ELCT 912: Advanced Embedded Systems

Transcription

1 ELCT 912: Advanced Embedded Systems Lecture 10: Applications for Programming PIC18 in C Dr. Mohamed Abd El Ghany, Department of Electronics and Electrical Engineering

2 Programming the PIC18 to transfer data serially 1. The TXSTA register is loaded with the value 20H, indicating asynchronous mode with 8-bit data frame, low baud rate, and transmit enabled. 2. Make TX pin of PORTC (RC6) an output for data to come out of the PIC 3. The SPBRG is loaded with value to set the baud rate for serial data transfer 4. SPEN bit in the RCSTA register is set HIGH to enable the serial port of PIC18 5. The character byte to be transmitted serially is written into the TXREG register. 6. Monitor the TXIF bit of the PIR1 register to make sure UART is ready for next byte 7. To transfer the next character, go to step 5 2

3 Programming the PIC18 to transfer data serially Write a C program for the PIC18 to transfer the letter G serially at 9600 baud, continuously. Use 8-bit data and 1 stop bit. Assume XTAL = 10 MHz. 3

4 Programming the PIC18 to transfer data serially Write a PIC18 C program to transfer the message YES serially at 9600 baud, 8-bit data, and 1 stop bit. Do this continuously. 4

5 Programming the PIC18 to receive data serially 1. The RCSTA register is loaded with the value 90H, to enable the continuous receive in addition to the 8-bit data size option 2. The TXSTA register is loaded with the value 00H to choose the low baud rate option. 3. SPBRG is loaded with a value to set the baud rate 4. Make the RX pin of PORTC (RC7) an input for data to come into the PIC The RCIF flag bit of the PIR1 register is monitored for a HIGH to see if an entire character has been received yet. 6. When RCIF is raised, the RCREG register has the byte. Its contents are moved into a safe place 7. To receive the next character, go to Step5 5

6 Programming the PIC18 to receive data serially Program the PIC18 in C to receive bytes of data serially and put them on PORTB. Set the baud rate at 9600, 8-bit data, and 1 stop bit. 6

7 Example Write an C18 program to send two different strings to the serial port. Assuming that SW is connected to pin PORTB.5, monitor its status and make a decision as follows: SW = 0; send your first name SW = 1; send your last name Assuming XTAL =10 MHz, baud rate of 9600, and 8-bit data Part 1 7

8 Example Part 2 8

9 Baud rates for BRGH =1 Note : for F osc = 10 MHz, we have SPBRG = (625000/ Baud Rate) 1 9

10 SPBRG Values for Various Baud Rates (XTAL = 10 MHz) 10

11 Baud rates error calculation For example, with the XTAL = 10 MHz and BRGH =0, we have the following for the 9600 baud rate: SPBRG value = (156250/9600)-1= = =15 Error = ( )/16 = 1.7% 11

12 Example Write a PIC18 C program to send the two messages Normal speed and High Speed to the serial port. Assuming that SW is connected to pin PORTB.0, monitor its status and set the baud rate as follows: SW = baud rate SW = baud rate Assuming XTAL =10 MHz for both cases. Part 1 12

13 Example Part 2 13

14 Programming Timer Interrupts Timer Interrupt Flag Bits and Associated Registers INTCON Register with Timer0 Interrupt Enable and Interrupt Flag 14

15 Programming Timer Interrupts The Role of Timer Interrupt Enable Flag (TMRxIE) 15

16 The Difference Between the RETURN and RETFIE Instructions Explain why we cannot use RETURN instead of RETFIE as the last instruction of ISR Both perform the same actions of popping off the top bytes of the stack into the program counter and making the PIC18 return to where it left off. However, RETFIE also performs the additional task of clearing the GIE flag, indicating that the servicing of the interrupt is over and the PIC18 now can accept a new interrupt. If you use RETURN instead of RETFIE as the last instruction of interrupt service routine, you simply block any new interrupt after the first interrupt, because the GIE would indicate that the interrupt is still being serviced. 16

17 Example Write a program to use Timer0 and Timer1 interrupts to generate square waves on pins RB1 and RB7 respectively, while data is being transferred from PORTC to PORTD Part 1 17

18 Example Part 2 18

19 Example Part 3 19

20 Example Part 4 20

21 PIC18 interrupt Programming in C Notes: The C18 compiler uses #pragma code to place code at a specific ROM address. Because the C18 does not pace an ISR at the interrupt vector table automatically, we must use assembly language instruction GOTO at the interrupt vector to transfer control to the ISR. 21

22 PIC18 interrupt Programming in C Now we redirect it from address location to another program to find the source of the interrupt and finally to the ISR. This is done with the help of the keyword interrupt as follows: 22

23 Example Write a program to use Timer0 and Timer1 interrupts to generate square waves on pins RB1 and RB7 respectively, while data is being transferred from PORTC to PORTD ( C version) Part 1 23

24 Example Part 2 24

25 Example Part 3 25

26 Example Part 4 26