Difference between revisions of "A4.8051 Communication Protocols:UART, RS232"

 
(39 intermediate revisions by 4 users not shown)
Line 1: Line 1:
 +
[[Category:8051_tutorials]]
 +
In this tutorial, we are going to discuss the serial/UART communication of 8051. After understating the basics of 8051 UART module, We will see how to use the ExploreEmbedded libraries to communicate with any of the UART devices.<br><br>
  
−
=UART Serial communication using 8051=
+
[[File:0 UART main.gif]]
−
{{Box|type=l_green_light|text=
+
−
We have covered the [[8051_timers|basics of timers]] in the previous tutorial, with that we can see how can a microcontroller communicate with a computer serially.
+
  
−
==The basics==
 
−
There are two ways to connect a microcontroller to a computer.
 
−
* Using RS232 protocol to the legacy serial port.
 
−
* Using a USB to serial convertor.
 
−
From microcontroller programming point of view, there is fundamentally no difference.
 
−
The communication between the computer and micrcontroller is asynchronous and full duplex.
 
−
*'''Asynchronous''': There is no fixed clock to synchronize transmission of data; rather a fix bit rate is specified at both transmitter and receiver, termed as '''Baud Rate'''. In simple words, it specifies number of bit received/transmitted in 1 second. With that transmitter and receiver get to know time duration for each bit transfer.
 
−
*'''Full Duplex'''Both computer and micrcontroller have the hardware capability to transmit and receive at the same time.
 
  
−
==The protocol==
+
=UART Registers=
−
'''RS232''':It is a legacy standard for serial communication.  The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pin-out of connectors.
+
The below table shows the registers associated with 8051 UART.<br>
−
==='''RS232 Signals'''===
+
{| class="table table-striped table-hover table-condensed table-bordered"
−
There are various signal for handshaking and communication in RS232 protocol. These are hardly used with modern computers, hence we will see '''Transmit(Tx)''' and '''Receive(Rx)''' Signals only.
+
|-class="info"
−
==='''Voltage Levels'''===
+
!Register || Description
−
Table below shows the voltage levels for RS232. They are significantly different from the TTL high(5v) and Low(0v). Hence a line driver like the MAX232 is required as shown in the schematic.
+
−
}}
+
−
{| class="wikitable" style="text-align:center;background-color:#87A96B;margin: 1em auto 1em auto;"
+
 
|-
 
|-
−
!Logic level||Voltages(V)
+
|SCON|| Serial Control Register
 
|-
 
|-
−
| 0 ||+3 to +25V
+
|TCON|| Timer Control Register for Baud Rate Generator
 
|-
 
|-
−
| 1 ||-3 to -25V
+
|TMOD|| Timer Mode Control for Baud Rate Generator
 
|-
 
|-
−
| Z ||+3 to -3V
+
|SBUFF||Serial Buffer holds the data to be transmitted and the data received
 
|}
 
|}
 +
<br><br>
  
 +
=UART Register Configuration=
 +
Now lets see how to configure the individual registers for UART communication.
 +
{| class="table table-striped table-hover table-condensed table-bordered"
 +
|-class="info"
 +
|SCON
 +
|-
 +
|7 || 6 ||  5 || 4 || 3 || 2 || 1 || 0 
 +
|-
 +
|SM0||SM1||SM2||REN||TB8||RB8||TI||RI
 +
|}
  
−
==8051 Serial communication Registers ==
+
{| class="table table-striped table-hover table-condensed table-bordered"
−
===SBUFF: Serial buffer register===
+
|-class="info"
−
{{Box|type=l_green_light|text=
+
!SM0||SM1||Operation||Description||Baud Rate Source
−
The important registers for serial communication are:
+
−
* '''SBUFF''', it is a serial buffer for data. It holds 8 bit data that needs to be transmitted or the data that is received. Since the serial port of 8051 is '''full duplex''', the microcontroller internally manages to transmit and receive data into the buffer register simultaneously.
+
−
}}
+
−
 
