SPI Interface for Cocotb

GitHub repository: https://github.com/schang412/cocotbext-spi

Introduction

SPI simulation framework for cocotb.

Installation

Installation from pip (release version, stable):

pip install cocotbext-spi

Installation from git (latest development version, potentially unstable):

pip install https://github.com/schang412/cocotbext-spi/archive/main.zip

Installation for active development:

git clone https://github.com/schang412/cocotbext-spi
pip install -e cocotbext-spi

Documentation and Usage

See the tests directory for complete testbenches using these modules.

SPI Signals

The SPI bus signals are bundled together into a SpiSignals class.

To create the object simply call it like a class and pass in arguments:

from cocotbext.spi import SpiConfig

spi_signals = SpiSignals(
    sclk = dut.sclk,     # required
    mosi = dut.mosi,     # required
    miso = dut.miso,     # required
    cs   = dut.ncs,      # required
    cs_active_low = True # optional (assumed True)
)

cocotb does not provide a way to generate signals that follow another one, so cs_active_low bool is implemented to support both active high and active low chip selects.

SPI Config

SPI Configuration parameters are bundled together into a SpiConfig class.

To create the object simply call it like a class and pass in arguments:

from cocotbext.spi import SpiConfig

spi_config = SpiConfig(
    word_width = 16,       # number of bits in a SPI transaction
    sclk_freq  = 25e6,     # clock rate in Hz
    cpol       = False,    # clock idle polarity
    cpha       = True,     # clock phase (CPHA=True means data sampled on second edge)
    msb_first  = True,     # the order that bits are clocked onto the wire
    data_output_idle = 1,  # the idle value of the MOSI or MISO line 
    frame_spacing_ns = 1,  # the spacing between frames that the master waits for or the slave obeys
                           #       the slave should raise SpiFrameError if this is not obeyed.
    ignore_rx_value = None # MISO value that should be ignored when received
)

All parameters are optional, and the defaults are shown above.

SPI Master

The SpiMaster class acts as an SPI Master endpoint.

To use this class, import it, configure it, and connect to the dut.

from cocotbext.spi import SpiMaster, SpiSignals, SpiConfig

spi_signals = SpiSignals(
    sclk = dut.sclk,     # required
    mosi = dut.mosi,     # required
    miso = dut.miso,     # required
    cs   = dut.ncs,      # required
    cs_active_low = True # optional (assumed True)
)

spi_config = SpiConfig(
    word_width = 16,     # all parameters optional
    sclk_freq  = 25e6,   # these are the defaults
    cpol       = False,
    cpha       = True,
    msb_first  = True
)

spi_master = SpiMaster(spi_signals, spi_config)

To send data into a design with SpiMaster, call write() or write_nowait(). Accepted data types are iterables of ints including lists, bytes, bytearrays, etc. Optionally, call wait() to wait for the transmit operation to complete. We can take a look at the data received back with read() or read_nowait()

# TX/RX transaction example
spi_master.write_nowait(0xFFFF)
await spi_master.wait()
read_bytes = await spi_master.read()
print(read_bytes)

# we can alternatively call (which has equivalent functionality)
await spi_master.write(0xFFFF)
read_bytes = await spi_masetr.read()

Constructor Parameters

• signals: SpiSignal
• config: SpiConfig

Methods

• write(data): send data (blocking)
• write_nowait(data): send data (non-blocking)
• read(count=-1): read count bytes from buffer, reading whole buffer by default (blocking)
• read_nowait(count=-1): read count bytes from buffer, reading whole buffer by default (non-blocking)
• count_tx(): returns the number of items in the transmit queue
• count_rx(): returns the number of items in the receive queue
• empty_tx(): returns True if the transmit queue is empty
• empty_rx(): returns True if the receive queue is empty
• idle(): returns True if the transmit and receive buffers are empty
• clear(): drop all data in the queue

SPI Slave

The SpiSlaveBase acts as an abstract class for a SPI Slave Endpoint.

To use this class, import it and inherit it. Then use the subclass as the slave and connect it to the dut.

from cocotbext.spi import SpiMaster, SpiSignals, SpiConfig

class SimpleSpiSlave(SpiSlaveBase):
    def __init__(self, signals):
        self._config = SpiConfig()
        self.content = 0
        super().__init__(signals)

    async def get_content(self):
        await self.idle.wait()
        return self.content

    async def _transaction(self, frame_start, frame_end):
        await frame_start
        self.idle.clear()

        self.content = int(await self._shift(16, tx_word=(0xAAAA)))

        await frame_end

spi_signals = SpiSignals(
    sclk = dut.sclk,
    mosi = dut.mosi,
    miso = dut.miso,
    cs   = dut.ncs
)
spi_slave = SimpleSpiSlave(spi_signals)

Implementation

All SPI Slave Classes should:

• inherit the SpiSlaveBase class
• define self._config adjust the values for:
  • word_width
  • cpha
  • cpol
  • msb_first
  • frame_spacing_ns
• implement a _transaction coroutine
  • the coroutine should take 3 arguments, self, frame_start and frame_end
  • the coroutine should await frame_start at the transaction start, and frame_end when done.
    • frame_start and frame_end are Rising and Falling edges of the chip select based on the chip select polarity
  • when the coroutine receives a frame_start signal, it should clear the self.idle Event.
    • self.idle is automatically set when _transaction returns
• when implementing a method to read the class contents, make sure to await the self.idle, otherwise the data may not be up to date because the device is in the middle of a transaction.

Simulated Devices

This framework includes some SPI Slave devices built in. A list of supported devices can be found in cocotbext/spi/devices and are sorted by vendor.

To use these devices, you can simply import them.

from cocotbext.spi.devices.TI import DRV8306

spi_signals = SpiSignals(
    sclk = dut.sclk,
    mosi = dut.mosi,
    miso = dut.miso,
    cs   = dut.ncs
)
spi_slave = DRV8306(spi_signals)

To submit a new device, make a pull request.