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Many of us have had a dream of building up a small general purpose board with a CPU, a bit of memory & some I/O lines. Then we could write programs to handle small applications. One easy way to do it would be to use the Stamp, a complete selfcontained microcontroller board the size of a large postage stamp. A look at the Stamp microcontroller board By BOB NICOL If you take the conventional approach to designing a micro­controller board, you have quite a few steps to go through. But once you had the basic design produced, it would make applica­tions a lot easier. Ideas could be tried out swiftly and easily, without the need to get parts together, before a start could be made. Such a device could be used on many projects and should a project fail, the board would not be wasted but used again with­out major changes. Once one starts building such a board, one will be up for paraphernalia to do the job efficiently – an assembler, an in-circuit emulator, and the need to put your own PC board together. And in the middle of all this, one is probably struggling to learn a new programming language. Now you will be exposed to the write, burn, try, debug, erase, edit, reburn, etc merrygo-round. And that time consuming procedure will occur for each new appli­cation of your board. With all the above to cope with, it is little wonder that few people turn the dream into reality – there is too much work involved. So here is a much less expensive and more comfortable alternative. It uses an EEPROM and 54  Silicon Chip reprograms in around a second! Called the Stamp, it is a small board, measuring only 60 x 35mm (not much bigger than a typical stamp). The board uses a PIC16C56 microcontroller and a memory chip, a ceramic resonator, a 5V regulator, two resistors, three capacitors, a transistor and a 14-pin connector. Also on the board are the battery connectors, a 3-pin header for programming by a PC-compatible computer, and a 10 x 14 plated-through hole work area. The complete circuit is shown in Fig.1. Made by Parallax, USA, the Stamp has 33 instructions to do some usual and unusual things with eight I/O lines. The Stamp is small and simple and comes close to being a programmable in­tegrated circuit. Take a look at the list of application notes in Table 1 and you will see that although the instruction set is small, sophisticated tasks can be achieved by the Stamp. At the time of writing, 19 application notes are sent with each Stamp programming package. Simplicity has been achieved by using the PIC16C56 which includes a BASIC-like interpreter. It has some familiar BASIC commands such as FOR, NEXT and LET, plus some unusual commands like MAX, MIN, PULSIN, PULSOUT, BUTTON and POT. An EEPROM is used to store a program. This same EEPROM may be used to store data from the PC when initially loading a pro­gram and via the WRITE command which allows the Stamp to write data into its own EEPROM. The exact amount of memo­ry available for data storage is determined by what is left spare in the 256 byte EEPROM after storing your program. One has to be careful not to overwrite the program with data. REM statements may be used in your program; these are not sent to the Stamp but kept in FILENAME.BAS which is stored by the Stamp editor/ programmer. To program the Stamp one needs an IBM compatible PC, a Stamp programming cable, software and the instruction book. To save the cost of a power supply, the Stamp may be powered by a 9V battery. Writing software To any one who has used a text editor, the editor/program­ming software supplied with the Stamp will have a familiar feel about it. It is a full screen editor with all the usual file handling PC PROGRAMMING CONNECTOR DATA 8 VCC VCC 4.7k 3 BUSY 2 16 15 VDD 4.7k RA0 17 18 RA1 RTCC RA3 RA2 1 11 IC1 PIC16C56XT OSC1 OSC2 4MHz MCLR 4 Q1 2N3906 RB0 6 VSS 5 OUT VCC (+5V) 2.2M 10 2.2M VSS 5 USER PROGRAMMABLE I/O PORT RB2 8 RB1 7 470k CS ORG 6 13 RB7 RB6 12 RB5 11 RB4 10 RB3 9 VCC Table 1: Applications 4 D0 3 D1 IC2 2 CLK 93LC56 IN COM 9V Fig.1: the circuit of the Stamp uses a PIC16C56 microcon­troller and a 93LC56 EEPROM. The 2N3906 is used for resetting the micro and is a surface mount device on the copper side of the board. functions, while movement through text on the screen is via the normal cursor keys, with HOME and END as well as PAGE-UP and PAGE-DOWN being appropriate commands. The editor does not use a mouse, however CUT, COPY, PASTE and SEARCH/REPLACE are easily accomplished using keyboard strokes. There are nine pages in the 63page instruction book ex­ p laining how to use the editor, four pages of introduction, 12 pages for explaining hardware, and 37 pages devoted to commands. Using some of the commands requires a little extra circuitry, as detailed below. BUTTON is the command used for connecting push buttons, keys, or switches to the Stamp. The Stamp’s in+5V 10k terpreter debounces the switch action; all the user needs to do is specify pin number, whether the transition will be high to low or vice versa, and auto repeat requirements. The two button circuits are shown in Fig.2. POT is a command which could be used to read the angular setting of a potentiometer. The command also works well with thermis­tors, light dependent resistors, etc. When using POT the program­mer needs to specify which pin the variable resistor is connected to and set a scale factor. There is a convenient setup facility in the program writing editor: just press <ALT>P and