+
−
{| class="wikitable" style="text-align:center;background-color:#87A96B;margin: 1em auto 1em auto;"
+
 
|-
 
|-
−
!colspan = '8'|SBUFF
+
|0||0||Mode 0|| 8-bit UART||1/12 the quartz frequency
 
|-
 
|-
−
|D7||D6||D5||D4||D3||D2||D1||D0
+
|0||1||Mode 1|| 8-bit UART||Determined by the timer 1
 
|-
 
|-
−
| || || || || || || ||
+
|1||0||Mode 2|| 9-bit UART||1/32 the quartz frequency
 +
|-
 +
|1||1||Mode 0|| 9-bit UART||Determined by the timer 1
 
|}
 
|}
  
−
===SCON: Serial Control Register===
+
*<b>SM2 - Serial port mode 2 bit</b>, also known as multiprocessor communication enable bit. When set, it enables multiprocessor communication in mode 2 and 3, and eventually mode 1. It should be cleared in mode 0.
−
{{Box|type=l_green_light|text=
+
*'''REN - Reception Enable bit''' enables serial reception when set. When cleared, serial reception is disabled.
−
*'''SCON; Serial Control register''' SCON is also an 8 bit register used for configuring different modes for serial communication as described below.
+
*'''TB8 - Transmitter bit 8'''. Since all registers are 8-bit wide, this bit solves the problem of transmitting the 9th bit in modes 2 and 3. It is set to transmit a logic 1 in the 9th bit.
−
}}
+
*'''RB8''' - Receiver bit 8 or the 9th bit received in modes 2 and 3. Cleared by hardware if 9th bit received is a logic 0. Set by hardware if 9th bit received is a logic 1.
 +
*'''TI''' - Transmit Interrupt flag is automatically set at the moment the last bit of one byte is sent. It's a signal to the processor that the line is available for a new byte to transmit. It must be cleared from within the software.
 +
*'''RI''' - Receive Interrupt flag is automatically set upon one-byte receive. It signals that byte is received and should be read quickly prior to being replaced by a new data. This bit is also cleared from within the software.
 +
 
  
−
{| class="wikitable" style="text-align:center;background-color:#87A96B;margin: 1em auto 1em auto;"
+
{| class="table table-striped table-hover table-condensed table-bordered"
 +
|-class="info"
 +
|TMOD
 
|-
 
|-
−
!colspan = '8'|SCON
+
!colspan="4"|Timer1||colspan="4"|Timer 0
 
|-
 
|-
−
|D7||D6||D5||D4||D3||D2||D1||D0
+
|7||6||5||4||3||2||1||0
 
|-
 
|-
−
|SM0||SM1||SM2||REN||TB8||RB8||TI||RI 
+
|Gate||C/T||M1||M0||Gate||C/T||M1||M0
 
|}
 
|}
−
{{Box|type=l_green_light|text=<br />
+
*<b>Gate Control</b>
−
*SM0 - Serial port mode bit 0 is used for serial port mode selection.
+
0 = Timer enabled<br>
−
*SM1 - Serial port mode bit 1.
+
1 = Timer enabled if INTx\ is high
−
}}
+
*<b>C/T:</b>Counter or Timer Selector
−
{| class="wikitable" style="text-align:center;background-color:#87A96B;margin: 1em auto 1em auto;"
+
0 = Internal count source (clock/12)<br>
 +
1 = External count source (Tx pin)
 +
*<b>M1-M0:</b>Mode Control
 +
00-Mode 0, 13 bit count mode<br>
 +
01-Mode 1, 16 bit count mode<br>
 +
10-Mode 2, Auto reload mode<br>
 +
11-Mode 3, Multiple mode
 +
 
 +
 +
{| class="table table-striped table-hover table-condensed table-bordered"
 +
|-class="info"
 +
|TCON
 
|-
 
|-
−
!SM0||SM1||Operation||Description||Baud Rate Source
+
|7 || 6 ||   5 || 4 || 3 || 2 || 1 || 0
 
