RPi::DigiPot::MCP4XXXX - Interface to the MCP4xxxx series digital potentiometers on the Raspbery Pi
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.
# 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
# shutdown (put to sleep) the potentiometer
Instantiates a new RPi::DigiPot::MCP4XXXX object, initiates communication with the SPI bus, and returns the object.
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.
Mandatory: Integer, represents the SPI bus channel that the potentiometer is connected to. 0 for /dev/spidev0.0 or 1 for /dev/spidev0.1.
Optional: Integer. The clock speed to communicate on the SPI bus at. Defaults to 1000000 (ie: 1MHz).
This method allows you to set the variable output on the potentiometer(s). These units have 256 taps, allowing that many different output levels.
Mandatory: Integer bewteen 0 for 0% output and 255 for 100% output.
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.
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.
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.
View the MCP4XXX datasheet.
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)
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.
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.
0001 set a new resistance value
0010 put potentiometer into 'shutdown' mode
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.
0001 potentiometer 0
0010 potentiometer 1 (MCP42xxx only)
0011 both 0 and 1 (MCP42xxx only)
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).
Here's an overview of the bits in order:
15-14: Unused ("Don't Care Bits", per the datasheet)
13-12: Command bits
9-8: Channel (built-in potentiomenter) select bits
7-0: Potentiometer tap setting data (0-255)
Steve Bertrand, <steveb at cpan.org>
<steveb at cpan.org>
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.
To install RPi::DigiPot::MCP4XXXX, copy and paste the appropriate command in to your terminal.
perl -MCPAN -e shell
For more information on module installation, please visit the detailed CPAN module installation guide.