Python Spidev

This project contains a python module for interfacing with SPI devices from user space via the spidev linux kernel driver.

This is a modified version of the code originally found here

All code is GPLv2 licensed unless explicitly stated otherwise.

More Details on Use with the Omega2

Full-duplex SPI transmissions are not supported. This is due to the MediaTek MT7688 SoC used in the Omega2 family. Half-duplex SPI transmissions ARE supported. See the xfer3 Half-Duplex Transmission section below.

See more details here:

• https://onion.io/2bt-brand-new-os-release/#spiimprovement
• http://community.onion.io/topic/3179/spi-bus-in-python/17

Installation on Omega2

Connect to the Omega's command line and run the following commands to install the Python3 module:

opkg update
opkg install python3-light python3-spidev

Legacy Python2 Support

Skip this section if you're using Python3

The spidev module is available for Python2 on Onion Firmware v0.3.x, the installation commands are slightly different:

opkg update
opkg install python-light python-spidev

Note this will only work if you're using Python2 on an Omega2 running Onion Firmware v0.3.x or v0.2.x

Usage

import spidev
spi = spidev.SpiDev()
spi.open(bus, device)
to_send = [0x01, 0x02, 0x03]
spi.xfer(to_send)

Settings

import spidev
spi = spidev.SpiDev()
spi.open(bus, device)

# Settings (for example)
spi.max_speed_hz = 5000
spi.mode = 0b01

...

Optionally change the settings:

• bits_per_word - change number of bits per word, expects integer value between 8 and 16
• cshigh - expects True or False
• loop - Set the "SPI_LOOP" flag to enable loopback mode, expects True or False
• no_cs - Set the "SPI_NO_CS" flag to disable use of the chip select (although the driver may still own the CS pin), expects True or False
• lsbfirst - expects True or False
• max_speed_hz- set maximum bus speed in hertz, expects long integer number
• mode - SPI mode as two bit pattern of clock polarity and phase [CPOL|CPHA], expects binary, min: 0b00 = 0, max: 0b11 = 3
• threewire - SI/SO signals shared, expects True or False

Methods

open() - Connect to SPI device

Connects to the specified SPI device, opening /dev/spidev<bus>.<device>

open(bus, device)

---

readbytes() - Read bytes from SPI bus

Read n bytes from SPI device. Returns list of bytes read by SPI controller

readbytes(n)

---

writebytes - Write bytes to SPI bus

Writes a list of values to SPI device.

writebytes(list of values)

---

xfer() - Full-Duplex SPI transaction, releasing CS

Performs an SPI transaction. Chip-select should be released and reactivated between blocks. Delay specifies the delay in usec between blocks. Returns list of bytes read by SPI controller.

xfer(list of values[, speed_hz, delay_usec, bits_per_word])

---

xfer2() - Full-Duplex SPI transaction, holding CS

Performs an SPI transaction. Chip-select should be held active between blocks. Returns list of bytes read by SPI controller.

xfer2(list of values[, speed_hz, delay_usec, bits_per_word])

---

xfer3 - Half-Duplex SPI transaction

Performs a half-duplex SPI transaction. Chip-select should be held active between blocks. Returns list of bytes read by SPI controller.

Use this function when the intent is to write a number of bytes and then immediately read a number of bytes (register reads for example)

xfer3(list of values to be written, number of bytes to read [, speed_hz, delay_usec, bits_per_word])

---

close() - Disconnect from Device

Disconnects from the SPI device.

close()

---

Differences between the xfer functions

Function	Recommended for use on Omega2	Chip-Select behaviour between blocks	SPI Transfer Type	Writes Successfully	Reads Successfully
xfer	Only for writing data	Released and reactivated	Full-duplex - sending and receiving data simultaneously	✅	❌
xfer2	Only for writing data	Held active	Full-duplex - sending and receiving data simultaneously	✅	❌
xfer3	✅	Held active	Half-duplex - send then receive data	✅	✅