|-
 
|-
−
|0||0||Mode 0|| 8bit ShiftReg||1/12 the quartz frequency
+
|TF1||TR1||TF0||TR0|| || || ||
−
|-style="background: blue; color: white"
+
|}  
−
|0||1||Mode 1|| 8 bit UART||Determined by the timer 1
+
−
|-
+
−
|1||0||Mode 2|| 9 bit UART||1/32 the quartz frequency
+
−
|-
+
−
|1||1||Mode 0|| 9 bit UART||Determined by the timer 1
+
−
|}
+
−
{{Box|type=l_green_light|text=<br />
+
−
*'''SM2 - Serial port mode 2 bit''', also known as multiprocessor communication enable bit. When set, it enables multiprocessor communication in mode 2 and 3, and eventually mode 1. It should be cleared in mode 0.
+
−
*'''REN - Reception Enable bit''' enables serial reception when set. When cleared, serial reception is disabled.
+
−
*'''TB8 - Transmitter bit 8'''. Since all registers are 8-bit wide, this bit solves the problem of transmitting the 9th bit in modes 2 and 3. It is set to transmit a logic 1 in the 9th bit.
+
−
*'''RB8''' - Receiver bit 8 or the 9th bit received in modes 2 and 3. Cleared by hardware if 9th bit received is a logic 0. Set by hardware if 9th bit received is a logic 1.
+
−
*'''TI''' - Transmit Interrupt flag is automatically set at the moment the last bit of one byte is sent. It's a signal to the processor that the line is available for a new byte transmit. It must be cleared from within the software.
+
−
*'''RI''' - Receive Interrupt flag is automatically set upon one byte receive. It signals that byte is received and should be read quickly prior to being replaced by a new data. This bit is also cleared from within the software.
+
  
−
==The Baud Rate==
+
*<b>TRx:</b> Timer x run control
−
As discussed earlier, both transmitter and receiver should agree upon a fixed data transfer rate known as baud rate. For the 8051 the '''Timer 1''' is used to generate the baud rate in '''Auto reload mode''.  
+
0 = Timer not running<br>
 +
1 = Timer running
 +
*<b>TFx:</b> Timer x OverFlow flag
 +
0 = Timer has not overflowed/rolled over<br>
 +
1 = Timer has overflowed/rolled over
 +
 
 +
=Baud Rate Calculation=
 +
The main criteria for UART communication is its baud rate. Both the devices Rx/Tx should be set to same baud rate for successful communication.<br>
 +
For the 8051 the '''Timer 1''' is used to generate the baud rate in '''Auto reload mode''.  
 
[[File:8051 Baud Rate.jpeg|thumbnail|x100px|Fig: Baud rate at 11.0592Mhz Crystal]]
 
[[File:8051 Baud Rate.jpeg|thumbnail|x100px|Fig: Baud rate at 11.0592Mhz Crystal]]
−
The crystal frequency '''Fclk''' is divided by 12 internally which is used to execute instructions also known as Machine Clock. Mclk. The timer again divides the Mclk by 32 and uses it as timer frequecny, say Tclk.
 
  
−
Usually a 11.0592 Mhz crystal oscillator is used to provide clock to 8051. The value seems to be odd but we see how it makes sense.
+
The crystal frequency '''Fclk''' is divided by 12 internally which is used to execute instructions also known as Machine Clock. Mclk. The timer again divides the Mclk by 32 and uses it as the timer frequency, say Tclk.
 +
 
 +
Usually, an 11.0592 Mhz crystal oscillator is used to provide the clock to 8051. The value seems to be odd but we see how it makes sense.
 
From above discussion<br/>
 
From above discussion<br/>
 
        
 
        
−
'''''Mclk = Fclk / 12 = 11.0592MHz/12 = 921.6KHz'''''<br/>
+
'''''Mclk = Fclk / 12 = 11.0592MHz/12 = 921.6KHz'''''<br>
  
−
'''''Tclk = Mclk/32= 921.6KHz/32 = 28.8KHz''''' <br\>
+
'''''Tclk = Mclk/32= 921.6KHz/32 = 28.8KHz''''' <br>
  
 
If we look at the standard baud rates used for serial communication shown in the table below, we can observe that all the baud rates are factors or multiples of the  Tclk (28.8K)! This results in low error rates and hence is a commonly used crystal with 8051.
 
