WJEC Assembler Code Tutorials

Please ensure you have read the Getting Started page before using these tutorials, so that you already know how to on-screen simulate and download a program.

WJEC / EDUQAS A Level Assembler – Overview

WJEC / EDUQAS A Level Assembler – Getting Started

Hardware

All tutorials can be simulated on-screen or downloaded to a real-life PICAXE-18M2 chip on the AXE056 or AXE091 boards (or on a home-made circuit on a breadboard).

If using the AXE056 project board to run a PIC16F88 (programmed via a conventional PIC programmer) instead of using the simpler PICAXE-18M2 system, please use this assembler template.

Tutorial 1 - Flash LED on and off

A student mnemonics reminder sheet can be downloaded here.

Type in and simulate / download the following example to flash an LED on pin B.1

Tip - to help check there are no typing mistakes click the ‘Syntax Check’ button.If you get a syntax error on line 3 make sure the PICAXE BASIC language pre-processor is switched off (installation step 5 here).

Key points:

All pins are inputs upon chip reset. TRISB startup value is therefore b'11111111'.
Any pin to be used as an output must therefore have it’s TRIS bit set to 0 during initialisation. Remember 1 looks like I=Input, 0 looks like O=Output.
Before converting to an output the PORTB bit should be cleared, so that the output always starts up in a known state.
TRISA and TRISB registers are on register memory page 1. All other registers used in these tutorials are on page 0. Use bsf/bcf STATUS,RP0 to navigate pages.
Use predefined 'wait1000ms' procedure to create time delay.
A label such as 'main' requires a colon when defining the program position, but does not use a colon when in a command (such as goto, call etc.)
Labels cannot be PICAXE commands such as 'loop', so instead use unique labels such as 'loop1', 'loop2' etc.

Tutorial 2 - Flash 3 LEDs on and off

Type in and simulate / download the following example to flash LEDs on pins B.2, B.1, B.0

Key points:

When a PORT is written to using movwf all output pins in the port are written simultaneously.

Tutorial 3 - Flash LED ten times then stop

Type in and simulate / download the following example to flash an LED on pin B.1 ten times

init: clrf PORTB ; clear PORTB output latches bsf STATUS,RP0 ; memory page 1 movlw b'11111111' ; set portA pins to input movwf TRISA ; write to TRIS register movlw b'00000000' ; set portB pins to output movwf TRISB ; write to TRIS register bcf STATUS,RP0 ; memory page 0 main: movlw d'10' ; move decimal 10 into working register movwf B1 ; write to B1 register loop1: bsf PORTB,1 ; output B.1 high call wait100ms ; delay 100 milliseconds bcf PORTB,1 ; output B.1 low call wait100ms ; delay 100 milliseconds decfsz B1,F ; decrement B1 and skip if zero goto loop1 ; not zero so loop back again loop2: goto loop2

Key points:

decfsz decrements the specified register value and skips if now zero.
decfsz needs to use target of F in this situation (F = result into same register file, W = result into the working register)
The never ending loop at the end of the program means a power reset is required to re-run the program.
If the watchdog timer is enabled a 'clrwdt' command is required within loop2 to prevent a timeout.

Tutorial 4 - Make LED follow input switch

Type in and simulate / download the following example to make LED on pin B.1 follow push switch on A.7. Make sure the switch has a 10k pull-down resistor so that it does not float when not pushed. To simulate an input left-click on the A.7 pin in the simulation panel.

Key points:

btfsc / btfss skip the next instruction if the specified bit is clear (=0) / set (=1)

Tutorial 5 - Flash LED after 5 switch presses

Type in and simulate / download the following example to flash the LED on pin B.1 after switch on A.7 is pressed 5 times. Make sure the switch has a 10k pull-down resistor so that it does not float when not pushed.

init: clrf PORTB ; clear PORTB output latches bsf STATUS,RP0 ; memory page 1 movlw b'11111111' ; set portA pins to input movwf TRISA ; write to TRIS register movlw b'00000000' ; set portB pins to output movwf TRISB ; write to TRIS register bcf STATUS,RP0 ; memory page 0 main: movlw d'5' ; move decimal 5 into working register movwf B1 ; write to B1 register loop1: call wait10ms ; delay 10 milliseconds to debounce btfss PORTA,7 ; test switch goto loop1 ; not high so loop back again loop2: call wait10ms ; delay 10 milliseconds to debounce btfsc PORTA,7 ; test switch is back off again goto loop2 ; not off yet so loop back again decfsz B1,F ; decrement B1 and skip if zero goto loop1 ; not zero yet loop back again bsf PORTB,1 ; output B.1 high call wait1000ms ; delay 1000 milliseconds bcf PORTB,1 ; output B.1 low goto main ; loop back to start

Key points:

To overcome switch bounce a small delay is used before testing for the switch off again
With each switch press the register B1 is decremented
decfsz decrements the specified register value and skips if result is now zero.

