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If Swan’s 2nd Law is: “ Microcontrollers can never have too many I/O channels” . . . what’s his first? Drum roll – Yet Another PICAXE! A lthough PICAXE microcontrollers are now well established and popular in numerous electronic projects, due in no small part to SILICON CHIP’s enthusiasm, many users know only of the entry level 08M, versatile 18X or powerful 28X. However, the recent arrival of a 20-pin PICAXE now takes the family to an awesome 14 members, although four earlier offerings are now discontinued and two “X2” fire breathers have yet to be released. This new PICAXE-20M (based upon a MicroChip PIC16F677) is really just a stretched 08M, as it offers similar memory and follows the same commands as the ever popular 08M and more recent 14M. Thankfully, its 10-a-side DIP packaging enables easier identification in the sea of chips lurking in many IC parts drawers, since its 20 pins make it noticeably longer than by Stan Swan common 14,16 or even 18-pin DIP logic ICs. The so-called 20M is mainly intended to meet the demand for further input and output (I/Os) channels than the four offered by the smaller 08M, or 5-6 of the 14M. Its provision of eight I/Os, arranged with inputs (0-7) on one side and outputs (0-7) on the other, looks to have strong appeal for projects where extra chips (such as Shift Registers) and wiring would otherwise be needed. Circuit size will naturally be reduced with just a single IC, yet greater flexibility will result, thanks of course to the PICAXE programmable features. All three “M” chips are especially well-suited for hobbyists and educational users, allowing enhanced design for just a few dollars more, when the need for extra I/O channels arises. The 14 PICAXE microcontrollers Summary – mid 2008 PICAXE type IC pins Memory ~progr. lines Outputs Inputs ADC (8-10bit) EEPROM memory (bytes) Speed (MHz) Best use? Microchip PIC type Approx. Cost (Aust$) 08 8 40 1-4 config. 1-4 config. 1x 4 bit low 128 less prog. 4 - 12F629 $5 08M 8 80 1-4 config. 1-4 config. 3 256 less prog. 4,8 Educ. 12F683 $6 14M 14 80 6 5 3-5 config 256 less prog. 4,8 Educ. 12F684 $7 20M 20 80 8 8 4 256 less prog. 4,8 Educ. 16F677 $8 18A 18 80 8 5 3 256 less prog. 4,8 - 16F819 $11 18X 18 600 8 5 3 256 plus I2C 4,8 Std. 16F88 $16 28X1 28 1000 9-17 0-12 0-4 256 plus I2C 4-20 Std 16F886 $22 28X2 28 2 x 1000 22 configurable I/O 0-12 256 plus I2C 4-40 Adv. 18F2420 $TBA 40X1 40 1000 9-17 3-7 256 plus I2C 4-20 Adv. 16F887 $28 0-12 I2C 4-40 Adv. 18F4420 $TBA 40X2 40 2 x 1000 8-20 33 configurable I/O 256 plus Earlier (2002-05) PICAXEs below are now considered obsolete, with supplies discontinued. 18 18 40 8 5 3x4bit low 128 less prog. 4 X 16F627 ($8) 28A 28 80 8 8 4 64 +256 28X 40X 28 40 40  Silicon Chip 600 600 9-17 8-16 0-12 8-20 4 3-7 4- X 16F872 ($10) 128 plus I2C 4- X 16F873A ($15) 128 plus I2C 4- X 16F874A ($26) siliconchip.com.au 1 2 3 4 PICAXE 08M +V SERIAL IN ADC 4 / OUT 4 / IN 4 INFRAIN 3 / IN 3 8 7 6 5 0V OUT O / SERIAL OUT /INFRAOUT IN 1 / OUT 1 / ADC 1 IN 2 / OUT 2 / PWM 2 / TUNE PICAXE 08M 1 14 2 13 3 4 5 6 PICAXE 14M +V SERIAL IN ADC 4 / IN 4 INFRAIN 3 / IN 3 IN 2 IN 1 ADC 0 / IN 0 12 11 10 9 8 7 2 SERIAL OUT SERIAL IN 3 5 10k 4 0V OUT 0 / SERIAL OUT /INFRAOUT OUT 1 OUT 2 OUT 3 OUT 4 OUT 5 3 PROGRAM EDITOR (TO PC SERIAL PORT) 20 2 19 3 18 4 5 6 7 PICAXE 20M 1 17 16 15 14 8 13 9 12 10 11 8 5 3V BATTERY 1 2 λ 5 λ 7 λ λ 0V SERIAL OUT OUT 0 / INFRAOUT OUT 1 OUT 2 OUT 3 OUT 4 OUT 5 OUT 6 OUT 7 3 8 λ λ 2 9 λ λ 10 λ 12 6 λ λ Pleasingly, the larger 14 and 20 “M” offerings retain the same programming and supply connections as the classic 08M, although only the 08M allows pins to be set either as Inputs or Outputs. I/O background Comparable to kids demanding the attention of a solitary teacher, it’s almost a byword with microcontroller projects that sensors and indicating devices will require more channels than are available. This is especially a problem with outputs and traditionally it has been addressed by multiplexing techniques, so that data signals are “juggled” around the displays at speeds 16-SEGMENT 4 S1 0 1 The three “M” PICAXES are very similar in pinouts and architecture, meaning little change in code as you move up the chain (in fact, the 08M is pin-for-pin for the top eight of the 14M). The main difference is all those beautiful extra inputs and output channels! 