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Sydne can me y readers et at the P Stan Swan IC Decemb AXE fair, er see pag 10th – e 43! If you’ll excuse our Latin . . . Picaxus Interruptus The new PICAXE-08M has been winning lots of new friends around the world. Sorry to interrupt but here’s yet another example of why that is! U nlike 2003’s popular PICAXE-08, 2004’s new 08M baby offers valuable interrupts to program flow. Such enhanced control has only been previously been available on the more costly Picaxes (typically the 18X), so this new command is quite a bonus, with significant potential for professional applications at a budget price. Although understanding Picaxe interrupts can be daunting, they turn out to be quite straightforward to use. However they’re not usually an issue for PICAXE beginners and hence can initially be neglected. If you’re still at the HIGH 1: PAUSE 100: LOW 1 stage then perhaps return to this article later, since their benefits often arise when trying to polish a programs action. OK - but what ARE these so-called “interrupts”? Interrupts to computer programs are similar to interruptions that plague our daily lives. If mid-way thru’ a thriller video your phone rings, then you’ll probably pause everything and take the call. (If it’s a Kiwi-made “Lord of the Rings” video you’ll no doubt welcome the chance to subdue your envy!) You’re effectively primed to break out of the viewing routine when such random phone calls arrive. Not all activities accommodate such branching of course, with perhaps gunfire the most dramatic – bullets hurtle to the target irrespective of the hunters Lashed up in the usual protoboard style, the PICAXE-08M ready to demonstrate its interrupt capabilities. It’s powered by a 3.6V nicad battery (blue cylinder). 78  Silicon Chip by Stan Swan 2nd thoughts after pulling the trigger. Most PICAXE programs are however essentially sequential in nature – first do this, now this, then this – with any breaks out of the sequenced flow first awaiting completion of an ongoing operation. Hence if a lengthy WAIT is underway, perhaps WAIT 60 (1 minute), then no further actions will occur until this period is timed out. Grr- naturally this may be frustrating! It’s comparable perhaps to customers twiddling their thumbs while serving staff finish their coffee break… Wouldn’t it be more convenient if your needs could be considered in a more streamlined manner? Tarah! – enter the interrupt command, or more correctly – a polled (“checked”) interrupt. Not all Picaxe commands can be interrupted, since some require the full attention of the micro controller once they have started to execute. For the Picaxe-08M these high priority non interruptible commands are – • INFRAIN2 - receiving IR data • INFRAOUT - sending IR data • NAP - brisk sleep (<2.3secs) • SLEEP - long “hibernation” sleep • SERIN - awaiting serial data • SOUND - production of simple sounds Annoyingly the power saving SLEEP is on this list, meaning very lengthy delays, with immense power saving benefits, still have to run their course. It’s maybe akin to hibernating animals only being awoken by the arrival of spring. WAIT (the shorter time delay) is interruptible, so delays up to a minute siliconchip.com.au are thus viable. The PLAY and TUNE commands can be interrupted partway through, after each note has been completed, even though a SOUND command cannot. When a PLAY or TUNE command is interrupted, any notes that haven’t been played will be discarded and execution after the RETURN from the ‘Interrupt:’ routine will continue at the statement following the PLAY or TUNE command that was interrupted. Argh – shades of musical chairs! Probably the most valuable contribution involves a PAUSE or WAIT command interruption that immediately terminates normal (short) delays. Hence a WAIT 30, for a ½ minute delay, will be chopped short as the interrupt is sensed. It’s almost a WAIT – “UNLESS” situation – wait for the next bus unless a mate offers you an earlier lift? Configuring your program – SETINT The SETINT has two arguments, a ‘compare with value’ (input) and an ‘input mask’ (mask) in that order. The syntax, although in cryptic binary (%), is in fact akin to the efficient presetting of normal Picaxe I/O channels with ‘LET PINS’ . Thus let pins = %00000000 