mirror of
https://github.com/KevinMidboe/python-gpiozero.git
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759a6a58e6
This PR adds a software SPI implementation. Firstly this removes the absolute necessity for spidev (#140), which also means when it's not present things still work (effectively fixes #185), and also enables any four pins to be used for SPI devices (which don't require the hardware implementation). The software implementation is simplistic but still supports clock polarity and phase, select-high, and variable bits per word. However it doesn't allow precise speeds to be implemented because it just wibbles the clock as fast as it can (which being pure Python isn't actually that fast). Finally, because this PR involves creating a framework for "shared" devices (like SPI devices with multiple channels), it made sense to bung Energenie (#69) in as wells as this is a really simple shared device.
107 lines
3.7 KiB
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107 lines
3.7 KiB
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===========
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SPI Devices
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===========
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.. currentmodule:: gpiozero
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SPI stands for `Serial Peripheral Interface`_ and is a mechanism allowing
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compatible devices to communicate with the Pi. SPI is a four-wire protocol
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meaning it usually requires four pins to operate:
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* A "clock" pin which provides timing information.
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* A "MOSI" pin (Master Out, Slave In) which the Pi uses to send information
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to the device.
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* A "MISO" pin (Master In, Slave Out) which the Pi uses to receive information
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from the device.
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* A "select" pin which the Pi uses to indicate which device it's talking to.
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This last pin is necessary because multiple devices can share the clock,
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MOSI, and MISO pins, but only one device can be connected to each select
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pin.
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The gpiozero library provides two SPI implementations:
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* A software based implementation. This is always available, can use any four
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GPIO pins for SPI communication, but is rather slow and won't work with all
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devices.
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* A hardware based implementation. This is only available when the SPI kernel
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module is loaded, and the Python spidev library is available. It can only use
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specific pins for SPI communication (GPIO11=clock, GPIO10=MOSI, GPIO9=MISO,
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while GPIO8 is select for device 0 and GPIO7 is select for device 1).
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However, it is extremely fast and works with all devices.
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.. _Serial Peripheral Interface: https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus
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.. _spi_args:
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SPI keyword args
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================
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When constructing an SPI device the are two schemes for specifying which pins
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it is connected to:
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* You can specify *port* and *device* keyword arguments. The *port* parameter
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must be 0 (there is only one user-accessible hardware SPI interface on the Pi
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using GPIO11 as the clock pin, GPIO10 as the MOSI pin, and GPIO9 as the MISO
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pin), while the *device* parameter must be 0 or 1. If *device* is 0, the
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select pin will be GPIO8. If *device* is 1, the select pin will be GPIO7.
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* Alternatively you can specify *clock_pin*, *mosi_pin*, *miso_pin*, and
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*select_pin* keyword arguments. In this case the pins can be any 4 GPIO pins
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(remember that SPI devices can share clock, MOSI, and MISO pins, but not
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select pins - the gpiozero library will enforce this restriction).
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You cannot mix these two schemes, i.e. attempting to specify *port* and
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*clock_pin* will result in :exc:`SPIBadArgs` being raised. However, you can
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omit any arguments from either scheme. The defaults are:
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* *port* and *device* both default to 0.
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* *clock_pin* defaults to 11, *mosi_pin* defaults to 10, *miso_pin* defaults
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to 9, and *select_pin* defaults to 8.
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Hence the following constructors are all equiavlent::
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from gpiozero import MCP3008
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MCP3008(channel=0)
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MCP3008(channel=0, device=0)
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MCP3008(channel=0, port=0, device=0)
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MCP3008(channel=0, select_pin=8)
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MCP3008(channel=0, clock_pin=11, mosi_pin=10, miso_pin=9, select_pin=8)
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Note that the defaults describe equivalent sets of pins and that these pins are
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compatible with the hardware implementation. Regardless of which scheme you
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use, gpiozero will attempt to use the hardware implementation if it is
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available and if the selected pins are compatible, falling back to the software
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implementation if not.
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Analog to Digital Converters (ADC)
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==================================
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.. autoclass:: MCP3004
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:members: channel, value, differential
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.. autoclass:: MCP3008
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:members: channel, value, differential
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.. autoclass:: MCP3204
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:members: channel, value, differential
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.. autoclass:: MCP3208
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:members: channel, value, differential
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.. autoclass:: MCP3301
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:members: value
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.. autoclass:: MCP3302
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:members: channel, value, differential
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.. autoclass:: MCP3304
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:members: channel, value, differential
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