[TUT] Newbie's Guide to AVR Interrupts

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Simon White
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1 This tutorial is about interrupt driven USART receive and transmit routines written in AVR assembly. The hardware is: Arduino Mega2560 Adafruit Ultimate GPS IBM PC Atmel JTAGICE3 Software: Atmel AS6.1 Windows XP sp3 PuTTY terminal emulator For more information about the Adafruit Ultimate GPS: I suggest that you read and understand the AVRFreaks tutorials on interrupts and UART/USART. By Dean Camera aka abcminiuser: [TUT] Newbie's Guide to AVR Interrupts [TUT] [SOFT] Using the USART - Interrupt driven serial comms By Kartman: [TUT][SOFT] The traps when using interrupts Refer to the datasheet for the ATmega2560. These routines, receive and transmit use USART3 on the Atmega2560. I will use USART0 to communicate with the PC. The JTAGICE3 is connected via JTAG. The Ultimate GPS continuously sends strings of ASCII characters terminated by a NL (code 10). Refer to the attached complete code example. Note that depending on processor selected you may need to use IN/OUT to access the io registers in.stead of LDS/STS This Tutorial is in 4 sections: 1. Receive data from the GPS via interrupts 2. Receive and echo to the PC 3. Receive, echo to PC and send message to GPS 4. Receive, echo to PC, send to GPS, process via interrupt. Section 1: All the receive function is in the receive interrupt, which posts a notice when a complete message has been received. The GPS ends each message with a NL (code 10). Variables which are used by the interrupt routines and need to be carefully accessed outside of the interrupt routines. inbuf assigned 120 bytes in the data segment

2 outbuf 120 bytes The following General Purpose IO registers: GPIOR0 flag for message received GPIOR1 index for the outbuf GPIOR2 index for the inbuf Define an input buffer in data segment..dseg inbuf:.byte 120 ; input buffer Begin the code segment with a jmp table, it must be exact. See the datasheet, this may be different for each processor..cseg.org 0x0000 ;Places the following code from address 0x0000 jmp RESET ; Reset Handler jmp EXT_INT0 ; IRQ0 Handler jmp USART3_RXC ; USART3, RX Complete Handler jmp USART3_UDRE ; USART3, UDR Empty Handler jmp USART3_TXC ; USART3, TX Complete Handler ; RESET: ldi r16, high(ramend); Main program start out SPH,r16 ; Set Stack Pointer to top of RAM ldi r16, low(ramend) out SPL,r16 cli ; disable interupts ldi r16, 103 sts UBRR3L, r16 sts UBRR3H, r16 sts UCSR3A, r16 ; Set baud rate ; baud rate calculations are in the datasheet, also some ;pre-calculated values depending on processor speed. ;9.6k baud to GPS, system clk 16Mhz ;connection to GPS ;high baud rate register ;according to the datasheet, USART3 register A should be ;zero for this use. ; set to zero to be sure ; Enable receiver and transmitter

3 ldi r16,0b1101_1000 ; rx & tx to gps (3 & 4),enable interrupt for rx & tx (6 & 7) sts UCSR3B,r16 ; Set frame format: 8data, 1stop bit, no parity ldi r16, 0b0000_0110 ; sts UCSR3C,r16 I am using GPIOR2 as the index into the input buffer. It is arbitrary, but whereever you put variables to be used by the interrupt routines you must be careful about changing them since the interrupt can fire at most any time. Note: here the GLOBAL interrupts are off. out GPIOR2,r16 ;set up counter for index into buffer ; finished setup of USART sei ;turn on global interrupts This is the main loop, doing nothing but checking and clearing the message received flag that is set in the receive interrupt routine. again: sbis GPIOR0,7 ;set on input message complete rjmp again cbi GPIOR0,7 ;clear the flag ;process in message here rjmp again Stay: rjmp stay ;catch if drop through ;ISR EXT_INT0: USART2_TXC: USART3_RXC: reti push r16 in r16,sreg push r16 push YH push YL push r18

4 == The index into the inbuf is retrieved from GPIOR2, incremented and check that not exceeding inbuf size. == ; in r16,gpior2 ;get index into buffer mov r18,r16 ;save for indexing inc r16 ;for next position cpi r16,120 ;limit input to 120 characters brne URX3a ;exceeded the limit so zero out sbi GPIOR0,7 ;flag message complete Point the Y register at the inbuf. Add the index, write character from the USART register UDR3 to inbuf. Check for end of message, the NL (code 10). === URX3a: ldi YH,high(inbuf) ;get address of the buffer ldi YL,low(inbuf) add YL,r18 ;index into the buffer clr r18 adc YH,r18 ;might be a carry lds r18,udr3 ;get the character from the USART buffer st Y,r18 ;put character into buffer cpi r18,10 ;check if character = NL (10) brne URX3b ;no continue ;end of message, so zero index for next message sbi GPIOR0,7 ;flag message complete URX3b: out GPIOR2,r16 ;save the index ; USART3_UDRE: USART3_TXC: pop r18 pop YL pop YH pop r16 out SREG,R16 pop r16 reti reti ;make sure the pops = the pushes & reverse order attached is a picture (jtag1) of the PC screen at a breakpoint showing message from the GPS.