14-SEGMENT 7 3 PICAXE 6 08M λ PICAXE 20M 7-SEGMENT 1 11 4 PICAXE 14M +V SERIAL IN ADC 7 / IN 7 IN 6 IN 5 IN 4 ADC 3 / IN 3 ADC 2 / IN 2 ADC 1 / IN 1 INFRAIN / IN 0 2 22k 5x7 MATRIX Here’s an example of a LED chaser using just a PICAXE 08M. In discrete logic this would require at least three or four chips to achieve. Although this looks elegant when compared to 1:1 LED driving, annoying flickering may be an issue and considerable hookup wiring is needed. greater than human persistence of vision (POV). POV makes LEDs appear to be solidly lit, even although they’re being rapidly pulsed (at ~100Hz) and offers a bonus of lowering average LED current demands. The colourfully-named “Charlieplexing” (after Maxim engineer Charlie Allen) also enables relatively few I/O pins to drive a large number of LEDs, utilising the Tri-state nature of micro channels, when a pin can be sink, source or be open circuit. With “n” outputs n x(n-1) LEDs can be controlled. So a 4-output (Pins 0,1,2,4) PICAXE-08M can control 4x(4-1) = 4x3 = 12 LEDs and still have a Pin 3 left for some kind of input. But before you get too excited, realise that multiplexing of any type requires all manner of fancy “digital jigsaw” coding and wiring (as shown above), and for just a dollar or so extra the larger PICAXEs may ease the pain! LED displays Four common types of LED displays. At left is the standard 7-segment (+DP) type we are using in this article. Next in line, extra segments are added to make a much-more versatile 14-segment (+DP) display. Third has the upper and lower segments broken in two to form a 16 segments, capable of displaying most – but not all – letters and numbers (eg, “D” and “O” can’t be displayed because it looks exactly the same as zero). Finally, the LEDs are broken down into a 5x7 matrix which is capable of displaying any letter, numeral and an array of symbols. It’s just a matter of how the LEDs are driven. siliconchip.com.au LEDs have been an enormous success story since their early 1970s introduction and ever-brighter types are now revolutionising lighting and displays. We’re all bombarded daily by single status LEDs on TV sets and phone chargers, to bar arrangements showing volume on audio gear. But aren’t colour LCDs now all the rage? Quite true, but when compared with older 16x2 mono LCDs, LED displays are very bright, much cheaper July 2008  41 COM A B C D E F G C COM A B C D E F G a b f g e c d dp DP 7-SEGMENT DISPLAY1 DATA1 DATA2 DATA3 DATA4 DATA5 DATA6 DATA7 DATA0 E B C a 7-SEGMENT DISPLAY1 b f g e A B C D E F G c d 8 8 E B DISPLAY1 8 DISPLAY2 a b c d e f g dp DP dp 7-SEGMENT DISPLAY2 a b c d e f g dp b g e c d dp COM B DISPLAY1 7-SEGMENT DISPLAY2 a f C E 8 DATA1 DATA2 DATA3 DATA4 DATA5 DATA6 DATA7 DATA0 +V DP A B C D E F G a b f g e c d dp DP COM B C E DISPLAY2 Here’s how the various segments in 7-segment displays are driven. At left is a “common anode” circuit – here the transistor controlling the device must be turned on as well as the data lines controlling the individual segments. For example, if data lines 1, 2, 7, 5 and 4 are activated, the display shows a “2”. At right is the same setup, this time for “common cathode” displays. These are in fact used more frequently than common anode types. ($2-$4), and although rather power hungry (~5mA each segment) are easily driven and understood. In dim lighting, when extra LCD backlighting