If we look at the standard baud rates used for serial communication shown in the table below, we can observe that all the baud rates are factors or multiples of the  Tclk (28.8K)! This results in low error rates and hence is a commonly used crystal with 8051.
−
}}
+
{| class="table table-striped table-hover table-condensed table-bordered"
−
{| class="wikitable" style="text-align:center;background-color:#87A96B;margin: 1em auto 1em auto;"
+
|-class="info"
−
|-
+
!Baud Rate|| Factor)
−
!Baud Rate|| Multiple (or Factor)
+
 
|-
 
|-
 
|2400||Tclk/'''12'''
 
|2400||Tclk/'''12'''
Line 109: Line 117:
 
|9600||Tclk/'''3'''
 
|9600||Tclk/'''3'''
 
|-
 
|-
−
|115200||Tclk*'''4'''
+
|14400||Tclk/'''2'''
−
|-
+
−
|14400||Tclk*'''5'''
+
 
|}
 
|}
−
=The Schematic=
 
−
{{Box|type=l_green_light|text=
 
−
As discussed earlier, we can connect the 8051 microcontroller using RS232 line driver or using a USB to Serial converter like CP2102, as shown in the figure.
 
−
}}
 
−
{{scroll box|width=650px|height=500px|text=<br />
 
−
[[File:8051 SERIALCOMMN Interface.PNG|x550px]]
 
−
}}
 
−
=The Code=
 
−
{{Box|type=l_green_light|text=
 
−
Again for this example we will divide the code into 2 '''c'''files. '''main.c''' contains the usage of the xplore labz '''UART.c''' library.
 
−
}}
 
−
===main.c===
 
−
{{Box|type=l_green_light|text=
 
−
The '''uart.h''' contains all the functions for initializing the UART, transmit/receive characters, strings and numbers.
 
−
The program receives a string and replies back the same string for ever!
 
−
As the program is very simple, let us see the '''uart.c''' in more detail.
 
−
}}
 
−
<syntaxhighlight>
 
−
#include <reg51.h>
 
  
−
#include "uart.h" //Xplore Labz UART library
+
The above factors should be loaded to Timer1(TH1) in Mode2 in order to generate the required baud rate.
 +
The final formula for baud rate is as below.
 +
Baudrate = Fosc/(32 * 12 * (256-TH1))
 +
$$TH1 = 256 - (Fosc/(32 * 12 * Baudrate))$$  //If( SMOD==0 in PCON register)
 +
$$TH1 = 256 - (Fosc/(32 * 6 * Baudrate))$$  //If( SMOD==1 in PCON register)
 +
 
 +
Now with Fosc = 11.0592Mhz, TH1 value for 9600 baudrate will be:
 +
TH1 = 256-(11.0592*10^6)/(32 * 12 * 9600) = 253 = 0xFD = -3
 +
 
 +
=Uart Init Steps=
 +
#Select the 8-bit , 1-Start and 1-Stop bit mode in SMOD
 +
#Configure the Timer1 for auto reload mode(Mode-2)
 +
#Load the baud rate generator value to TH1
 +
#Start the Timer for baudrate generation
 +
<html><script src="https://gist.github.com/SaheblalBagwan/c7da45b94e94fb6c9278db95b3e559c2.js"></script></html>
 +
<br><br>
 +
 
 +
=Steps To Send Char=
 +
#Load the new char to be transmitted int SBUF.<br>
 +
#Wait till the char is transmitted. TI will be set when the data in SBUF is transmitted.
 +
#Clear the TI for next cycle.
 +
<html>
 +
<script src="https://gist.github.com/SaheblalBagwan/f98eb4474a489eebf80fb3667c99d8a8.js"></script>
 +
</html>
 +
<br><br>
 +
 
 +
=Steps To Receive Char=
 +
#Wait till the Data is received. RI will be set once the data is received in SBUF register.
 +
#Clear the receiver flag(RI) for next cycle.
 +
#Copy/Read the received data from SBUF register.<br>
 +
<html>
 +
<script src="https://gist.github.com/SaheblalBagwan/567a0845d21299df010a2c27c80df232.js"></script>
 +
</html>
 +
<br><br>
 +
 
 +
=Code=
 +
Below is the sample code to Transmit and receive the chars at 9600 baudrate with 11.0592Mhz clock.
 +
<html>
 +
<script src="https://gist.github.com/SaheblalBagwan/022405d82a54424f638c4a06b52082b7.js"></script>
 +
</html>
 +
 
 +
=Using ExploreEmbedded Libraries=
 +
In the above tutorial we discussed how to configure and use the inbuilt 8051 UART.<br>
 +
Now we will see how to use the ExploreEmbededd UART library.<br>
 +
For this you have to include the uart.c/uart.h files and associated gpio/stdutils files.<br>
 +
*Note:Refer the uart.h file for more info.
  