5 Section 2: Here the echo to the PC via USART0 is added. The Atmega2560 has 4 USARTS and the Arduino Mega2560 connects USART0 to USB. Code to be changed is the initialization of USART0 and the writing to UDR0. Changed or added lines are marked with CC Initialization code: ===== cli ; disable interupts ; Set baud rates ldi r16, 25 ;38.4k baud to pc from ATmega2560 datasheet CC sts UBRR0L, r16 ;connection to PC CC ldi r16, 103 ;9.6k baud to GPS sts UBRR3L, r16 ;connection to GPS sts UBRR0H, r16 ;high baud rate CC sts UBRR3H, r16 ;high baud rate sts UCSR0A, r16 ; set to zero to be sure CC sts UCSR3A, r16 ; set to zero to be sure ; Enable receiver and transmitter

6 ldi r16,0b0000_1000 ; only transmitter to pc & no interrupt CC sts UCSR0B,r16 ; CC ldi r16,0b1101_1000 ; rx&tx to gps and enable interrupt rx & tx sts UCSR3B,r16 ; Set frame format: 8data, 1stop bit, no parity ldi r16, 0b0000_0110 ; sts UCSR0C,r16 ; CC sts UCSR3C,r16 out GPIOR2,r16 ;set up counter for index into buffer ; finished setup of USART == In the receive interrupt routine: == URX3a: ldi YH,high(inbuf) ;get address of the buffer ldi YL,low(inbuf) add YL,r18 ;index into the buffer clr r18 adc YH,r18 ;might be a carry lds r18,udr3 ;get the character from the USART buffer sts UDR0,r18 ;echo to PC CC st Y,r18 ;put character into buffer cpi r18,10 ;check if character = NL (10) brne URX3b ;no continue ;yes, end of message, so zero index for next message sbi GPIOR0,7 ;flag message complete URX3b: ==================================== See second attachment (scr1) for screenshot of output to PC. Notice that the first message was not complete.

Section 3. Receive data from GPS, echo to PC and send message to GPS. ; Some constant control messages are stored in the program memory segment. jmp USART3_TXC ; USART3, TX Complete Handler antoff:.

7 Section 3. Receive data from GPS, echo to PC and send message to GPS. ; Some constant control messages are stored in the program memory segment. jmp USART3_TXC ; USART3, TX Complete Handler antoff:.db "$PGCMD,33,0*6D",13,10 ;turn pgtop messages off rmconly:.db "$PMTK314,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0*29",13,10,10 udrate:.db "$PMTK220,200*2C",13,10,10 RESET: ldi r16, high(ramend); Main program start The first section of added code is to move the message from Program Memory to the outbuf. movmsg: sei ldi YH,HIGH(outbuf) ldi YL,LOW(outbuf) ldi ZH,HIGH(rmconly<<1) ldi ZL,LOW(rmconly<<1) ; finished setup of USART ;turn on global interrupts ;point the Y register at the outbuf ;get the address of message in program memory

8 again: lpm r16,z+ st Y+,r16 cpi r16,10 brne movmsg rcall sendmsg ;load from program memory ;store in data segment, outbuf ;was the character a NL? At this point the outbuf contains the rmconly message. This will turn off other messages such as GPGGA, GPVTG, GPGSA, but not the PGTOP messages reporting the antenna status. The sendmsg routine is in the main program and sends the first byte of the message, the rest of the messages are sent from the transmit complete interrupt routine. Note: If you plan to send messages without some delay between them, you will need to flag the end of message in some way. sendmsg: ldi YH,HIGH(outbuf) ldi YL,LOW(outbuf) ld r16,y sts UDR3,r16 ldi r17,1 out GPIOR1,r17 ret ;send to GPS, message in the outbuf. ;save the index in GPIOR1 ;only the first character is sent from the ;main ;program, the rest are sent from the transmit ;complete interrupt routine. ;point to outbuf ;get the first character ;send it ;set index to 2nd character ;and save ================= At this point the outbuf contains the rmconly message. This will turn off other messages from the GPS such as GPGGA, GPVTG, GPGSA, but not the PGTOP messages reporting the antenna status. Here is the transmit complete interrupt routine USART3_TXC: push r16 in r16,sreg ;Transmit complete interrupt routine ;the last character transmission is complete ;so can send the next character

9 push r16 push r17 push YH push YL ; in r17,gpior1 ;get index into message cpi r17,0 ;if zero nothing to send (will happen on last ;character ;transmitted breq endtxc1 ldi YH,HIGH(outbuf) ;get pointer to outbuf ldi YL,LOW(outbuf) cpi r17,120 ;check for too many characters sent breq endtxc ;need a zero byte to add to get the carry add YL,r17 ;add the index to the Z register adc YH,r16 ;might be a carry inc r17 ;point the index to the next character ld r16,y ;get the character to send from program memory sts UDR3,r16 ;send it to the GPS cpi r16,10 ;end if character = NL (10) brne endtxc1 ;not end endtxc: clr r17 ;end, so the index is now zero, and next TX ;complete interrupt can be ignored. endtxc1: out GPIOR1,r17 ;save the index ; pop YL pop YH pop r17 pop r16 out SREG,R16 pop r16 reti compare the attached scr2.jpg with scr1.jpg to see that the command was received and the messages other than PGTOP and GPRMC were turned off.

COMP2121: Microprocessors and Interfacing

COMP2121: Microprocessors and Interfacing Lecture 19: Interrupts II http://www.cse.unsw.edu.au/~cs2121 Lecturer: Hui Wu Session 1, 2006 Overview AVR Interrupts Interrupt Vector Table System Reset Watchdog Timer Timer/Counter0 Interrupt Service