SPI using pigpio

The Rapsberry Pi has a SPI interface on its GPIO header. With the spidev driver the FPGA can be used by using only 4 wires (MOSI, MISO, CLK, and CS). This driver is designed specifically for the Raspberry Pi for boosting speed and reducing jitter. It relies on the library pigpio, which is dynamically loaded when the module is laoded by Litex-CNC.

Configuration

The configuration of the FPGA consist of the definition of the pins where MOSI, MISO, CLCK, and CS are connected to. Below is the example used for the HUB75HAT.

"connection": {
    "connection_type": "spi",
        "mosi": "j5:14",
        "miso": "j5:12",
        "clk":  "j5:15",
        "cs_n": "j5:11"
}

HAL

Use the following connection string for pigpio:

loadrt litexcnc connections="pigpio:<CS_channel>:<speed>"

Only the SPI 0 is supported. The second SPI device on the Rapsberry Pi is currently not supported by the driver. The CS_channel must be either 0 or 1, because SPI 0 has only two channels.

To determine the speed of the SPI communication, one can use the component litexcnc_pigpio_speed_test. This component will increase the speed in 500 kHz steps until identification of the Litex-CNC firmware is not correctly received any longer. For the speed test one must use halcmd:

loadrt litexcnc_pigpio_speed_test

Will, for example, give the following output:

sps=20977.7: 11 bytes @ 1000000 bps (loops=10000, average time=47.670 us, maximum time=703.812 us)
sps=31240.3: 11 bytes @ 1500000 bps (loops=10000, average time=32.010 us, maximum time=735.044 us)
sps=40977.2: 11 bytes @ 2000000 bps (loops=10000, average time=24.404 us, maximum time=582.933 us)
sps=50786.4: 11 bytes @ 2500000 bps (loops=10000, average time=19.690 us, maximum time=102.997 us)
sps=60441.5: 11 bytes @ 3000000 bps (loops=10000, average time=16.545 us, maximum time=123.978 us)
sps=69732.0: 11 bytes @ 3500000 bps (loops=10000, average time=14.341 us, maximum time=101.089 us)
sps=77709.7: 11 bytes @ 4000000 bps (loops=10000, average time=12.868 us, maximum time=92.030 us)
sps=87851.0: 11 bytes @ 4500000 bps (loops=10000, average time=11.383 us, maximum time=50.068 us)
Failed transmission at 5000000 Hz

In the case above, a speed of 4,000,000 is recommended.

Warning

At this moment there is a bug in litexcnc_pigpio_speed_test, causing it to hang as soon as the test is finished. You can close halcmd by pressing CTRL+Z. This however will prevent closing the connection to the GPIO as it should. Restart of the Raspberry Pi is required to close these resources. Not restarting would lead to an increase in jitter; running the test twice without restart shows constant average time, but a severe increases the maximum tim of the communication.