−
/* start the main program */
+
<html>
−
void main()
+
<script src="https://gist.github.com/SaheblalBagwan/d657c1091766d4b938cd904cb1a504b0.js"></script>
−
{
+
</html>
−
    char msg[50];
+
[[File:Pic16f877a uart.PNG]]<br><br>
−
    UART_Init(); // function to initialzie UART
+
−
+
−
  while(1)
+
−
    {
+
−
 
+
−
UART_TxString("\n\n\rEnter a new String: ");
+
−
UART_RxString(msg); 
+
−
        UART_TxString("\n\rEntered String: ");
+
−
        UART_TxString(msg); //Transmit the received string
+
−
    }  
+
  
−
}
+
=Downloads=
−
</syntaxhighlight>
+
Download the sample code and design files from [https://github.com/ExploreEmbedded/8051_DevelopmentBoard this link].
−
==Basic functions uart.c==
+
−
{{Box|type=l_green_light|text=
+
−
The entire [[8051_Family_C_Library#Uart.c:_8051_Uart_C-Libraray|'''uart.c''']] library contains various functions let us discuss the few used in this tutorial.
+
−
<syntaxhighlight>
+
−
void UART_Init()
+
−
{
+
−
      TMOD |=0x20; //Timer1 in Mode2.
+
−
      TH1=-3;    // 9600 Baud rate at 11.0592MHz
+
−
      SCON=0x50; // Asynchronous mode 8-bit data and 1-stop bit
+
−
      TR1=1;    //Turn ON the timer.
+
−
}
+
−
</syntaxhighlight>
+
−
This function initializes the UART module in 8051.
+
−
*Timer 1  is setup in 8-bit auto reload mode as discussed
+
−
earlier
+
−
*To get the baud rate of 9600 at 11.0592MHz crystal, we can see from the baud rate table that it requires a factor of three. Hence TH1 is loaded with -3 so that 28.8KHz is divided by 3 to get the exact baud rate.
+
−
*SCON is configured in widely used mode :Asynchronous mode 8-bit data and 1-stop bit
+
−
*The timer is turned on.
+
  
  
−
}}
+
Have an opinion, suggestion , question or feedback about the article let it out here!
 +
{{DISQUS}}

Latest revision as of 10:13, 2 September 2016

In this tutorial, we are going to discuss the serial/UART communication of 8051. After understating the basics of 8051 UART module, We will see how to use the ExploreEmbedded libraries to communicate with any of the UART devices.

0 UART main.gif


UART Registers

The below table shows the registers associated with 8051 UART.

Register Description
SCON Serial Control Register
TCON Timer Control Register for Baud Rate Generator
TMOD Timer Mode Control for Baud Rate Generator
SBUFF Serial Buffer holds the data to be transmitted and the data received



UART Register Configuration

Now lets see how to configure the individual registers for UART communication.

SCON
7 6 5 4 3 2 1 0
SM0 SM1 SM2 REN TB8 RB8 TI RI
SM0 SM1 Operation Description Baud Rate Source
0 0 Mode 0 8-bit UART 1/12 the quartz frequency
0 1 Mode 1 8-bit UART Determined by the timer 1
1 0 Mode 2 9-bit UART 1/32 the quartz frequency
1 1 Mode 0 9-bit UART Determined by the timer 1
  • SM2 - Serial port mode 2 bit, also known as multiprocessor communication enable bit. When set, it enables multiprocessor communication in mode 2 and 3, and eventually mode 1. It should be cleared in mode 0.
  • REN - Reception Enable bit enables serial reception when set. When cleared, serial reception is disabled.
  • TB8 - Transmitter bit 8. Since all registers are 8-bit wide, this bit solves the problem of transmitting the 9th bit in modes 2 and 3. It is set to transmit a logic 1 in the 9th bit.
  • RB8 - Receiver bit 8 or the 9th bit received in modes 2 and 3. Cleared by hardware if 9th bit received is a logic 0. Set by hardware if 9th bit received is a logic 1.
  • TI - Transmit Interrupt flag is automatically set at the moment the last bit of one byte is sent. It's a signal to the processor that the line is available for a new byte to transmit. It must be cleared from within the software.
  • RI - Receive Interrupt flag is automatically set upon one-byte receive. It signals that byte is received and should be read quickly prior to being replaced by a new data. This bit is also cleared from within the software.


