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El Principito LuleKooks asked How can I control a digital resistor with a matricial keyboard?
I have to type x number with a matricial keyboard in order to control a digital resistor. Example: If I type "1", r=100ohms; If I type "15", r=1.5Kohms. I'm trying to do it with a GAL or an EPROM, but the thing is I haven't got a clue about how to do the program I need to control the resistor. Any help, please?
And got the following answer:
It is far more likely that you would use a small microprocessor or micro-controller, along the lines of a PIC. It is less effort in the long run. A matrix keyboard is typically four columns and 4 rows for 16 keys or states. By scanning the columns one at a time and reading the 4 rows at each column, determine which key is pressed. This needs i/o with 4 outputs and 4 inputs from the uP. If you search for "PIC source code matrix keyboard" you should find examples of programming. This kind of sequential task is more difficult with a gal or eprom, it becomes a challenge. When the sequence of digits is complete, usually an "enter" key is pressed (perhaps the "#" key. This lets the software know to start processing the number. The digits will be converted from the matrix codes (X,Y representing each key identity) to whatever code the digital resistor uses, then sent to the resistor. The use of a uP makes it easier to add a display, such as a 16 character LCD display, or even 7 segment LEDs, to verify what number is received. Digital potentiometers come in many forms, so you start with the data sheets. They usually have some sort of bit serial interface, like I2C or one-wire. Once again this is difficult to achieve with a GAL or eprom, as it is sequential. The uP can format and send the correct code to the device using a single output i/o line or so as for the protocol your digi-pot has. The value is possible a binary number, converted from the key code. The source code for these (at least in generic form for one-wire, I2C etc ) is also likely to be found on the internet. Further details will come from the data sheet for your device. The link below shows a selection of digi-pot devices from one manufacturer. Note they are potentiometers! A similar device is a digital to analogue converter which produces a programmable voltage or current output, and can usually be used as a potentiometer too with the reference driven by your signal. It is possible to convert these outputs to a programmable resistance using op-amps (analogue feedback circuits). The resistor produced in this way is not floating, but constrained by power supply and ground connections. The supplies could be isolated, or may be suitable for a load resistance. Another approach is to use relays to switch combinations of resistors. The relay C/O contacts can be arranged to combine resistors in an R - 2R network, so a binary code gives a combined value. In this way 4 relays gives 16 values, 8 relays give 256 values, etc. Another way is to switch individual resistors to get the desired value. This could end up needing a lot of relays. Relays can provide individual "floating" resistance values like a substitution box, which seems to be what you want. Here a gal could be used to expand the digital i/o outputs from the uP by decoding several binary lines into many individual lines to operate relays. Instead of relays it may be possible to use CMOS analogue switches within limits, but these will mean the resistances are constrained by power supply again. Relay contacts are more accurate, but do have contact bounce. It is also possible to get the uP to control some sort of mechanical multi-position switch that selects a value from a number of resistors. This could be a motor driven switch. Imagine even motors driving the switches on a resistance substitution/decade box.