SPI Using Spidev library python

Technical Support
1

Hello everyone!

I’m currently exploring EtherCAT communication on the Toradex IMX7 Colibri evaluation board using the spidev library. I have a query regarding the necessity of managing the chip select (CS) line while transmitting data over SPI.

Is it mandatory to toggle the chip select line high and low for correct data transmission? Additionally, I’m seeking guidance on how to effectively send an array of data using the spidev library.

For instance, I’m considering using the spi.xfer() method for data transmission. Could someone provide insights on the proper usage of this method to send data arrays over SPI?

Your assistance and expertise are greatly appreciated!

Thank you."

Example format (which i using currently):

SPI setup

spi = spidev.SpiDev()
spi.open(2, 0) # Assuming SPI bus 0, device 0
spi.max_speed_hz = 1000000 # Adjust as needed
spi.mode = 3

def Etc_Read_Reg(address, length):
Result = ULONG() # Initialize Result as ULONG instance
Addr = UWORD() # Initialize Addr as UWORD instance and set address
xfrbuf = (ctypes.c_uint8 * 7)() # Create buffer for SPI transfer

Addr.LANWord = address

xfrbuf[0] = COMM_SPI_READ  # SPI read command
xfrbuf[1] = Addr.LANByte[1]  # Address high byte
xfrbuf[2] = Addr.LANByte[0]  # Address low byte

print()
print("Entered Read_reg")
print()
print("xfrbuf before filling: ", end="")
for i in range(3):
    print("{:02X} ".format(xfrbuf[i]), end="")          

for i in range(length):
    xfrbuf[i + 3] = DUMMY_BYTE  # Fill dummy bytes after address bytes

print("\nxfrbuf after filling: ", end="")
for i in range(7):
    print("{:02X} ".format(xfrbuf[i]), end="")

response = spi.xfer(xfrbuf)

# response = spi.xfer2(list(xfrbuf))  # Perform SPI transfer and receive response

for i in range(length):
    Result.LANByte[i] = response[i + 3]  # Assign received bytes to Result.LANByte[i]
return Result.LANLong
2

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Hello,

I understand you’re exploring SPI communication using the spidev library on a Toradex iMX7 Colibri board and have questions about chip select management and data transmission.

Regarding the necessity of toggling the chip select (CS) line, it is generally required for correct data transmission over SPI. The CS line is used to signal the beginning and end of a data frame, ensuring that the peripheral device knows when to listen and when to ignore the bus. Most SPI peripherals require the CS line to be pulled low at the start of the transaction and set high at the end. However, the spidev library and your SPI device’s driver might manage this for you automatically when you use the xfer() or xfer2() methods, depending on how the SPI bus is configured in your device tree or SPI setup. It’s important to consult your specific SPI device’s datasheet for its CS handling requirements.

For sending an array of data using the spidev library, your approach using the spi.xfer() method is correct. This method allows you to send and receive data in a full-duplex manner. The data to be sent is passed as a list or array, and the method returns a list of the same length containing the data received during the transaction. Your example code correctly demonstrates setting up the SPI device, configuring its parameters, and preparing a buffer for transmission. When using spi.xfer(xfrbuf), the xfrbuf array is sent out over SPI, and the response from the SPI device is captured in the response variable.

Here’s a simplified example of sending an array of data and receiving the response:

import spidev

# SPI setup
spi = spidev.SpiDev()
spi.open(2, 0)  # Open SPI bus 2, device 0
spi.max_speed_hz = 1000000
spi.mode = 3

# Data to send
data_to_send = [0x01, 0x02, 0x03, 0x04]  # Example data

# Transmit data and receive response
response = spi.xfer(data_to_send)

print("Response: ", response)

In this example, data_to_send is an array of bytes to be transmitted. The spi.xfer() method sends this data and captures the response from the SPI device in the response variable.

Remember, the effectiveness of your SPI communication

Please, also check if these links can help you solve your problem:

SPI (Linux) | Toradex Developer Center.

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