrupt enabled

PWM0 CONTROL/STATUS REGISTER ( PWM0CSR):

0000 OP0

0

CMPF00

- CMPF0:

It indicates that the upcounter value matches the DCR register value. When it is,
0: Upcounter value does not match DCR value.
1: Upcounter value matches DCR value.

PWM OUTPUT CONTROL REGISTER (PWMCR):

000

0

0

00

OE0

- OE0: When set to 1, PWM0 output enabled

Timer Initialization:

Write to DCRx registers with required value. Then Output Compare functionality must be en­abled. To do this, reset the OE bit in the PWMCR register. This will disable PWM output.

To configure fcounter = fcpu, set CK1=1 & CK0=0, and to enable the CMPF interrupt, set
CMPIE=1 in the ATCSR register.

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

4 UART IMPL EMENTATION

4.1 BAUD RATE DEFINITION

The Autoreload timer is used to generate the baud rate. Autoreload timer clock (fCOUNTER)
is same as fcpu clock, which is 8 MHz internal clock.

The description here below is the example to show how this baud is generated:
Example:

- baudrate = 2400

- fcpu = 8MHz (so clock period is 125 ns)

- In the application fCOUNTER = fcpu

So to get 2400 baud the “Prescaler” required is,

Prescaler = fcpu / baud

6

Prescaler = 8 * 10

= 0.0033333333 * 10

/ 2400

6

= 3333.33 decimal
= D05 Hex (= 1 bit delay count)

So in software DCRx will be

DCR0H = 0D + CNTRH

DCR0L = 05 + CNTRL
to generate one bit delay when 2400 baud is required
For half bit period “Prescaler” is,

Prescaler = one bit / 2

= 1666.66 (decimal)

= 682 Hex (= 1/2 bit delay)
DCR0H = 06 + CNTRH
DCR0L = 82 + CNTRL

The software provided with this application note has been functionally tested in the range
2400 to 19200 baud.

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

4.2 MAJORITY VOTI NG SYSTEM

The receive section is complex compared to the transmit section. So, to avoid possible errors
in detecting exact bit status because of line noise etc, this program uses a majority-voting
system. When the ST7 is in receive mode, i t reads (samples) each bit three times at the
middle of the bit time. To determine its exact state, it compares the number of 1’s with the
number of 0’s. If there are more ‘1‘s than ‘0‘s, the bi t received is a ‘1‘, else it is a ‘0‘. ( see Figure

3.)

Figure 3. Majority voting system

START

1st sample

012 3

3rd sample

2nd sample

4

5

67

STOP

This section is only used for the receive part. You can change it to suit your requirements.

4.3 STATUS HANDLING

To keep track of status during program execution, software uses variable, “sci_status”.
Bits 0-4 of this variable are used to hold the system status.

SPBRREBSTE

Definition of each flag:
SP: Reception mode sampling phase
BR: Byte Received flag
RE: Reception Enable
BS: Byte Sent flag
TE: Transmission Enable

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

4.4 TRANSMIT & RECEIVE IMPLEMENTATION

Transmission mode gets executed after power on. To transmit data, the program uses port pin
PA3 in output mode with “Output Compare” interrupt. One bit is transmitted during each inter­rupt, generated after a bit delay when the 12-bit upcounter (CNTR) reaches the value (=1 bit
delay) stored in the DCR0H and DCR0L registers. The value for duty cycle resister (DCR0L/
DCR0H) depends on the required baud rate & clock frequency.

For receive mode, the program uses port pin PA7 in input mode along with its interrupt &
output compare f unctionality. To s ense STA RT bit, interr upt “ei1” is used, configured a s
“Falling edge only”. In this interrupt routine, program sets the ATR - output compare value for
half* bit delay & di sable “ei1” interrupt to use PA7 in nor mal input mode to receive enti re frame.
This half bit dela y is used to sa mple bit at m iddle of bit. After r eceiving START bi t in output
compare interrupt, the same interrupt routine gets executed each time a bit must be received.
To receive each bit correctly majority voting system is implemented in this interrupt routine.

The interrupt strategy used in software for transmit & receive allows to have other applications
to work at the same time.

Note:
– Error handling: This program does not handle communication errors (e.g. Frame Errors).
– No handshaking is implemented.
– For better performance at high baud rates it is r ecommended to use character transmission

rather than string transmission.

*User can adjust this value depending on baud rate.

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

5 HARDWARE SETUP

A general system configuration is shown in Figure 1. The ST7FLITE0 is connected to an RS­232 line driver chip which is in turn connected to any RS-232-compatible device. The RS-232
line driver is needed to convert t he 5-volt logic le vel of th e ST7FLITE 0 to the proper R S-232
line voltages, and vice versa.

Figure 4. shows a specific e xample of a hardware s etup required for UA RT in which the
ST7FLITE0 processor and RS-232 Driver/ Receiver are used. You can use different driver/re­ceiver in your application. For example you can use a MC14C88/ 89 as a low cost solution.

The Receive Data pin (RD) of the serial port of the PC mu st correspond to the PA3 pin of the
ST7FLITE0, and the Transmit Data pin (TD) to PA7 pin.

Be sure that the th ree main de vices ( PC, ST7, MA X232 ) have the sam e elec trical refe rence
(GND). For a detailed description of MAX232, please refer to the datasheet.