switches all outputs off while %00000010 turns just pin 1 high These binary digits read R-L, with the left most being 7. With interrupts, the ‘input mask’ defines which pins are to be checked to see if an interrupt should be generated. %00000001 will check input pin 0 %00000010 will check input pin 1 %00000100 will check input pin 2 %00001000 will check input pin 3 These can be usefully combined to check a number of input pins together ... %00000011 will check input pins 1 and 0 %10000100 will check input pins 7 and 2 (useful for the larger Picaxe –18X) Having decided which pins you want to use for the interrupt, the value determines the second parameter of the SETINT command. Once a SETINT is active, the PICAXE monitors the pins you have specified in ‘input mask’ where a ‘1’ is present, ignoring other pins. Every time the PICAXE checks these input pins it creates an 8-bit value which reflects the value of siliconchip.com.au IO PINS (CHANNELS) (TO PC CON2 DB9 SERIAL PORT) +4.5V (3xAA) 100kΩ NTC 2 22k 3 10k 5 1 2 3 IC1 PICAXE-08M 4 7 0 6 1 5 2 8 8 3 4 4 1 RED LED PIEZO λ 10kΩ GREEN LED λ LEDS K A SC 2004 Picaxus interRuptus CHANNEL 0: PROG OR OUT CHANNEL 3: IN ONLY The circuitry – programming, I/O, etc – remains the same for the 08M as the 08. the pins it did read with others being forced to zero. Example: an input mask of %00001100 will check pins 3 and 2 and create a value of %0000ab00 where bit ‘a’ will be 1 if pin 3 is high and 0 if low, and bit ‘b’ will be 1 if pin 2 is high and 0 if low. PROGRAM LISTING The ‘compare with value’, the first argument of the SETINT command, is what this created value is compared with, and if the two match, then the interrupt will occur, if they don’t match then the interrupt won’t occur. If the ‘input mask’ is %00001100 (meaning pins 3 and 2), then the valid Available for downloading from www.picaxe.orcon.net.nz/int08m.bas ‘Demo “barebones” Picaxe-08M polled interrupt handling. Ver 0.9 28th Oct 2004 ‘Overall effect is a Chernobyl alarm- a monitoring status beep & LED flash every ‘minute if cool, but alarm instantly sounds if NTC sensor thermistor gets too warm! ‘NB-works reliably as is, but possibly refine with READADC & PIN status check ? ‘ ‘Triggered by raised temps(~60C) when NTC R falls to ~30k,since Ch.1 input then ‑ ‘goes high. Returns to low (0) when it cools <60C. NTC & R voltage divider involves ‘+supply--(10k)--Pin 1--(100k NTC)--ground. Alter 10k &/or NTC value to suit your ‘own alert temp (Jaycar 100k <at> 25C used here).Red LED Ch.2 & Green LED Ch.4, with ‘piezo on Ch.0.Program hosted for download at =>www.picaxe.orcon.net.nz/int08m.bas ‘Protoboard layout =>www.picaxe.orcon.net.nz/pcxs08m.jpg (as used 08/M datalogger) ‘via Stan. SWAN => s.t.swan<at>massey.ac.nz for Dec. 04 “Silicon Chip” article. ‘------------------------------------------------------------------------------SETINT %00000010,%00000010 ‘Go to Interrupt routine when Pin 1 goes high statusloop: sound 0,(100,10) high 4 pause 500 low 4 wait 60 goto statusloop ‘Normal cool monitoring routine ‘Audible beep for operator if room very bright ‘\ ‘ Brief green LED flash in case noisy room ‘/ ‘Temperature status interval (wait 60 ~1min) ‘Continue “all OK” safety status looping interrupt: sound 0,(100,10) high 2 pause 100 low 2 SETINT %00000010,%00000010 RETURN ‘Beep/LED code when thermistor hot ‘Urgent beeps ‘\ ‘ Rapid attention getting red LED flashing ‘/ ‘re-enable interrupt ‘allow the interrupts December 2004  79 ‘compare with value’ can be one of the following 4 binary options... %00000000 - Pin 3 = 0 and pin 2 = 0 %00000100 - Pin 3 = 0 and pin 2 = 1 %00001000 - Pin 3 = 1 and pin 2 = 0 %00001100 - Pin 3 = 1 and pin 2 = 1 So if you want to generate an interrupt whenever Pin 3 is high and Pin 2 is low, the ‘input mask’ is %00001100 and the ‘compare with value’ is %00001000, giving a SETINT command of – SETINT %00001100,%00001000 Thus the interrupt will then occur if, and only if, pin 3 is high (1) and pin 2 is low(0). Other combinations – maybe pin 3 low with pin 2 high - will not trigger the interrupt. Processing the interrupt Interrupts are not enabled until the first SETINT is encountered. After that, any input interrupt condition which is met causes a subroutine jump to a routine which must be labelled ‘Interrupt:’. The RETURN of that