NAME RPi::DigiPot::MCP4XXXX - Interface to the MCP4xxxx series digital potentiometers on the Raspbery Pi DESCRIPTION This distribution allows you to interface directly with the MCP41xxx and MCP42xxx series digital potentiomenters attached to the SPI bus on the Raspberry Pi. The MCP41xxx units have a single built-in potentiometer, where the MCP42xxx units have two. Both series will operate on either 3.3V or 5V, as the potentiometers do not send anything back to the Pi's GPIO. This software requires wiringPi to be installed, as we use its SPI library to communicate to the potentiometer over the SPI bus. SYNOPSIS # GPIO pin number connected to the potentiometer's # CS (Chip Select) pin my $cs = 18; # SPI bus channel my $chan = 0; my $dpot = RPi::DigiPot::MCP4XXXX->new($cs, $chan); # potentiometer's output level (0-255). # 127 == ~50% output my $output = 127; # set the output level $dpot->set($output); # shutdown (put to sleep) the potentiometer $dpot->shutdown; METHODS new Instantiates a new RPi::DigiPot::MCP4XXXX object, initiates communication with the SPI bus, and returns the object. Parameters: $cs Mandatory: Integer, the GPIO pin number that connects to the potentiometer's Chip Select `CS' pin. This is the pin we use to start and finish communication with the device over the SPI bus. $channel Mandatory: Integer, represents the SPI bus channel that the potentiometer is connected to. `0' for `/dev/spidev0.0' or `1' for `/dev/spidev0.1'. $speed Optional: Integer. The clock speed to communicate on the SPI bus at. Defaults to `1000000' (ie: `1MHz'). set This method allows you to set the variable output on the potentiometer(s). These units have 256 taps, allowing that many different output levels. Parameters: $data Mandatory: Integer bewteen `0' for 0% output and `255' for 100% output. $pot Optional: Integer, instructs the software which of the onboard potentiometers to set the output voltage on. `1' for the first potentiometer, `2' for the second, and `3' to change the value on both. Defaults to `1'. NOTE: Only the MCP42xxx units have dual built-in potentiometers, so if you have an MCP41xxx unit, leave the default `1' set for this parameter. shutdown The onboard potentiometers allow you to shut them down when not in use, resulting in electricity usage. Using `set()' will bring it out of sleep. Parameters: $pot Optional: Integer, the built-in potentiometer to shut down. `1' for the first potentiometer, `2' for the second, and `3' to change the value on both. Defaults to `1'. NOTE: Only the MCP42xxx units have dual built-in potentiometers, so if you have an MCP41xxx unit, leave the default `1' set for this parameter. TECHNICAL INFORMATION View the MCP4XXX datasheet. OVERVIEW The MCP4xxxx series digital potentiometers operate as follows: - CS pin goes LOW, signifying data is about to be sent - exactly 16 bits are sent over SPI to the digipot (first 8 bits for control second 8 bits for data) - CS pin goes HIGH, signifying communication is complete There must be exactly 16 bits of data clocked in, or the commands and data will be thrown away, and nothing accomplished. Here's a diagram of the two bytes combined into a single bit string, showing the respective positions of the bits, and their function: |<-Byte 1: Control->|<-Byte 0: Data->| | | | fcn: | command | channel | data | |---------|---------|----------------| bit: | 7 6 5 4 | 3 2 1 0 | 7 6 5 4 3 2 1 0| -------------------------------------- ^ ^ | | MSB (bit 15) LSB (bit 0) CONTROL BYTE The control byte is the most significant byte of the overall data being clocked into the potentiometer, and consists of a command nibble and a channel nibble. COMMAND The command nibble is the most significant (leftmost) 4 bits of the control byte (bits 7-4 in the above diagram). The following diagram describes all possible valid values. Bits Value ------------- 0000 NOOP 0001 set a new resistance value 0010 put potentiometer into 'shutdown' mode 0011 NOOP CHANNEL The channel nibble is the least significant 4 bits (rightmost) of the control byte (bits 3-0 in the above diagram). Valid values follow. Note that the MCP41xxx series units have only a single potentiometer built in, there's but one valid value for them. Bits Value ------------- 0001 potentiometer 0 0010 potentiometer 1 (MCP42xxx only) 0011 both 0 and 1 (MCP42xxx only) DATA BYTE The data byte consists of the least significant 8 bits (rightmost) of the 16 bit combined data destined to the potentiometer. Both the MCP41xxx and MCP42xxx series potentiometers contain 256 taps, so the mapping of this byte is simple: valid values are `0' (0% output) through `255' (100% output). REGISTER BIT SEQUENCE Here's an overview of the bits in order: `15-14': Unused ("Don't Care Bits", per the datasheet) `13-12': Command bits `11-10': Unused `9-8': Channel (built-in potentiomenter) select bits `7-0': Potentiometer tap setting data (0-255) AUTHOR Steve Bertrand, `' LICENSE AND COPYRIGHT Copyright 2017 Steve Bertrand. This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License. See http://dev.perl.org/licenses/ for more information.