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PICAXE – the new millennium 555? Stan (the man) Swan, electronics lecturer from Massey University in Wellington (NZ), leads the charge with the innovative PICAXE-08, the PIC for everyman. Stan reckons this PIC is really easy to use, even for old-timers! T he legendary electronic industry workhorse IC, the 8-pin 555 timer, was first introduced (unpatented!) by Signetics in 1972. It subsequently featured in countless circuits. Virtually every monthly electronics magazine (SILICON CHIP included) continues to have further tips, hints and innovative tweaks for them. In fact, entire “555 Cook Books” have evolved. In today’s silicon chip age, the 30-year endurance of such a humble device is a remarkable tribute to its sound initial design, reliability and cheapness. It’s been the largest volume IC sold every year and the original design, although enhanced with lower powered CMOS versions, remains unmodified. To put this in historical context, consider that the 555 dates from the first pocket calculators, VCRs and colour TV sets, and that today’s 21st century electronic appliances (DVDs, mobile phones, home PCs, GPS, etc) were then the stuff of science fiction. Given its white-hot rate of change, thirty years in electronics compares to perhaps three hundred in normal engineering fields (steam engines to space shuttles?) and by any measure the 555 seems due for a successor. Enter the PIC The 1990s arrival of the Microchip PICs (Peripheral Interface Controllers or Programmable ICs), with their PC interfacing, cheapness and non-volatile RAM, initially showed most promise. PICs abound in modern electronic devices, including bread makers, washing machines, sewing machines, PCs, mobile phones, digital cameras and even toys. Most PICs now are Flash RAM, with the “F” (rather than an earlier “C”), indicating block memory cell erasure when electrons pierce a thin dielectric under Fowler-Nordheim tunnelling. Phew! Data remains in the IC without any battery backup, yet can be easily be edited or retrieved. An irksome trade-off between cost and programming difficulties exists however. Bare PICs (such as the popular PIC16F84) are cheap and powerful but too complicated for many enthusiasts and educators. They may be great for controlling smart washing machines and ideal for mass production but they intimidate beginners simply wanting to flash a few LEDs! In contrast, the Parallax BASIC “Stamp” variations were better suited to education and developmental work, since they were easy to high-level program in BASIC. But their costs were usually much Although both the 555 and PICAXE too high to justify production work are 8-pin DIP, their internals differ and their large footprint was – well radically. Note that the “I/O pin” num– postage-stamp sized. bers are NOT the same as the chip More recently, high level editors “leg” numbers (especially 3 & 4!). Part 1: Introduction to the PICAXE – by Stan Swan* 8  Silicon Chip www.siliconchip.com.au It’s a PIC that even your dog could understand. . . and PIC intro kits have also abounded but with costs or assembly complexity still rather daunting for newcomers. In an admirable blend of industry and education, akin perhaps to Victorian ventures when schools aligned with local firms, the UK Oil and Gas Industry recently funded a West Country firm, Revolution Education, in a remarkable PIC venture. The firm already handles a more powerful and larger PICAXE “18” and “28” range but it’s their new 8-pin PICAXE-08 that looks set to take the “555 successor” mantle. PICAXE-08s are bubble-gum cheap, powerful, have non-volatile memory and are delightfully easy to use. They PC-connect via a 3-wire D9 serial cable and program under a free Windows high level editor using 35 BASIC “plain English” commands such as “high, low, nap, goto, sound, if…then, sleep” etc. Even your dog could understand these! The command set is intentionally similar to that of BASIC “Stamps”, meaning that circuits and code already produced The Revolution Education Starter Pack board uses soldered contacts and although allowing neat assembly, can be heat damaged and is inconvenient for prototyping. A suggested layout on 300-hole protoboard is shown at right – much more convenient for the hobbyist and experimenter. The PICAXE-08 has unusual