SPI — `pymcu.hal.spi`#

from pymcu.hal.spi import SPI

SPI bus communication in master mode. Available for AVR (ATmega328P).

class `SPI`#

`SPI(mode=0, cs="", baudrate=4000000, polarity=0, phase=0, lsb_first=0)`#

Initializes the hardware SPI peripheral. mode is 0 for controller and 1 for peripheral; cs is an optional chip-select pin, auto-asserted on select().

baudrate, polarity, phase and lsb_first are the clock and the frame. polarity and phase are the two bits that name the SPI mode (mode 0 is 0, 0; mode 3 is 1, 1), and anything but 0 or 1 for either is refused where the bus is constructed. The defaults are what the literal control-register value used to be: mode 0, MSB first, fosc/4, which is 4 MHz at a 16 MHz clock, so a bus that asks for nothing emits the same two register writes it always did.

They used to be nowhere, so busio.SPI.configure() recorded a baudrate, a polarity and a phase and reprogrammed none of them: a display asked for mode 3 at 8 MHz ran mode 0 at 4 MHz, silently.

configure(...) reprograms a bus that is already running; the pin directions are already set, so only the two control registers are written.

frequency() returns the bit rate the hardware actually produces. The dividers are powers of two – 2, 4, 8, 16, 32, 64, 128 – and the chosen one never exceeds the request, because a peripheral rated for 1 MHz must not be clocked at 2. Asking for 3 MHz at a 16 MHz clock gets 2 MHz, and reporting 3 MHz back would be a number the pin never carried.

Pinout (ATmega328P / Arduino Uno):

Pin	Arduino	Function
`PB5`	D13	SCK
`PB4`	D12	MISO
`PB3`	D11	MOSI
`PB2`	D10	SS / CS

Methods#

Method	Description
`select()`	Drive SS low (begin transaction)
`deselect()`	Drive SS high (end transaction)
`transfer(data: uint8) -> uint8`	Full-duplex byte exchange
`write(data: uint8)`	Send byte (discard received byte)
`__enter__()`	Alias for `select()` — called by `with spi:`
`__exit__()`	Alias for `deselect()` — called by `with spi:`

Examples#

Context manager (recommended)#

from pymcu.hal.spi import SPI

spi = SPI()

with spi:
    spi.write(0xAB)
    b = spi.transfer(0x00)

with spi: calls select() on enter and deselect() on exit, even if the block returns early.

74HC595 shift register#

from pymcu.hal.spi import SPI
from pymcu.time import delay_ms
from pymcu.types import uint8

def main():
    spi = SPI()
    data: uint8 = 0x01

    while True:
        with spi:
            spi.write(data)
        data = (data << 1) | (data >> 7)   # rotate left
        delay_ms(100)

SoftSPI (bit-bang)#

Use SoftSPI for arbitrary GPIO pins (no hardware SPI constraint):

from pymcu.hal.softspi import SoftSPI
from pymcu.hal.avr.gpio import Pin
from pymcu.types import uint8

spi = SoftSPI(sck=Pin("PD4", Pin.OUT), mosi=Pin("PD5", Pin.OUT),
              miso=Pin("PD6", Pin.IN), cs=Pin("PD7", Pin.OUT))

with spi:
    b: uint8 = spi.transfer(0x55)

SPI — pymcu.hal.spi#

class SPI#

SPI(mode=0, cs="", baudrate=4000000, polarity=0, phase=0, lsb_first=0)#