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PICAXE APPLICATION SPECIAL PICAXE-08 PICAXE Port Expansion Everyone is raving about the PICAXE-08. It’s tiny, cheap and is so easy to program that even beginners can do it. There is one major drawback, though: it only has five pins available for input and output ports. Not any more, it ain’t! S ure we could use the PICAXE-18 or PICAXE-28 when we need more inputs and outputs (both of which have significantly more i/o pins) but that would defeat the purpose of using a small, low-cost chip. Fortunately there is a simple solution. By using 74xx165 by David Lincoln and the 74xx595 shift registers, we can expand the number of input and output ports in multiples of eight. Fig.1 shows how. In Fig.1a a 74xx165 is being used to expand the number of inputs to eight. This requires three of the PICAXE ports; one for serial data, one for clocking The author’s Port Expansion unit, lashed up on a Protoboard. This has both inputs (from a telephone keypad) and outputs (monitored by the LED display. The chips top left of the photo are a true RS232 interface. 22  Silicon Chip www.siliconchip.com.au Figs. 1a (left) and 1b (right) show the input and output port expansion (respectively) for a PICAXE-08. Only one 8-port expansion chip is shown but these can be further cascaded as required. the shift register, and one for latching the data into the shift register. The 74xx165 also has a serial output port making it possible to daisy chain multiple 165’s to achieve even more inputs. To read 8 input lines from a single 74xx165 the PICAXE-08 code is: Symbol Symbol Symbol latchin = 4 datain = pin3 clk = 2 Main: High latchin Loop: Gosub bytein Goto loop ‘Read a byte into b1 ‘Loop forever Bytein: ‘Reads a byte of data into b1 Pulsout latchin, 1 ‘Latch the input register Let b1 = 0 ‘Initialise data to zero For b0 = 0 to 7 ‘Count to 8 Let b1 = b1 * 2 ‘Shift left If datain = 0 then nobit ‘Test for a data bit Let b1 = b1 + 1 Nobit: Pulsout clk, 1 ‘Clock the shift register Next b0 return In Fig.1b a 74xx595 is being used to expand the number of outputs to eight. Once again three PICAXE ports are needed; one for serial data, one for clocking the shift register, and one for latching the data. Like the 74xx165, the 74xx595 also has a serial output port, again making it possible to daisy chain multiple 595’s to achieve even more outputs. To output 8 bits from a single 74xx595 the PICAXE-08 code is: Symbol Symbol Symbol dataout = 0 clk =2 latchout = 1 Main: For b3 = 0 to 255 Let b2 = b3 Gosub byteout Next b3 Goto main Byteout: For b0 = 0 to 7 Let b1 = b2 & 1 If b1 = 1 then outhi Low dataout Goto clockout Outhi: High dataout Clockout: Pulsout clk, 1 Let b2 = b2 / 2 Next b0 Pulsout latchout, 1 return ‘Output the numbers 0 thru 255 ‘Loop forever ‘Output the byte in b2 — b2 is destroyed in the process ‘Count to 8 ‘Mask off low order bit Test for output bit ‘Set output bit to zero ‘Set output bit to one ‘Clock the shift register ‘Shift right ‘Latch the output register Expanding both the input and output ports at the same Fig.2 combines both of the above circuits into one, giving both input and output port expansion for the ’08. www.siliconchip.com.au June 2003  23 time can be achieved by combining the circuits of Figs.1a and 1b into the circuit of Fig.2. Only five PICAXE ports are required because the clock line can be shared by the input and output shift registers. That’s just as well, because the PICAXE-08 has more than four and less than six ports . . . Now that we have eight inputs and eight outputs to play with we can run some experiments. By connecting push button switches to the expanded input ports and LEDs to the expanded output ports, we can show that our port expansion unit is working. Fig.3 shows what’s needed. Goto clockout Outhi: High