supply voltage positions: Pin 1 is +ve and pin 8 is ground. The jumper allows one channel to be used for I/O as well as normal programming. Wire colours follow normal resistor colour coding to help identify channels, ie, black 0, brown 1, red 2, orange 3, yellow 4. Lo, the PIXAXE! www.siliconchip.com.au February 2003  9 PICAXE-08 COMMANDS (Note similarity to Parallax BASIC Stamp PBASIC) Several “pseudo” commands too, especially SYMBOL (assigns new word to a value) and INPUT = pin Rem, semicolons (;) or apostrophe (‘) precede remarks/comments, & also colons (:) as usual in BASIC. DIGITAL OUTPUT. HIGH Switch an output pin high (on). Example: High 2 turns pin 2 on. LOW Switch an output pin low (off). Example: Low 3 turns pin 3 off. TOGGLE Toggle (alter) the hi/lo state of an output pin. OUTPUT Set a pin as an output. Example: Output 1 makes pin 1 an output. INPUT Set a pin as an input. REVERSE Reverse the I/O state of a pin. PULSOUT Output a timed pin inverted pulse. Example: Pulsout 0,3 - pin 0, 30 microseconds. ANALOG OUTPUT PWM Provide a pulse width modulation output. Example: Pwm 1,20,8 - pin 1,20/255 duty,   8 cycles. SOUND Make sound(s) 0 = quiet, 255 = hiss. Example: Sound 4,(100,10) - sound pin  4,~5kHz,~100ms. DIGITAL INPUT IF...THEN Jump to new program line, depending on condition. Example: If b3 < b2 then ledoff PULSIN Measure input pulse duration (µs). Example: Pulsin 4,0,w2 - pin 4 input,   logic low triggered. ANALOG INPUT READADC Read analog channel (0 - 160) into a variable. Example: Read 1,b2 - read channel 1 into b2 PROGRAM FLOW FOR...NEXT Establish a for-next loop. Example: For b2=0 to 100 step 2   counts even numbers. BRANCH Jump to address specified by output if in range (akin to ON x GOTO). GOTO Jump to address. Example: If b0=5 goto daylight   goes to daylight routine if b0=5. GOSUB Jump to subroutine at address specified. Example: Gosub test heads to test subroutine. RETURN Returns to main program from gosub routine. 10  Silicon Chip VARIABLE MANIPULATION (LET) Assigns a value to a variable & does limited L-R maths. Example: Let w0=b2*22/7 – Pi LOOKUP Lookup indexed data specified by offset & store. Example: lookup 1,(6,7,8) - takes 7 LOOKDOWN Search values for a target’s match number & store in variable, akin to $trings RANDOM Generate a pseudo-random number. SERIAL I/O SEROUT Serial data output (to 2400bps). Example: Serout 0,n2400,(65) - sends    ASCII 65 (=A). SERIN Serial data input - many qualifiers! Example: Serin 0,n2400,(“A”) - waits for    ASCII 65. INTERNAL EEPROM ACCESS (program & data storage; take care since program overwriting may occur). EEPROM Store data in EEPROM before download. Example: eeprom 0,(“hello”) - starts <at>   location 0. READ Read data EEPROM into variable. Example: Read 255,b2    get location of last program instruction. WRITE Write variable into data EEPROM. Example: Write 220,b3 - stores byte b3 into   address 220. POWER DOWN NAP Enter low power 20µA mode for short period ( <2.3 secs). Example: Nap 3 - sleeps for 144ms. SLEEP Enter low power mode long period (±1%). Example: Sleep 3600 - sleeps for 1 hr   (max 65535s). END Power down until reset (indefinite sleep). MISCELLANEOUS PAUSE Wait up to 65535ms (65.5 ) ~ 1ms overhead. Example: Pause 100 - pauses ~100ms = 0.1s. WAIT Wait for up to 65 seconds. DEBUG Displays variable value on attached PC screen. Example: Debug b0 - shows b0 value on screen. Ch.0 Ch. 3 Ch.1 PROGR or switchable OUT IN only Low res. Analog I/O www.siliconchip.com.au A typical 555 oscillator circuit to flash a single LED. As you can see, even this simple circuit requires more components than the PICAXE. And that’s all this circuit can do! may readily convert to PICAXE use. PICAXE-08s are based on the new Microchip 8-pin PIC12-F629s but with the BASIC interpreter squeezed on-board. Such interpreters allow easy code tweaking, since the program doesn’t need compiling before each run, although a slight execution overhead (about one millisecond) exists. They have in-built 4MHz oscillators, 1K code space, 64 bytes RAM, 128 bytes EEPROM (enough for about 40 lines of code), five I/O pins and valuable low-resolution analog-to-digital conversion (ADC). Battery needs are a flexible – 2.5V to 6V at just 2mA, yet they’re able to supply 20mA at each pin to drive LEDs or piezo speakers, etc. Perhaps the