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UART Serial Communication with PIC Microcontrollers: Theory and Practice


Pic Serial Communication Assembly Code: A Beginner's Guide




Serial communication is a method of transferring data bit by bit over a serial bus. It is widely used in embedded systems, telecommunication, and data transmission applications. In this article, we will learn how to use serial communication with PIC microcontrollers using assembly code.




Pic Serial Communication Assembly Code



PIC microcontrollers are popular and versatile devices that can be programmed in different languages, such as C, BASIC, or assembly. Assembly language is the lowest level of programming, where each instruction corresponds to a machine code that the microcontroller can execute directly. Assembly language gives us full control over the hardware and allows us to optimize the performance and memory usage of our programs.


One of the advantages of PIC microcontrollers is that they have built-in modules for serial communication, such as UART (Universal Asynchronous Receiver Transmitter), SPI (Serial Peripheral Interface), or I2C (Inter-Integrated Circuit). In this article, we will focus on UART, which is one of the most common and simple serial communication protocols.


What is UART?




UART stands for Universal Asynchronous Receiver Transmitter. It is a hardware device that converts parallel data into serial data and vice versa. UART can operate in asynchronous mode, where the sender and receiver do not share a common clock signal, or in synchronous mode, where they do.


UART has two main pins: TX (transmit) and RX (receive). The TX pin sends serial data out of the microcontroller, while the RX pin receives serial data into the microcontroller. The data is sent and received in frames, which consist of a start bit, 8 data bits, and a stop bit. Optionally, a parity bit can be added for error detection.


The baud rate is the speed of serial communication, measured in bits per second (bps). The baud rate of the sender and receiver must match for successful communication. The baud rate can be calculated as follows:


baud rate = Fosc / (4 * (SPBRG + 1))


where Fosc is the oscillator frequency of the microcontroller and SPBRG is a register that determines the baud rate.


How to use UART with PIC assembly code?




To use UART with PIC assembly code, we need to configure some registers and enable some interrupts. Here are the main steps:


  • Configure PORTA as I/O port and set all pins as output.



  • Configure CMCON register to disable analog comparator.



  • Configure PIE1 register to enable or disable peripheral interrupts.



  • Configure INTCON register to enable or disable general interrupts.



  • Configure SPBRG register to set the desired baud rate.



  • Configure TXSTA register to set the transmission parameters.



  • Configure RCSTA register to enable serial port and receive mode.



  • Create an interrupt service routine to handle received data.



  • Create a main loop to send data or wait for button press.



The following code shows an example of using UART with PIC assembly code. It uses two PIC16F877A microcontrollers connected via TX and RX pins. One microcontroller acts as a transmitter and sends a byte when a button is pressed. The other microcontroller acts as a receiver and displays the received byte on an LCD screen.


;TRANSMITTER CODE


PROCESSOR 16F876A


INCLUDE


__config _XT_OSC & _WDT_OFF & _PWRTE_OFF & _CP_OFF & _LVP_OFF & _BODEN_OFF


TMP_TX EQU 0x20 ;VALUE TO HOLD TRANSMITTED VALUE


ORG 0x000


GOTO MAIN


ORG 0x004 ;INTERRUPT VECTOR


GOTO INTERRUPT


MAIN:


;---USE PORTB AS I/O


CLRF PORTB


BSF STATUS,RP0 ;SWITCH TO BANK1


MOVLW D'255' ;USE PORTB AS I/O


MOVWF TRISB ;SET ALL PINS OF PORTB AS INPUT


BCF OPTION_REG,7


;---CONFIGURE PERIPHERAL INTERRUPTS


MOVLW B'00000100' ;DISABLE ALL PERIPHERAL INTERRUPTS EXCEPT TRANSMITTER


MOVWF PIE1 ;PERIPHERAL INTERRUPT ENABLE/DISABLE


;---CONFIGURE GENERAL INTERRUPTS


MOVLW B'01000000' ;DISABLE ALL INTERRUPTS EXCEPT PERIPHERAL


MOVWF INTCON ;INTERRUPT CONTROL REGISTER


;---CONFIGURE SPBRG FOR DESIRED BAUD RATE


MOVLW D'51' ;WE WILL USE 4800bps


MOVWF SPBRG ;BAUD AT 4MHZ


;---CONFIGURE TXSTA


MOVLW B'00100100' ;CONFIGURE TXSTA AS :


MOVWF TXSTA ;