dataout Clockout: Pulsout clk, 1 Let b2 = b2 / 2 Next b0 Pulsout latchout, 1 return ‘Set output bit to one ‘Clock the shift register ‘Shift right ‘Latch the output register Almost anything that recognises or runs from a TTL-compatible signal can be connected to the expanded input and output terminals. Fig.4 shows how to connect a 7-segment LED display and Fig.5 shows how to connect a telephone pushbutton keypad. Here is the PICAXE code to display the numbers 0 through 9 on the 7 segment display: Symbol Symbol Symbol Fig.3: this little test setup can be built to show that all is working properly. To copy 8 bits from input to output, the PICAXE-08 code is: Symbol Symbol Symbol Symbol Symbol latchin = 4 datain = pin3 clk = 2 dataout = 0 latchout = 1 Main: ‘Read a byte into b1 and output it from b2 High latchin Loop: Gosub bytein ‘Read a byte Let b2 = b1 ‘Copy input to output Gosub byteout ‘Write a byte Goto loop ‘Loop forever Bytein: ‘Reads a byte of data into b1 Pulsout latchin, 1 ‘Latch the input register Let b1 = 0 ‘Initialise data to zero For b0 = 0 to 7 ‘Count to 8 Let b1 = b1 * 2 ‘Shift left If datain = 0 then nobit ‘Test for a data bit Let b1 = b1 + 1 Nobit: Pulsout clk, 1 ‘Clock the shift register Next b0 return Byteout: ‘Output the byte in b2 — b2 is destroyed in the process For b0 = 0 to 7 ‘Count to 8 Let b1 = b2 & 1 ‘Mask off low order bit If b1 = 1 then outhi ‘Test for output bit Low dataout ‘Set output bit to zero 24  Silicon Chip dataout = 0 clk =2 latchout = 1 Main: ‘Output the numbers 0 thru 9 on a 7 segment display For b3 = 0 to 9 Lookup b3, ($BE, $82, $DC, $D6, $E2, $76, $7E, $92, $FE, $F2), b2 Gosub byteout Pause 500 Next b3 Goto main ‘Loop forever Byteout: For b0 = 0 to 7 Let b1 = b2 & 1 If b1 = 1 then outhi Low dataout Goto clockout Outhi: High dataout Clockout: Pulsout clk, 1 Let b2 = b2 / 2 Next b0 Pulsout latchout, 1 return ‘Output the byte in b2 — b2 is destroyed in the process ‘Count to 8 ‘Mask off low order bit ‘Test low order bit ‘Set output bit to zero ‘Set output bit to one ‘Clock the shift register ‘Shift right ‘Latch the output register The following code will read the buttons pressed on a telephone keypad and display the result on the LED display. To do this we need 10 output lines, more than can be achieved with a single 74xx595, so a second ’595 is daisy chained to the first to give 8 extra outputs (see Fig.5). A similar technique could be used to daisy chain ’195 shift registers to give more inputs. ‘Keypad input - 7 seg. display output ‘ ‘ b0 bit counter ‘ b1 temp work data ‘ b2 display data ‘ w4 output data www.siliconchip.com.au Fig.4: here’s how a 7segment LED display is connected to the ’595. Any of the “garden variety” common cathode LED displays could be used. ‘ ‘Digit 0 1 2 3 4 5 6 7 8 9 * # ‘Segment code $BE, $82, $DC, $D6, $E2, $76, $7E, $92, $FE, $F2, $68, $EB ‘ ‘b2 bit 76543210 ‘Segment b g f a e d c dp ‘ symbol clk = 2 symbol latchout = 1 symbol dataout = 0 symbol datain = pin3 symbol latchin = 4 main: high latchin loop: let w4 = b2 + $100 gosub inout lookup b1, (b2, $82, $E2, 0, $92, 0, 0, 0, $68), b2 if b1 <> 0 then loop let w4 = b2 + $200 gosub inout lookup b1, (b2, $DC, $76, 0, $FE, 0, 0, 0, $BE), b2 if b1 <> 0 then loop let w4 = b2 + $400 gosub inout lookup b1, (b2, $D6, $7E, 0, $F2, 0, 0, 0, $EB), b2 goto loop inout: wordout: ‘Output the word in w4 - destroys w4 for b0 = 0 to 15 let b1 = w4 & 1 if b1 = 1 then outhi low dataout goto clockout outhi: high dataout clockout: pulsout clk, 1 let w4 = w4 / 2 next b0 pulsout latchout, 1 bytein: ‘Read a byte into b1 pulsout latchin, 1 let b1 = 0 for b0 = 0 to 7 let b1 = b1 * 2 if pin3 = 0 then nobit let b1 = b1 + 1 nobit: pulsout clk, 1 next b0 return SC Fig.5: input from a telephone keypad and output to a 7-segment LED display. www.siliconchip.com.au June 2003  25