PICAXE name represents a miner’s digging tool with “Silicon – I’ve struck Silicon!” the cry, instead of “Gold!” Gold prices are irrelevant however, since PIXAXE08s sell for an astound- On the left is the classic 555 oscillator, very cheap but with all aspects hardware-dependent. Contrast this with larger footprint and much more costly BASIC “Stamp” approach at right. In the centre is the smaller PICAXE approach which offers the best of both worlds: software control and “bubble gum” cheapness. ingly low $3 each. Being so cheap, they can be even left in the final soldered circuit. Later software tweaking can be attended to with a 3-wire serial connection if need be. It’s recommended that all PICAXE circuits have such a simple inclusion and the associated 10kΩ and 22kΩ resistor pair. Folks, if you’ve been meaning to get into PICs but thought the process akin to unravelling DNA sequences, then PICAXE chips are the answer. Although Revolution Education (UK) market a $25 “Starter Pack” Introducing the PICNIK box – the “Peripheral Interface Con-troller Nifty Intro Kit.” The protoboard fits neatly into a parts box and along with the battery pack and sundry other components, offers a one-stop PICAXE experimenter’s kit. www.siliconchip.com.au (along with other project boards) that could be ideal for dedicated circuits, this involves fine soldering and inconvenience when away from the workbench. For more flexibility, solderless protoboards are better, since these allow easy component swapping and reuse, neater circuit layouts, and no burnt fingers! It’s a PICNIK! With increased productivity in mind, I’ve developed the PICNIK (Peripheral Interface Controller Nifty Intro Kit?) box. Emily, a Year 11 student, uses her notebook PC and a PICNIK to create an instant solderless PIC workshop. It’s suitable for education, non-specialist workrooms or home use. February 2003  11 What are the goodies in the PICNIK Box . . . PICAXE-08 IC Revolution Education, UK (see website) Approx. $3 each in a tube of 5 Small solderless “Wish” protoboard Sold by most electronics outlets. 4 AA battery holder and battery snaps Ensure PICAXE is not run from 9V! 4 AA cells (Power demands are very light, so cheap non-alkalines should do). Solid core hookup wire Multi-coloured (avoid stranded wire on protoboards). Wire snips & strippers Even fingernail nippers can be used! Small piezo speaker Allows direct sound production. (Don’t use a normal 8Ω speaker). Assorted LEDs Easily driven by the PICAXE. 330Ω dropping resistors LEDs can be driven directly but it’s wise to use these. Light Dependent Resistor (LDR) Allows easy ADC action via a simple voltage divider with further resistors. 3-wire serial cable Perhaps from an old serial mouse and PC motherboard header pins. Other tools & materials Hot-melt glue gun, wide heatshrink tubing (clear), photocopied labels. PICAXE Editor and .PDF manuals from CD, or free download from Rev-Ed (also at www.picaxe.orconhosting.net.nz). The protoboard neatly fits into a semi-transparent parts box and along with the battery pack and sundry other components, offers a one-stop PICAXE experimenter’s kit. Few of its items are exotic, and with junk box fossicking, the total bill of materials shouldn’t set you back more than $30. Both educational users and experienced circuit developers should find this a most cost effective approach. We hope to use the PICNIK box over the next few months to get you as enthusiastic about these new chips as we are! What’s this? No serial port? Don’t have a serial port in that spiffy new notebook? The computer industry has perhaps been rather TOO keen to run with USB ! Although wonderfully convenient and well supported now by modern peripherals, USB-only machines leave serial RS-232 “legacy” devices in limbo – including our PICAXE. This may also be frustrating for GPS, data logging and PDA applications. Help is at hand, however, since USB to serial adaptors ( but NOT vice versa) are available, althought they may be quirky and costly (A$60 range) and need software drivers. Undemanding “3 wire” serial needs, such as this PICAXE-08 ,should be supported by almost any such adaptor but if conversion problems arise it’s suggested you just rescue that older serial-port-fitted Win95 PC from the broom cupboard! 