SPI (Serial Peripheral Interface)

2025-06-30 embedded · basic · sp

A technical introduction to the SPI - low-speed four-wire master-slave full-duplex serial protocol.
SPI is widely used as a faster alternative to UART/I2C for transferring data between an intelligent controller and a less intelligent peripheral device.

SPI

SPI - Serial Peripheral Interface A protocol in which there is one leader (master or controller) and one or more slaves (or peripheral device)
Usually operates in full-duplex (both nodes transmit data simultaneously)
Message exchange occurs in frames. SPI has no agreement on frame formats, number of transmitted bits and voltage levels

SPI Design

the CS(Chip Select)/SS(Slave Celect) channel is used to select the slave
clock signals on the SCLK line are used for synchronization
MOSI (Master Out Slave In) data is transmitted from the master to the slave
MISO (Master In Slave Out) data is transmitted from the slave to the master

switch the SC line low(0), indicating that information will be transferred to this slave
starts the SCLK signal
transfer occurs on (MOSI / MISO)
switch CS back to high(1)

SCLK

after CS is set to low(0), the master starts generating a clock signal - this allows the slave to not have a clock signal, and used master clock
the clock is usually in Mhz
the clock in this case tells the slave at what moment to start reading the voltage level on the data line.

The clock signal in SPI by default idle can be in a low(0) or high(1) state, and data can be read on the rising or falling edge of the signal.

MOSI

Used to send data from a master to a slave After CS is pulled low(0) and SCLK is enabled, MOSI is used to transfer data from the master to the slave as an 8-bit bytes, where the first bit can be the least significant bit (LSB, MSB).
Multiple bytes can be transferred while CS is low(0).

MISO

Used to send data from a slave to a master

Often used to respond to data received from MOSI.
Since only the master can hold the clock, it must know exactly how many bytes the slave wanted to send over MISO.
This can be a fixed number, or a specified number that the master will respond to the slave’s request with.

SPI Adjustment

Clock Polarity

CPOL - the clock signal generated by the master can be:

low(0) at idle, active at high(1) level (CPOL = 0. “non-inverted”) (Leading (first edge)- rising)
high(1) at idle, active at low(0) level (CPOL = 1. “inverted”) (Leading (first edge)- falling) CPOL indicates which of these options is used

For both types of clock pulse polarity, you can focus on

leading edge of the clock pulse (Leading (first edge))
trailing edge of the pulse (Trailing (second)edge)

Clock Phase

CPHA - determines on which edge the nodes will read data

CPHA - 0 means that data will be read on the rising edge of each clock signal
CPHA - 1 means that data will be read on the falling edge of each clock signal

SPI Mode

The operation of the master and slave must be coordinated via the SPI operating mode.
The mode is an integer combination of CPOL and CPHA

Mode	CPOL	CPHA
0	0	0
1	0	1
2	1	0
3	1	1

Multi-slave configuratuion

Independent Slaves

In this configuration, the master has a separate CS line for each slave.
The remaining channels are shared on the bus.
The method is relatively simple until there are many slaves.

Cooperative Slaves / Daisy Chain

With this configuration, the master has only one CS line, which is used by all slave devices.
With this configuration, data on MOSI is always sent to the first slave, and data transfer to another slave occurs through MISO of the first slave that received data on MOSI, and so on to transfer data to the next slave.
If a slave wants to send data to the master through its MISO channel, it also sends it to the next slave in the chain, and so on until it reaches the last one, whose MISO channel is not connected to the next slave, but is connected to the master.
The cooperation mode reduces the required number of lines, but it is more complex and may not be supported by some devices.