Figure 4. Test Setup

TERMINAL

PC

seri al port

GND

TD
RD

GROU ND

GND

OUTIN

OUT

MAX232

ST7FLITE0

GND

PA7 pin

IN

PA3 pin

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

6 FUNCTIONAL SOFTWARE FLOW

The character format used is “ASCII”. Before starting communication you should configure the
communication parameters to your requirements. You can select the baudrate, number of bits
and STO P b its in d iffe r en t c omb i nati ons. H ow e ver , for co r rec t c om mun i cati on , th e r ecei ver
must have a reception baud rate strictly equal to the transmission

baud rate of the transmitter. If not, the communication will be corrupted. So you have to con
figure “Hyperterminal” correctly.

To configur e th e dev ice , you m us t se t d el_1b h, del_1 bl , del _sam pl, d el_sa mp h, STO P_BIT
and TxRx_data_lnth correctly in your software.

After power on, the ST7FLITE0 goes in to transmit mode. It initiates com munication by trans­mitting character “$” in ASCII format & then goes into receive mode. The PC then receives this
character on its serial port, which can be viewed in the Hyperterminal window.

Now you have to press a key (charac ter) to trans mit. After pressing a c haracte r key, it gets
transmitted and the ST7 receives it. The ST7 then s ends same received c haracter to PC. This
sequence can be repeated continuously (Refer to the flow charts below).

6.1 MAIN SOFTWARE ROUTINES

The UART software consists of main 4 routines:
– Tx_data
– Rx_data
– EI1_Interrupt
– OPCOMP_int
– Tx_data: This routine is used to transmit data in bit format stored in variable “TX_byte”. It

also takes care of synchronization & the output compare interrupt. The AT timer is initialized
to the application configuration inside this routine.

– Rx_data: This routine is used to receive data on the input pin in bit format & stores it in

“RX_byte”. To detect a “START” bit it initializes the PA7 pin in interrupt mode.

– EI1_Interrupt: This “ei1” interrupt routine gets executed only once during receive data when

PA7 receives a “START” (High to Low edge) bit. It also initiates the AT timer for half bit du­ration.

– OPCOMP_int: This routine gets executed in both transmit & receive mode. During

Tx_dataroutine execution this gets executed for each data bit transfer. And it is actually used
to transmit data on the TxD (PA3) pin along with START & STOP bits.

During receive mode it gets executed for each data bit received including the START & STOP
bits.

The majority-voting system is implemented in the same section of code.

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6.2 SOFTWARE FLOW CHARTS

The flowcharts below represent the application program flow.

Figure 5. Main Flowchart

MAIN

Initialize port & variables

Initiate Transmission

RX_data

tx_byte = rx_byte

TX_data

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Figure 6. Output Compare Interrupt

t

OPCOMP_int

Init DCRx with CNTRx

Go on “Transmit routine”

Is it 1st. bit?

no

yes

Transmit/Receive

Go on “Receive routine”

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Figure 7. Transmit routine

SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

TX_data

Set “TE” flag

Set synchronization

delay

Set clock & enable out­put compare interrupt

RIM

“BS” flag set

?

yes

SIM

ret

no

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

Figure 8. Receive data routine

RX_data

Set “RE” flag

Init required variable

Put “RxD” in input inter­rupt mode. Set EICR reg.

RIM

“BR” flag set

?

yes

SIM

ret

no

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Figure 9. Output Compare Tran smit flow

Transmit routine

Set “BS” flag in

“sci_status” var.

Disable interrupt & stop

timer clock

b

bit_count=

“0”
?

no

yes

”FRAME”

sent?

no

All “data”
sent ?

no

Transmit “data” bit

Load “CNTR” with 1 bit
delay count

yes

Send “START” bit

a

yes

Send “STOP”bit

a

Clear prev. interrupt &
set compare interrupt

iret

b

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

Figure 10. Output Compare Receive Flow

Receive routine

Read & Check for
“STOP” bit condition

Full“frame”

Received?

no

Read “RxD” & apply

majority voting system

Need

“START”

?

no

yes

Need

“STOP” ?

no

Store received data

yes

yes

c

Read & Check for
“START” bit condition

a

c

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Set “BR” flag

Disable interrupt & stop

timer clock

b

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

Figure 11. EI1 Interrupt rout ine

EI1_int

Read CNTRx

Set DCRx for 0.5 bit

delay

Set “SP” flag & put “RxD”
in normal input mode

Clear pending interrupt

Enable output compare
interrupt & select clock

iret

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7 TEST PROCEDURE

– Connect both the communication & terminal equipment as shown in fig.4
– Open & configure “Hyperterminal “application [Port, Baud rate & etc.].
– Power on ST7
– On the terminal you should get “$” as a starting character (If not then check link between

terminal and the ST7 and check the communication settings)
– Then press a key to send a character
– The same character should be returned by the ST7 (check “Hyperterminal” window)
– Continue by repeating the previous two steps

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

8 SOFTWARE

The complete software can be found on the ST internet webs ite in zipped file format. It is in­tended for use only as an example. It is up to you to adapt it to your specific application.

The source file is for guidance only. STMicroelectronics shall not be held liable for any direct,
indirect or consequential damages with respect to any claims arising from use of this software.

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SOFTWARE UART USING ST7 12-BIT AUTORELOAD TIMER

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SHALL NOT BE HELD LIABLE FOR ANY DIRECT, INDIRECT OR CONSEQUENTIAL DAMAGES WITH RESPECT TO
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THE INFORMATION CONTAINED HEREIN IN CONNECTION WITH THEIR PRODUCTS.”

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