Serial Peripheral Interface (SPI) is a synchronous serial data protocol used by microcontrollers for communicating with one or more peripheral devices quickly over short distances. It can also be used for communication between two microcontrollers.
With an SPI connection, there is always one master device (usually a microcontroller) which controls the peripheral devices. Typically there are three lines common to all the devices:
- MISO (Master In Slave Out) – The Slave line for sending data to the master,
- MOSI (Master Out Slave In) – The Master line for sending data to the peripherals,
- SCK (Serial Clock) – The clock pulses which synchronize data transmission generated by the master
and one line specific for every device:
- SS (Slave Select) – the pin on each device that the master can use to enable and disable specific devices.
When a device’s Slave Select pin is low, it communicates with the master. When it’s high, it ignores the master. This allows you to have multiple SPI devices sharing the same MISO, MOSI, and CLK lines.
Coding in Arduino IDE
To write code for a new SPI device you need to note a few things:
- What is the maximum SPI speed your device can use? This is controlled by the first parameter in SPISettings. If you are using a chip rated at 15 MHz, use 15000000. Arduino will automatically use the best speed that is equal to or less than the number you use with SPISettings.
- Is data shifted in Most Significant Bit (MSB) or Least Significant Bit (LSB) first? This is controlled by a second SPI Settings parameter, either MSBFIRST or LSBFIRST. Most SPI chips use MSB first data order.
- Is the data clock idle when high or low? Are samples on the rising or falling edge of clock pulses? These modes are controlled by the third parameter in SPISettings.
The SPI standard is loose and each device implements it a little differently. This means you have to pay special attention to the device’s datasheet when writing your code.
Mode of Transmission
Generally speaking, there are four modes of transmission. These modes control whether data is shifted in and out on the rising or falling edge of the data clock signal (called the clock phase), and whether the clock is idle when high or low (called the clock polarity). The four modes combine polarity and phase according to this table:
|Mode||Clock Polarity (CPOL)||Clock Phase (CPHA)||Output Edge||Data Capture|
Once you have your SPI parameters, use SPI.beginTransaction() to begin using the SPI port. The SPI port will be configured with all of your settings. The simplest and most efficient way to use SPISettings is directly inside SPI.beginTransaction(). For example:
SPI.beginTransaction(SPISettings(14000000, MSBFIRST, SPI_MODE0));
If other libraries use SPI from interrupts, they will be prevented from accessing SPI until you call
SPI.endTransaction(). The SPI settings are applied at the begin of the transaction and
SPI.endTransaction() don’t change SPI settings. Unless you, or some library, call beginTransaction a second time, the setting is maintained. You should attempt to minimize the time between before you call
SPI.endTransaction(), for best compatibility if your program is used together with other libraries which use SPI.
With most SPI devices, after
SPI.beginTransaction(), you will write the slave select pin LOW, call
SPI.transfer() any number of times to transfer data, then write the SS pin HIGH, and finally call
- MOSI: Digital Pin 51
- MISO: Digital Pin 50
- SCK: Digital Pin 52
- SS (Slave): Digital Pin 53
- SS (Master): –
- Level: 5V