you will be guided through a setting up routine. Measurement of the resistor value is achieved by measuring the time taken +5V TO I/O PIN 10k TO I/O PIN Fig.2: these are two circuits for use with the BUTTON command. The button can switch the input high or low. 5-50k TO I/O PIN 0.1 0.1 Fig.3: the POT command uses this circuit to charge a 0.1µF ca­pacitor and the charging time is measured by the micro. (1) LCD user interface terminal (2) Interfacing an A/D converter (3) Hardware solution for keypads (4) Controlling and testing servos (5) Practical pulse measurements (6) A serial stepper controller (7) Sensing temperature with a thermistor (8) Sending messages in Morse code (9) Constructing a dice game. (10) Sensing humidity and temp­ erature (11) Wireless infrared communication (12) Cheap sonar rangefinding with the Stamp (13) Using (extra) serial EEPROMs (14) Networking multiple Stamps (15) Using PWM for analog output (16) Keeping Stamp private (17) The solar powered Stamp (18) One pin, many switches (19) Using BUTTON effectively to charge the capacitor in the circuit shown in Fig.3. PWM is virtually the reverse of the POT command. PWM with appropriate circuitry will give an analog representation of a value contained in a variable. This variable could have been accessed by the Stamp from a serial data input, a variable resis­tor input from POT, or could be derived from calculations done on variables, constants and look-up tables. The circuit used is an RC integrator as shown in Fig.4 but it will need to be followed by an op amp buffer. SERIN is the Stamp’s command for reception of serial data from another Stamp, a PC, logging device, a modem or whatever. The programmer needs to specify pin number and speeds from 300-2400 baud are available. One may invert the mark/space of the bits; this makes it easy to use other interface ICs where a more sophisticated system may be in use. The SERIN command may be set to wait for a specific character or string before doing anything. The circuit (Fig.5) is simply a resistor between the ±10V serial input and the designated I/O pin. SEROUT is exactly the reverse of SERIN and may be used to send mesDecember 1994  55 Table 2: Stamp Command Set Branching IF THEN BRANCH GOTO GOSUB Looping FOR.NEXT Numerics (LET) Compare and conditionally branch Branch to address specified by offset Branch to address Branch to Subroutine, up to 16 allowed. Establish a FOR-NEXT loop Perform variable manipulation, such as A=5, B=A+2, etc. Possible operations are add, subtract, multiply, divide, maxlim­it, minlimit, and logical operations, AND, OR, XOR, ANDNOT, ORNOT and XORNOT. Note variables handle integers only. LOOKUP Lookup data specified by offset, and store in variable. This instruction provides the means to make a look up table. LOOKDOWN Find target’s match number (0-N) and store in a vari­able. RANDOM Generate a pseudo random number. Digital I/O OUTPUT Make a pin an output LOW Make pin output low HIGH Make a pin output high TOGGLE Make a pin an output, and toggle its state PULSOUT Output a timed pulse, by inverting a pin, for a time. INPUT Make a pin an input PULSIN Measure an input pulse REVERSE If pin is an output, make it an input, or if output, make it an input. BUTTON Debounce button, perform an auto repeat, and branch to address if button in target state. Serial I/O SERIN Serial input with optional qualifiers and variables for storage of received data. If qualifiers are given, then the instruction will wait until they are received before filling variables or continuing to the next instruction. Baud rates of 300, 600, 1200, and 2400 are possible. Data received must be with no parity, 8 data bits and 1 stop bit. SEROUT Send data serially. Data format the same as SERIN command. Analog I/O PWM Output PWM, then return to input. This can be used to output analog voltages (0-5V) using a capacitor and resistor. POT Read a 5-50kΩ potentiometer and scale the result. Sound SOUND Play notes. Note 0 is silence, notes 1-127 are ascending tones, and notes 128-255 are white noises. EEPROM Access EEPROM Store data in EEPROM before downloading BASIC program. READ Read EEPROM byte into variable. Time PAUSE Pause execution for 0-65536 milliseconds. Power Control NAP Nap for a short period. Power consumption is reduced. SLEEP Sleep for 1-65536 seconds. Current consumption is reduced to about 20uA. END Sleep until the power cycles, or the PC connects. Current consumption is reduced to 20uA. Program Debugging DEBUG Send variables to PC for viewing. 56  Silicon Chip sages to a network of Stamps, pulling each Stamp into use as needed. Again the circuit is dead simple, as shown in Fig.6. SOUND puts out a tone on the specified pin. Pitch may be specified in the command line or may be taken from a variable. A conventional speaker may be driven by putting a capacitor in series as shown in Fig.7, while a piezo speaker can be directly connected without the capacitor. As with any program writing exercise, the jobs that may be done with the Stamp are limited only by the user’s imagination, skill with the commands, and the facilities of the hardware supplied. To help the user get started, 19 application notes are supplied with the Stamp programming kit. These are listed in Table 1. One of these application notes is reproduced here. This application note presents a program in Parallax BASIC that ena­bles the BASIC Stamp to operate as a simple user interface termi­nal. Many systems use a central host computer to control remote