class SPI – a controller-driven serial protocol¶
SPI is a serial protocol that is driven by a controller. At the physical level there are 3 lines: SCK, MOSI, MISO.
See usage model of I2C; SPI is very similar. Main difference is parameters to init the SPI bus:
from pyb import SPI
spi = SPI(1, SPI.CONTROLLER, baudrate=600000, polarity=1, phase=0, crc=0x7)
Only required parameter is mode, SPI.CONTROLLER or SPI.PERIPHERAL. Polarity can be 0 or 1, and is the level the idle clock line sits at. Phase can be 0 or 1 to sample data on the first or second clock edge respectively. Crc can be None for no CRC, or a polynomial specifier.
Additional methods for SPI:
data = spi.send_recv(b'1234') # send 4 bytes and receive 4 bytes
buf = bytearray(4)
spi.send_recv(b'1234', buf) # send 4 bytes and receive 4 into buf
spi.send_recv(buf, buf) # send/recv 4 bytes from/to buf
Constructors¶
- class pyb.SPI(bus, ...)¶
Construct an SPI object on the given bus.
bus
can be 1 or 2, or ‘X’ or ‘Y’. With no additional parameters, the SPI object is created but not initialised (it has the settings from the last initialisation of the bus, if any). If extra arguments are given, the bus is initialised. Seeinit
for parameters of initialisation.The physical pins of the SPI buses are:
SPI(1)
is on the X position:(NSS, SCK, MISO, MOSI) = (X5, X6, X7, X8) = (PA4, PA5, PA6, PA7)
SPI(2)
is on the Y position:(NSS, SCK, MISO, MOSI) = (Y5, Y6, Y7, Y8) = (PB12, PB13, PB14, PB15)
At the moment, the NSS pin is not used by the SPI driver and is free for other use.
Methods¶
- SPI.deinit()¶
Turn off the SPI bus.
- SPI.init(mode, baudrate=328125, *, prescaler, polarity=1, phase=0, bits=8, firstbit=SPI.MSB, ti=False, crc=None)¶
Initialise the SPI bus with the given parameters:
mode
must be eitherSPI.CONTROLLER
orSPI.PERIPHERAL
.baudrate
is the SCK clock rate (only sensible for a controller).prescaler
is the prescaler to use to derive SCK from the APB bus frequency; use ofprescaler
overridesbaudrate
.polarity
can be 0 or 1, and is the level the idle clock line sits at.phase
can be 0 or 1 to sample data on the first or second clock edge respectively.bits
can be 8 or 16, and is the number of bits in each transferred word.firstbit
can beSPI.MSB
orSPI.LSB
.ti
True indicates Texas Instruments, as opposed to Motorola, signal conventions.crc
can be None for no CRC, or a polynomial specifier.
Note that the SPI clock frequency will not always be the requested baudrate. The hardware only supports baudrates that are the APB bus frequency (see
pyb.freq()
) divided by a prescaler, which can be 2, 4, 8, 16, 32, 64, 128 or 256. SPI(1) is on AHB2, and SPI(2) is on AHB1. For precise control over the SPI clock frequency, specifyprescaler
instead ofbaudrate
.Printing the SPI object will show you the computed baudrate and the chosen prescaler.
- SPI.recv(recv, *, timeout=5000)¶
Receive data on the bus:
recv
can be an integer, which is the number of bytes to receive, or a mutable buffer, which will be filled with received bytes.timeout
is the timeout in milliseconds to wait for the receive.
Return value: if
recv
is an integer then a new buffer of the bytes received, otherwise the same buffer that was passed in torecv
.
- SPI.send(send, *, timeout=5000)¶
Send data on the bus:
send
is the data to send (an integer to send, or a buffer object).timeout
is the timeout in milliseconds to wait for the send.
Return value:
None
.
- SPI.send_recv(send, recv=None, *, timeout=5000)¶
Send and receive data on the bus at the same time:
send
is the data to send (an integer to send, or a buffer object).recv
is a mutable buffer which will be filled with received bytes. It can be the same assend
, or omitted. If omitted, a new buffer will be created.timeout
is the timeout in milliseconds to wait for the receive.
Return value: the buffer with the received bytes.