12  Silicon Chip References and updated material For convenience these are hot-linked at website http://picaxe.orconhosting.net.nz/refs.htm http://picaxe.orconhosting.net.nz Author’s pictorial page outlining PICAXE-08 features http://picaxe.orconhosting.net.nz/progedit Free program editor and PDF manuals (6 .cab files ~ 8.5 MB) www.hippy.freeserve.co.uk/picaxe.htm Overview of entire PICAXE family www.rev-ed.co.uk Revolution Education, Bath UK, microcontroller products www.picaxe.co.uk Rev-Ed’s PICAXE products and programming editor www.techsupplies.co.uk Rev-Ed’s online technical products shop – global sales! www.jpixton.dircon.co.uk/pic/history.html PIC history page www.microchip.com Microchip PICmicro products and data. Who is Stan Swan? We first met Stan Swan in his "WiFi" article in SILICON CHIP back in November 2002. Stan is a New Zealander who first wrangled electrons with 1960s valve-era ham radio. He is a career educator teaching Electronics and Computer Technology at the Wellington (NZ) campus of Massey University. His enthusiasm for “hands on” appropriate technology covers such diverse fields as PC interfacing, alternative energy resources, wireless data comms, digital photography and Internet applications. He credits Ray Doty’s “Wordless Workshop “ and the lucid electronics articles of US writer Forrest M. Mims III as especially formative influences. * Stan’s email address is s.t.swan<at>massey.ac.nz www.siliconchip.com.au Your first PICAXE application: a simple LED flasher The 555 timer oscillates under time periods established by external resistor and capacitor values. If a LED is used at the output, perhaps on a battery-powered bike light, the flash rate, duty cycle and battery drain can be juggled by changing these components. This may be very inconvenient, especially if only physically larger capacitors are available! In contrast, a PICAXE LED flasher uses software tweaks for fine tuning. Flash rates that are the most attention-getting or duty cycles that prolong battery life are simply “cut and tried” at the PC keyboard. Not only is this more versatile but the parts count, cost and circuit size are all reduced. First attach the programming serial cable, connect a LED (plus suitable dropping resistor (see below) to output 2 and power up the PICAXE-08. Run the editor (download free from www.rev-ed. co.uk or http://picaxe.orconhosting.net.nz) on the PC, enter the following code and send it to the PICAXE. The comments (preceded by ‘) are not saved on the IC but are essential documentation for your future reference. Colons (:) identify the defined routine – helpful names are best. ledflash: high 2 pause 5 low 2 pause 50 goto ledflash ‘LED flash rate & mark/space experiment ‘turn on output pin 2 – LED lights up ‘keep it on for 5 milliseconds ‘turn off pin 2 – LED goes out ‘keep it off for 50ms ‘repeat routine The value of RD, the series resistor, depends on the supply voltage and the type of LED you are using. In fact, it may not even be required as there is a 20mA limit on the PICAXE output. But we are used to using current-limiting resistors with LEDs so let’s keep it that way and avoid accidents! For a red LED (which requires about 2V <at> 10mA), with a 6V supply, RD = (6-2)/.01, or 400Ω (330Ω would be fine). For a white LED (3.6V <at> 25mA), RD = (6-3.6)/0.025 = 100Ω. With an ultra-bright white LED, normally taking 25mA at 3.6V, this DIY serial leads can be made using 10:1 duty cycle pulsing still gave the header pins rescued from old impression of a steady light, yet avermotherboards or modems. First, age current dropped to just a few mA. solder the three serial wires, then This means batteries should last 10 dribble hot-melt glue onto them. times as long. The inbuilt slight interAllow this to cool somewhat and preter overhead could mean the timing while still warm and pliable, work it is stretched by a millisecond or so. with your fingers into a neat “plug”. NEXT MONTH: www.siliconchip.com.au The D9 serial plug has its pins 5, 3 & 2 going to PICAXE legs 8 [ground], 2 (via a 22kΩ resistor) [serial in] and 7 via a jumper (S1) [serial out]. A small photocopied label slid under protective clear heat-shrink tubing will identify the plug after final hot-air shrinking. A more ambitious application of the PICNIK box. Build your solderless kit up now and get to know its features in anticipation! February 2003  13