functions. At various locations, users communicate with the main system via small terminals that display the system status and will accept inputs. The BASIC Stamp’s ease of programming and built-in support for serial communications make it a good can­didate for such user interface applications. The liquid crystal display (LCD) used in this project is based on the popular Hitachi 44780 controller IC. These chips are the heart of LCDs ranging in size from two lines of four charac­ters (2 x 4) to 2 x 40. When power is first applied, the BASIC program initialises the LCD. It sets the display to print from left to right and enables an underline cursor. To eliminate any stray characters, the program clears the screen. After initialisation, the program enters a loop, waiting for the arrival of a character via the 2400 baud RS-232 interface. When a character arrives, it is checked against a short list of special characters (Backspace, control C and RETURN). If it is not one of these, the program prints it on the display and re-enters the waiting for data loop. If a backspace is received, the program moves the LCD cursor back one space, prints a blank (space) character to blot out the character that was there, and then moves back again. The second move back is necessary because the LCD automatically advances the cursor. If a control C is received, the program issues a clear instruction to the LCD, which responds by filling the screen with blanks and returning the cursor to the left most position. If a RETURN character is received, the program interprets the message as a query, requiring a response from the user. It enters a loop, waiting for the user to press one of the four push buttons. When he does, the program sends the character (0 through 3), representing the button number back to the host system. It then re-enters its waiting loop. Because of all this processing, the user interface cannot receive characters sent rapidly at the full baud rate. The host program must put a little breathing space between characters; perhaps a 3ms delay. If you reduce the baud rate to 300 baud and set the host terminal to 1.5 or 2 stop bits, you may avoid the need to program a delay. From an electronic standpoint, the circuit employs a couple of tricks. The first involves the RS-232 communication. The Stamp’s processor, the PIC16C56, is equipped with static protection diodes on its input/output pins. When the Stamp re­ceives RS-232 data which typically swings between -12V and +12V, these diodes serve to limit the voltage actually seen by the PIC’s internal circuitry to 0V and +5V. The 22kΩ resistor limits the current through the diodes to prevent damage. Sending serial output without an external driver circuit exploits another loophole in the RS-232 standard. While most RS-232 devices expect the signal to swing between at least -3V and +3V, most will accept the 0 and +5V output of the PIC without problems. This setup is less noise immune than circuits that follow the RS-232 rules. If you add a line driver/receiver such as a MAX232, remember that these devices also invert the signals. You’ll need to change the baud mode parameter in the instructions SERIN and SEROUT to T2400 where T stands for true signal polari­ty. If greater noise immunity is required, or the interface will be at the end of a long cable, use an RS-422 driver receiver. This will require the same changes to SERIN and SEROUT. Another trick allows the sharing of input and output pins be- 10k FROM I/O PIN ANALOG OUTPUT 1 Fig.4: this integrator is used for the PWM command but will probably need to be followed by an op amp buffer for many appli­cations. FROM I/O PIN 10k TO OTHER STAMPS Fig.6: a loading resistor is all that is required to implement the SEROUT (serial data out) command. TO ±10V 22k SERIAL I/O INPUT PIN Fig.5: just one resistor is needed to implement the SERIN (serial data in) command. FROM I/O PIN 10 40  Fig.7: by using the SOUND command, any of the I/O pins may be used to drive a 40Ω speaker via a capacitor. Note that if an 8Ω speaker is used, a series resistor of 33Ω will be required and this will inevitably reduce the available sound level. Fig.8: the BASIC Stamp Programming package includes a number of application notes, including one that enables the Stamp to operate a simple LCD user interface terminal – see text. tween the LCD and the push­butttons. What happens if the user presses the buttons while the LCD is receiving data? Nothing. The Stamp can sink enough current to prevent the 1kΩ pullup resistors from affecting the state of its active output lines. And when the Stamp is receiving input from the switches, the LCD is disabled, so its data lines are in a high impedance state. These points allow the LCD and the switches to share the data lines without interference. Finally, note that the resistors are shown on the data side of the switches, not on the +5V side. This is an inex- pensive precaution against damage or interference due to electrostatic discharge from the user’s fingertips. Currently the Stamp is available in Australia at three levels: Starter level is a programming kit containing software, instruction book, a programming cable and one Stamp. This is priced at $270 including sales tax. For a second stage, extra Stamp modules are available at $79.85 each including sales tax. Postage and packing on all orders is $8.00. Send all orders to MicroZed Computers, PO Box 634, Armidale, NSW 2350. Phone (067) 72 2777 or SC fax (067) 72 8987. December 1994  57