can draw significant currents, LED displays are supreme, especially for “at a glance” monitoring. If arranged in suitable patterns, traditionally in seven segments with all the LED cathodes joined (hence common cathode or CC), they can spell out numbers and letters that may be quite sufficient for simple status and alerts. Each character is just built up of suitably lit segments, with these traditionally referred to as a,b,c,d,e,f,g and dp(decimal point). The letter or number “0” will use segments a,b,c,d,e,f , while a “3” has a,b,c,d,g illuminated. Aside from displaying the numbers 0-9, with suitable program tweaking even basic characters (E, F, L, J, O, P, H, PROGRAMMING RESISTORS 4.7k 3 5 10k SERIAL IN 22k +V PROGRAM EDITOR (TO PC SERIAL PORT) DS 18B20 OUT ADC7/ INPUT 7 INPUT6 INPUT5 INPUT4 PICAXE 20M 20 10 ADC2/ INPUT2 1 DS18B20 OUT +V 0V SC 2008 ADC3/ INPUT3 ADC1/ INPUT1 INFRAIN/ INPUT0 2 1 19 3 18 4 17 5 16 6 PICAXE 20M 15 7 14 8 13 9 12 10 11 20 OUTPUT1 # OUTPUT2 # OUTPUT3 OUTPUT4 OUTPUT5 OUTPUT6 OUTPUT7 Picaxe 20m LED THERMOMETER # # # 8 2 S, A, I, U and C etc) and sequences such as HI and LO can be shown. The real world is full of crucial information (eg, lift floors etc) shown in such a cryptic manner. 14 and even 16-segment LEDS are available for more specialised displays but LED matrix arrays (such as the 5x7 shown) usually offer more character versatility – we will come back to these on a later project. But why the appeal of LEDs with PICAXEs? Driving a classic 7-segment display, when faced with just a few output lines, normally requires extra ICs like the DIP16 CMOS 4511, specifically designed for the task. Such approaches however involve considerable wiring, extra component costs and loss of display flexibility. Hence it’s VERY satisfying to see the 20Ms “gang of eight” outputs, as these immediately appeal for direct but versatile 7-segment use. It’s akin perhaps to hav+4.5-5V ing your very own personal teacher. (EG 3 x ‘AA’, You can’t beat 1:1 if the luxury is 4 x NiCd or # VALUE DEPENDS NiMH, etc) ON DISPLAY USED available – and each display LED is AND SUPPLY thus fed from a dedicated output, with 0V – 330 Ω TYPICAL an 8th available for the decimal point. SERIAL OUT # Voila – it’s almost as if 20Ms and OUTPUT0/ 7-segment displays were made for INFRAOUT # each other! DP A B C D E F G a b f g e c d dp COM # ACTUAL PINOUTS DEPEND ON 7-SEGMENT DISPLAY USED The new PICAXE-20M, with eight I/Os, is ideal to drive a 7-segment LED display. With a DS18B20 temperature sensor on the input, it makes a great thermometer. 42  Silicon Chip An application – and an answer to Stan’s first law! The 20M inputs can monitor almost any regular sensor using normal READADC, and it would be straightforward to connect a thermistor, LDR or the like in a simple voltage divider, with values being shown on the outputs perhaps via multiple LED bars. However, readers may well recall Swan’s 1st Law –“You can never have too many thermometers” and with this in mind, an enhanced but simple PICAXE 20M + 7 segment LED + DS18B20 digital thermometer circuit has evolved. siliconchip.com.au ‘PICAXE-20M,7 seg LED & DS18B20 demo. Stan.SWAN + PICAXE “Forum” ‘Uses single 7 seg display,sequencing digits so 24°C temp=”2” then “4” etc ‘Ranges very cold subzero (preceeded with flashing -ve) to ~99°C within 1/2°C ‘Suits further tweaking for info.display needs or enhanced battery life etc ‘204/256 bytes.Much tighter programming possible- EEPROM,symbol etc ‘Program download at => www.picaxe.orconhosting.net.nz/20m7segds.bas ‘Breadboard layout => www.picaxe.orconhosting.net.nz/20m7segds.jpg ‘--------------------------------------------------------------------------temp20m:readtemp 7,b0 ‘DS18B20 temp reading at 20M input 7 if b0>128 then gosub negtemps ‘Sub zero temps value correction b1= b0/10 b2= b0//10 if b1=0 then units ‘divide orig temp to get tens value ‘divide orig temp so remainder yields units value ‘suppress “0” if temps between ±9°C,so “4” & not “04” tens: ‘tens numeral test,with first digit suppressed if 0 on b1 gosub zero,one,two,three,four,five,six,seven,eight,nine wait 1:pins=%00000000 ‘blanks all 7 segs to ensure tens & units pause 200 ‘don’t run together if similar- 11,22,33 etc units: ‘units numeral test on b2 gosub zero,one,two,three,four,five,six,seven,eight,nine wait 2 ‘units digit extra hold on,as likely of most interest pins=%00000000:wait 1 ‘blanks for DP heartbeat between each temp.display for b3 = 1 to 3:pins=%00000001:wait 1:pins=%00000000:wait 1:next b3 goto temp20m ‘Bit order follows 20M outputs 7,6,5,4,3,2,1,0 (or 7 seg g,f,e,d,c,b,a,+DP) ‘if wired as on 20M breadboard demo using DSE Z4104 7 seg LED zero: pins=%01111110:return ‘0 shows one: pins=%01100000:return ‘1 shows two: pins=%10110110:return ‘2 shows three:pins=%10011110:return ‘3 shows four: pins=%11001100:return ‘4 shows five: pins=%11011010:return ‘5 shows six: pins=%11111010:return ‘6 shows seven:pins=%00001110:return ‘7 shows eight:pins=%11111110:return ‘8 shows nine: pins=%11001110:return ‘9 shows negtemps:’DS18B20 subzero negative temps routine + flashing -ve alert for b3 = 1 to 2:pins=%10000000:pause 200:pins=%00000000:pause 200:next b3 b0 = b0 - 127:return ‘b0 now correctly able to show subzero temps Stan’s traditional breadboard layout of the PICAXE-20M “thermometer”. The temperature sensor is bottom right. Here’s the commented (‘) code for the Thermometer. It’s not too long to type out – or you can download it if you wish. The ever popular DS18B20, which reads temperatures over a wide range to within ½°C, returns direct °C values for monitoring by the READTEMP command. Rather than use a second 7-segment display, a skinflint approach has been followed, where the display is just sequenced. So a temperature of 24° is shown as “2” then a”4”, with suitable blanking to prevent confusion. The DS18B20 measurement range is from an amazing –55°C to + 125°C, although peaking at 99°C in our case, with sub-zero temperatures preceded by a flashing “-”. Instead of coding with tedious multiple HIGH/LOW commands to build up a 7 segment pattern, outputs are setup in the efficient but cryptic %10101010 style. These are serviced g,f,e,d,c,b,a,dp so the command PINS=%00001110 shows a “7”. Even more efficient coding techniques could be used but the current approach is very readable while leaving scope for ample hacking and display tweaking. may then be required. The LED brightness relates to the values of these resistors, so alter the normal 330W droppers to suit, perhaps even using >1000W if the display is too bright in a darkened room. Such higher values will have the benefit of prolonging battery life (presently perhaps ~100 hours), which will naturally relate to the numbers of segments being lit. The circuit in fact works perfectly well on just 3V (although the display will be dimmer), and when thus compacted the entire design could even be housed in a small sealed waterproof plastic container. The DS18B20 can conveniently be extended off circuit and perhaps sealed in a rugged probe, as naturally the circuitry itself may need protecting if in liquids or at thermal extremes. Circuit layout Using a 7-segment display with side pins, the entire circuit readily fits on a small standard solderless breadboard. Larger or bottom-pinned LED displays will rather cramp the dropping resistor arrays and flying leads or a second board siliconchip.com.au Conclusion At ~$8 the new 20M looks well suited initially just in a cost effective display niche based on its 8 outputs, since these greatly streamline 7-segment LED driving, while offering display versatility. For convenience clickable links for code downloading, resources and references are all hosted at www.picaxe. orconhosting.net.nz/20M.htm SC July 2008  43