Tutorial 6 - Using an interrupt

Type in and simulate / download the following example to flash the LED on pin B.1 every second. An interrupt on A.7 (switch) going high activates a call to the ISR sub procedure as soon as the switch is pushed. During the interrupt the LED on B.2 will flash quickly. Make sure the A.7 switch has a 10k pull-down resistor so that it does not float when not pushed.

init: clrf PORTB ; clear PORTB output latches bsf STATUS,RP0 ; memory page 1 movlw b'11111111' ; set portA pins to input movwf TRISA ; write to TRIS register movlw b'00000001' ; B.0 as input, other portB pins to output movwf TRISB ; write to TRIS register bcf STATUS,RP0 ; memory page 0 main: bsf INTCON,INT0IE ; set external interrupt enable bsf INTCON,GIE ; enable all interrupts loop1: bsf PORTB,1 ; output B.1 high call wait1000ms ; delay 1000 milliseconds bcf PORTB,1 ; output B.1 low call wait1000ms ; delay 1000 milliseconds goto loop1 ; loop forever W_SAVE EQU B20 interrupt: movwf W_SAVE ; save working register into register so can restore later btfss INTCON,INT0IF ; check correct interrupt has occurred retfie ; no, so return and re-enable GIE loop2: bsf PORTB,2 ; output B.2 high call wait100ms ; delay 100 milliseconds bcf PORTB,2 ; output B.2 low call wait100ms ; delay 100 milliseconds btfsc PORTA,7 ; NB- change to PORTB,0 for a 16F88 goto loop2 ; no so loop again bcf INTCON,INT0IF ; clear interrupt flag movf W_SAVE,W ; restore saved value back to working register retfie ; return and re-enable GIE

Key points:

To enable an interrupt both the GIE (Global Interrupt Enable) and INT0IE (int 0 interrupt enable) bits must both be set. However to prevent glitches GIE should only be set after other IEs have already been set, not in the same instruction.
On the PICAXE-18M2 the external interrupt is on A.7 (not B.0 as for a PIC16F88)
When the interrupt occurs it immediately sets the appropriate flag (INT0IF) and then calls the ISR (interrupt service routine) labelled interrupt:
Within the ISR it is essential to save the working register value, as the value will be corrupted (by being used) within the ISR. Note that a simplified version is shown here - in real projects it is also necessary to save some other registers like STATUS.
Before returning from the ISR the interrupt flag (INT0IF) should be cleared and the working register value restored.
The retfie command re-enables the interrupt enable bit automatically (i.e. sets INTCON,GIE) upon the return

Tutorial 7 (extension) - Read analogue values

Type in and simulate / download the following example to read LDR (A.0), pot (A.1) and temperature (A.2) and then display the pot (A.1) value in binary on 8 LEDs (portB). If using the real-life chip 'call debug' will send all 3 results to the computer screen for display, if simulating right-click on the pin in the simulation panel to alter the analogue value.

init: clrf PORTB ; clear PORTB output latches bsf STATUS,RP0 ; memory page 1 movlw b'11111111' ; set portA pins to input movwf TRISA ; write to TRIS register movlw b'00000000' ; set portB pins to output movwf TRISB ; write to TRIS register bcf STATUS,RP0 ; memory page 0 main: call readadc0 ; readadc A.0 into register B0 call readadc1 ; readadc A.1 into register B1 call readtemp2 ; read temperature A.2 into register B2 call debug ; send values to computer screen (real-life chip) movf B1,W ; get ADC on pin A.1 result movwf PORTB ; display as binary on 8 LEDs call wait100ms ; delay 100 milliseconds goto main

Key points:

Predefined readadc/readtemp subroutines read the analogue values
Predefined debug routines sends values serially up cable for computer display

Tutorial 8 (extension) - Switch LED on when analogue value reaches a threshold

Type in and simulate / download the following example to make LED on pin B.1 switch on at an analogue threshold of 100 on A.1 (pot).

init: clrf PORTB ; clear PORTB output latches bsf STATUS,RP0 ; memory page 1 movlw b'11111111' ; set portA pins to input movwf TRISA ; write to TRIS register movlw b'00000000' ; set portB pins to output movwf TRISB ; write to TRIS register bcf STATUS,RP0 ; memory page 0 main: call readadc1 ; read analogue A.1 into register B1 call debug ; send values to computer screen movf B1,W ; mov register B1 into working register sublw d'100' ; subtract W from 100 btfsc STATUS,C ; if Carry is clear ADC >= 100 bsf PORTB,1 ; output B.1 high btfss STATUS,C ; if Carry is set ADC < 100 bcf PORTB,1 ; output B.1 low goto main

Key points:

Note that sublw is subtract working register from literal (result = literal - WREG), not the other way around
if a subtraction overflows (ie is < 0 ) the C carry bit in STATUS is set, otherwise it is cleared. By testing the C bit we therefore know if the reading is higher or lower than the threshold value (set to d'100' in this example).