TMOD
Timer1 Timer 0
7 6 5 4 3 2 1 0
Gate C/T M1 M0 Gate C/T M1 M0
  • Gate Control

0 = Timer enabled
1 = Timer enabled if INTx\ is high

  • C/T:Counter or Timer Selector

0 = Internal count source (clock/12)
1 = External count source (Tx pin)

  • M1-M0:Mode Control

00-Mode 0, 13 bit count mode
01-Mode 1, 16 bit count mode
10-Mode 2, Auto reload mode
11-Mode 3, Multiple mode


TCON
7 6 5 4 3 2 1 0
TF1 TR1 TF0 TR0
  • TRx: Timer x run control

0 = Timer not running
1 = Timer running

  • TFx: Timer x OverFlow flag

0 = Timer has not overflowed/rolled over
1 = Timer has overflowed/rolled over

Baud Rate Calculation

The main criteria for UART communication is its baud rate. Both the devices Rx/Tx should be set to same baud rate for successful communication.
For the 8051 the Timer 1' is used to generate the baud rate in Auto reload mode.

Fig: Baud rate at 11.0592Mhz Crystal

The crystal frequency Fclk is divided by 12 internally which is used to execute instructions also known as Machine Clock. Mclk. The timer again divides the Mclk by 32 and uses it as the timer frequency, say Tclk.

Usually, an 11.0592 Mhz crystal oscillator is used to provide the clock to 8051. The value seems to be odd but we see how it makes sense. From above discussion

Mclk = Fclk / 12 = 11.0592MHz/12 = 921.6KHz

Tclk = Mclk/32= 921.6KHz/32 = 28.8KHz

If we look at the standard baud rates used for serial communication shown in the table below, we can observe that all the baud rates are factors or multiples of the Tclk (28.8K)! This results in low error rates and hence is a commonly used crystal with 8051.

Baud Rate Factor)
2400 Tclk/12
4800 Tclk/6
9600 Tclk/3
14400 Tclk/2

The above factors should be loaded to Timer1(TH1) in Mode2 in order to generate the required baud rate. The final formula for baud rate is as below. Baudrate = Fosc/(32 * 12 * (256-TH1)) $$TH1 = 256 - (Fosc/(32 * 12 * Baudrate))$$ //If( SMOD==0 in PCON register) $$TH1 = 256 - (Fosc/(32 * 6 * Baudrate))$$ //If( SMOD==1 in PCON register)

Now with Fosc = 11.0592Mhz, TH1 value for 9600 baudrate will be: TH1 = 256-(11.0592*10^6)/(32 * 12 * 9600) = 253 = 0xFD = -3

Uart Init Steps

  1. Select the 8-bit , 1-Start and 1-Stop bit mode in SMOD
  2. Configure the Timer1 for auto reload mode(Mode-2)
  3. Load the baud rate generator value to TH1
  4. Start the Timer for baudrate generation



Steps To Send Char

  1. Load the new char to be transmitted int SBUF.
  2. Wait till the char is transmitted. TI will be set when the data in SBUF is transmitted.
  3. Clear the TI for next cycle.



Steps To Receive Char

  1. Wait till the Data is received. RI will be set once the data is received in SBUF register.
  2. Clear the receiver flag(RI) for next cycle.
  3. Copy/Read the received data from SBUF register.



Code

Below is the sample code to Transmit and receive the chars at 9600 baudrate with 11.0592Mhz clock.

Using ExploreEmbedded Libraries

In the above tutorial we discussed how to configure and use the inbuilt 8051 UART.
Now we will see how to use the ExploreEmbededd UART library.
For this you have to include the uart.c/uart.h files and associated gpio/stdutils files.

  • Note:Refer the uart.h file for more info.

Pic16f877a uart.PNG

Downloads

Download the sample code and design files from this link.


Have an opinion, suggestion , question or feedback about the article let it out here!

Retrieved from "https://exploreembedded.com/wiki/index.php?title=A4.8051_Communication_Protocols:UART,_RS232&oldid=10499"