;8 BIT TRANSMISSION - 6.BIT


;TRANSMIT ENABLED - 5.BIT


;ASYNCHRONOUS MODE - 4.BIT


;ENABLE HIGH SPEED BAUD RATE - 2.BIT


BCF STATUS,RP0 ;SWITCH TO BANK0


MOVLW B'10000000' ;ENABLE SERIAL PORT


MOVWF RCSTA ;RECEIVE STATUS REG


CLRF TMP_TX


BSF INTCON,7 ;ENABLE ALL UNMASKED INTERRUPTS


BSF TXSTA,5 ;ENABLE USART TRANSMIT


MAIN_LOOP: ;CONTINOUS LOOP


BTFSC PORTB,4 ;CHECK IF THE BUTTON IS PRESSED


GOTO MAIN_LOOP ;IF NOT GOTO CONTINOUS LOOP


INCF TMP_TX ;INCREMENT TMP_TX BY ONE


MOVF TMP_TX,W ;LOAD TMP_TX TO W REGISTER


MOVWF TXREG ;MOVE W TO TXREG TO TRANSMIT THE DATA


GOTO MAIN_LOOP ;CONTINOUS LOOP


INTERRUPT:


BCF INTCON,7 ;DISABLE ALL INTERRUPTS


BTFSS PIR1,4 ;CHECK IF THE TXIF FLAG IS SET


GOTO QUIT_INT ;IF NOT RETURN BACK TO THE MAIN LOOP


BCF PIR1,4 ;CLEAR THE FLAG


RETFIE


QUIT_INT:


RETFIE


END


;RECEIVER CODE


PROCESSOR 16F876A


INCLUDE


__config _XT_OSC & _WDT_OFF & _PWRTE_OFF & _CP_OFF & _LVP_OFF & _BODEN_OFF


TMP_RX EQU 0x20 ;VALUE TO HOLD RECEIVED VALUE


COL_CNT EQU 0x21


LINE_CNT EQU 0x22


ORG 0x000


GOTO MAIN


ORG 0x004 ;INTERRUPT VECTOR


GOTO INTERRUPT


INCLUDE


MAIN:


CALL LCD_INIT ;FIRST OF ALL WE HAVE TO INITIALIZE LCD


CLRF LINE_CNT


MOVLW 0xFF


MOVWF COL_CNT


;---USE PORTA AS I/O


CLRF PORTA


MOVLW D'7' ;USE PORTA AS I/O


MOVWF CMCON ;


BSF STATUS,RP0 ;SWITCH TO BANK1


CLRF TRISA ;SET ALL PINS OF PORTA AS OUTPUT


BCF OPTION_REG,7


;---CONFIGURE PERIPHERAL INTERRUPTS


MOVLW B'00100000' ;DISABLE ALL PERIPHERAL INTERRUPTS EXCEPT RECEIVER


MOVWF PIE1 ;PERIPHERAL INTERRUPT ENABLE/DISABLE


;---CONFIGURE GENERAL INTERRUPTS


MOVLW B'01000000' ;DISABLE ALL INTERRUPTS EXCEPT PERIPHERAL


MOVWF INTCON ;INTERRUPT CONTROL REGISTER


;---CONFIGURE SPBRG FOR DESIRED BAUD RATE


MOVLW D'51' ;WE WILL USE 4800bps


MOVWF SPBRG ;BAUD AT 4MHZ


;---CONFIGURE TXSTA


MOVLW B'00100100' ;CONFIGURE TXSTA AS :


MOVWF TXSTA ;


;8 BIT TRANSMISSION - 6.BIT


;TRANSMIT ENABLED - 5.BIT


;ASYNCHRONOUS MODE - 4.BIT


;ENABLE HIGH SPEED BAUD RATE - 2.BIT


BCF STATUS,RP0 ;SWITCH TO BANK0


MOVLW B'10000000' ;ENABLE SERIAL PORT


MOVWF RCSTA ;RECEIVE STATUS REG


CLRF TMP_RX


BSF INTCON,7 ;ENABLE ALL UNMASKED INTERRUPTS


BSF RCSTA,4 ;ENABLE USART RECEIVE


MAIN_LOOP: ;CONTINOUS LOOP


GOTO MAIN_LOOP


INTERRUPT:


BCF INTCON,7 ;DISABLE ALL INTERRUPTS


BTFSS PIR1,5 ;CHECK IF THE RCIF FLAG IS SET


GOTO QUIT_INT ;IF NOT RETURN BACK TO THE MAIN LOOP


MOVF RCREG,W ;MOVE THE RECEIVED BYTE TO


Conclusion




In this article, we have learned how to use serial communication with PIC microcontrollers using assembly code. We have seen how to configure the UART module and how to send and receive data using interrupts. We have also created a simple application that demonstrates the communication between two PIC microcontrollers.


Serial communication is a useful and powerful technique that can enable us to communicate with other devices, such as sensors, LCDs, computers, or other microcontrollers. By using assembly code, we can optimize our programs and have full control over the hardware. However, assembly code can also be complex and tedious to write and debug. Therefore, it is important to understand the basics of serial communication and UART before attempting to use it in our projects.


We hope you have enjoyed this article and learned something new. If you have any questions or comments, please feel free to share them below. Happy coding! 4e3182286b


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