SPI is a serial bus standard established by Motorola and supported in silicon products from various manufacturers. SPI interfaces are available on popular communication processors. SPI has been a widely used serial interface for many peripheral components such as Flash memory, EEPROM, data acquisition sensors, LCD display, etc. SPI is a synchronous serial data link that operates in full duplex (signals carrying data go in both directions simultaneously).

Overview of SPI Interface

The interconnection between two SPI devices always happens between a master device and a slave device. Compared to some peripheral devices like sensors which can only run in slave mode The master is the active part in this system and has to provide the clock signal a serial data transmission is based on. The slave is not capable of generating the clock signal and thus can not get active on its own. The slave just sends and receives data if the master generates the necessary clock signal. The master however generates the clock signal only while sending data. That means that the master has to send data to the slave to read data from the slave. The slave just sends and receives data if the master generates the necessary clock signal. The master however generates the clock signal only while sending data. That means that the master has to send data to the slave to read data from the slave.

Devices communicate using a master/slave relationship, in which the master initiates the data frame. When the master generates a clock and selects a slave device, data may be transferred in either or both directions simultaneously. In fact, as far as SPI is concerned, data are always transferred in both directions. It is up to the master and slave devices to know whether a received byte is meaningful or not. So a device must discard the received byte in a “transmit only” frame or generate a dummy byte for a “receive only” frame.

Input/Output Signals (Pins) of the Serial Peripheral Interface SPI

SPI specifies four signals: clock (SCLK); master data output, slave data input (MOSI); master data input, slave data output (MISO); and slave select (/SS). Figure 1 shows these signals in a single-slave configuration. SCLK is generated by the master and input to all slaves. MOSI carries data from master to slave. MISO carries data from slave back to master. A slave device is selected when the master asserts its /SS signal.

MOSI – Master Output Slave Input
MISO – Master Input Slave Output
SCLK – Clock signal from master to slave
SS – Slave select from master to slave

Multi Slave Systems – /SS pin Functionality

If multiple slave devices exist, the master generates a separate slave select signal for each slave.

The master generates slave select signals using general-purpose discrete input/output pins or other logic. This consists of old-fashioned bit banging and can be pretty sensitive. You have to time it relative to the other signals and ensure, for example, that you don’t toggle a select line in the middle of a frame.

While SPI doesn’t describe a specific way to implement multi-master systems, some SPI devices support additional signals that make such implementations possible. However, it’s complicated and usually unnecessary, so it’s not often done.

A pair of parameters called clock polarity (CPOL) and clock phase (CPHA) determine the edges of the clock signal on which the data are driven and sampled. Each of the two parameters has two possible states, which allows for four possible combinations, all of which are incompatible with one another. So a master/slave pair must use the same parameter pair values to communicate. If multiple slaves are used that are fixed in different configurations, the master will have to reconfigure itself each time it needs to communicate with a different slave.

The ability to connect several devices to the same SPI-bus is based on the fact that only one master and only one slave is active at the same time. The MISO, MOSI and SCK lines of all the other slaves are tri-stated (configured as input pins of a high impedance with no pull up resistors enabled). A false implementation (e.g. if two slaves are activated at the same time) can cause a driver contention which can lead to a CMOS latch up state and must be avoided. Resistances of 1 to 10 k ohms in series with the pins of the SPI can be used to prevent the system from latching up. However this affects the maximum usable data rate, depending on the loading capacitance on the SPI pins.

Serial Peripheral Interface SPI at a Higher Level

SPI does not have an acknowledgment mechanism to confirm receipt of data. In fact, without a communication protocol, the SPI master has no knowledge of whether a slave even exists. SPI also offers no flow control. If you need hardware flow control, you might need to do something outside of SPI.

Slaves can be thought of as input/output devices of the master. SPI does not specify a particular higher-level protocol for master-slave dialog. In some applications, a higher-level protocol is not needed and only raw data are exchanged. An example of this is an interface to a simple codec. In other applications, a higher-level protocol, such as a command-response protocol, may be necessary. Note that the master must initiate the frames for both its command and the slave’s response.