routine cause execution to continue from the point where the interrupt occurred. With interrupts enabled the PICAXE monitors the input pins specified by a SETINT to determine if an interrupt has occurred, and if so a subroutine jump to the ‘Interrupt:’ routine is made. The input pins are checked (“polled”) just before each PICAXE statement is executed, but not while that statement is executing. This means that the interrupt must exist for long enough for the PICAXE to sense, else it’ll be missed if it appears only while the PICAXE is busy doing something else. When the ‘Interrupt:’ routine is entered, the SETINT is disabled and must be re-enabled for further interrupts to be responded to. Interrupts are re-enabled by issuing another SETINT command which is activated when the RETURN of the interrupt routine is executed It is important that the condition which initially caused the interrupt has ‘disappeared’ or when interrupts are re-enabled and the RETURN is executed, another interrupt will be immediately seen. . It’s rather like your mates queuing up to give you a lift – you only need a single ride at that time but could do with another ride tomorrow if the bus is again late. If the interrupt was caused by a button push, then the interrupt routine should check if that button is still held, 80  Silicon Chip and only re-enable the interrupt and continue when it has been released. The following code demonstrates how to handle an interrupt caused by a button switch which sets Input Pin 3 to 1 when pushed (perhaps via a voltage divider) and to 0 (maybe via a pull down resistor) when it is released ... SETINT %00001000,%00001000 ‘Go to Interrupt routine on Pin 3 High loop: Main program code GOTO loop interrupt: Code to do something when button pushed buttonhold: IF pin3 = 1 THEN buttonhold ‘stay here if button still held on SETINT %00001000,%00001000 ‘re-enable interrupt RETURN ‘allow the interrupts There – that wasn’t too bad! Further sophisticated extensions are possible with clever use of diodes. With lateral thinking a second Picaxe, devoted perhaps to a memory hungry task, could even be switched on by an interrupt driven 08M. PICAXES can “source” (supply) some 20mA a pin – easily enough to supply the ~5mA needs of a second bare 08. Transistors could amplify this source current of course. Chernobyl alert Here’s short program that illustrates 08M interrupts at work in a realistic application. This uses almost exactly the same hardware layout as our October 08M NTC data logger, and pulses an “all OK” green status LED every minute, unless the temperature suddenly becomes too high (Chernobyl?). Urgent LED flashes (red = hot) and distinctive tones should be immediately given in such circumstances. Naturally holding on (if still hot) and resetting (once temperatures fall to safe levels again) needs considering too. If a classic PICAXE sequential routine had been used, it would first need to work through a WAIT (here 1 minute) before encountering the commands to detect the temperature rise and sound alarms. Yikes – your reactor could have melted down by then… In our case, a quick blast from a hot air gun readily triggers the alarm at about 60°C. Rather surprisingly, normal PICAXE commands that monitor pin status (readadc, pin etc) were found redundant in this example, as the NTC/10kW voltage divider, under a 3 AA (4.5V) supply, was alone enough to determine 0/1 logic states for the interrupt settings at input pin 1. Your mileage may vary ! References: 1: Picaxe Programming Editor help (F1 & .pdf) provides interrupt help 2: Generous interrupt insights were provided by “Hippy” from the online Picaxe support forum –         www.rev-ed.co.uk/picaxe/forum 3: www.picaxe.orcon.net.nz/pcxs08m.jpg shows the 08M NTC/LED layout, with just a single resistor changed (47kW to 10kW) for this example. 4: www.picaxe.orcon.net.nz/int08m.bas hosts the Chernobyl Alert program. A quick burst from a hot air gun readily heats the sensor thermistor enough to alert the snoozing PICAXE. This alarm temperature can be easily altered (here ~60°C) by changing the thermistor and voltage dividing 10kW resistor values. Note – although neglected on this simple 3.6V breadboard, the 2 LEDs strictly should use dropping resistors, especially on higher supply voltages. siliconchip.com.au