mirror of
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143 lines
4.4 KiB
ReStructuredText
143 lines
4.4 KiB
ReStructuredText
===========
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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 there 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:: MCP3001
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:members: value
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.. autoclass:: MCP3002
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:members: channel, value, differential
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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:: MCP3201
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:members: value
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.. autoclass:: MCP3202
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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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Base Classes
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============
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The classes in the sections above are derived from a series of base classes,
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some of which are effectively abstract. The classes form the (partial)
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hierarchy displayed in the graph below:
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.. image:: images/spi_device_hierarchy.*
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The following sections document these base classes for advanced users that wish
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to construct classes for their own devices.
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AnalogInputDevice
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=================
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.. autoclass:: AnalogInputDevice
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:members:
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SPIDevice
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=========
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.. autoclass:: SPIDevice
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:members:
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