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, `<steveb at cpan.org>'
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.
