Serial Peripheral Interface


The Serial Peripheral Interface is a synchronous serial communication interface specification used for short-distance communication, primarily in embedded systems. The interface was developed by Motorola in the mid-1980s and has become a de facto standard. Typical applications include Secure Digital cards and liquid crystal displays.
SPI devices communicate in full duplex mode using a master-slave architecture with a single master. The master device originates the frame for reading and writing. Multiple slave-devices are supported through selection with individual slave select, sometimes called chip select, lines.
Sometimes SPI is called a four-wire serial bus, contrasting with [|three-], two-, and one-wire serial buses. The SPI may be accurately described as a synchronous serial interface, but it is different from the Synchronous Serial Interface protocol, which is also a four-wire synchronous serial communication protocol. The SSI protocol employs differential signaling and provides only a single simplex communication channel. SPI is one master and multi slave communication.

Interface

The SPI bus specifies four logic signals:
MOSI on a master connects to MOSI on a slave. MISO on a master connects to MISO on a slave. Slave Select is the same functionality as chip select and is used instead of an addressing concept. Pin names are always capitalized as in Slave Select, Serial Clock, and Master Output Slave Input.
These signal names can be used to label both the master and slave device pins plus the signal lines between them in an unambiguous way. While they are the most popular on new products, older products can have nonstandard SPI port pin-names:
Serial Clock:
Master Output → Slave Input :
Master Input ← Slave Output :
Slave Select:
The SPI bus can operate with a single master device and with one or more slave devices.
If a single slave device is used, the SS pin may be fixed to logic low if the slave permits it. Some slaves require a falling edge of the chip select signal to initiate an action. An example is the Maxim MAX1242 ADC, which starts conversion on a high→low transition. With multiple slave devices, an independent SS signal is required from the master for each slave device.
Most slave devices have tri-state outputs so their MISO signal becomes high impedance when the device is not selected. Devices without tri-state outputs cannot share SPI bus segments with other devices without using an external tri-state buffer.

Data transmission

To begin communication, the bus master configures the clock, using a frequency supported by the slave device, typically up to a few MHz. The master then selects the slave device with a logic level 0 on the select line. If a waiting period is required, such as for an analog-to-digital conversion, the master must wait for at least that period of time before issuing clock cycles.
During each SPI clock cycle, a full-duplex data transmission occurs. The master sends a bit on the MOSI line and the slave reads it, while the slave sends a bit on the MISO line and the master reads it. This sequence is maintained even when only one-directional data transfer is intended.
Transmissions normally involve two shift registers of some given word-size, such as eight bits, one in the master and one in the slave; they are connected in a virtual ring topology. Data is usually shifted out with the most significant bit first. On the clock edge, both master and slave shift out a bit and output it on the transmission line to the counterpart. On the next clock edge, at each receiver the bit is sampled from the transmission line and set as a new least-significant bit of the shift register. After the register bits have been shifted out and in, the master and slave have exchanged register values. If more data needs to be exchanged, the shift registers are reloaded and the process repeats. Transmission may continue for any number of clock cycles. When complete, the master stops toggling the clock signal, and typically deselects the slave.
Transmissions often consist of eight-bit words. However, other word-sizes are also common, for example, sixteen-bit words for touch-screen controllers or audio codecs, such as the TSC2101 by Texas Instruments, or twelve-bit words for many digital-to-analog or analog-to-digital converters.
Every slave on the bus that has not been activated using its chip select line must disregard the input clock and MOSI signals and should not drive MISO although some devices need external tristate buffers to implement this.

Clock polarity and phase

In addition to setting the clock frequency, the master must also configure the clock polarity and phase with respect to the data. Motorola SPI Block Guide names these two options as CPOL and CPHA respectively, a convention most vendors have also adopted.
The timing diagram is shown to the right. The timing is further described below and applies to both the master and the slave device.
The MOSI and MISO signals are usually stable for the half cycle until the next clock transition. SPI master and slave devices may well sample data at different points in that half cycle.
This adds more flexibility to the communication channel between the master and slave.

Mode numbers

The combinations of polarity and phases are often referred to as modes which are commonly numbered according to the following convention, with CPOL as the high order bit and CPHA as the low order bit:
For "Microchip PIC" / "ARM-based" microcontrollers :
SPI modeClock polarity
Clock phase
Clock edge
0001
1010
2101
3110

For PIC32MX: SPI mode configure CKP, CKE and SMP bits. Set SMP bit and CKP, CKE two bits configured as above table.
For other microcontrollers:
ModeCPOLCPHA
000
101
210
311

Another commonly used notation represents the mode as a tuple; e.g., the value '' would indicate CPOL=0 and CPHA=1.
Note that in Full Duplex operation, the Master device could transmit and receive with different modes. For instance, it could transmit in Mode 0 and be receiving in Mode 1 at the same time.

Independent slave configuration

In the independent slave configuration, there is an independent chip select line for each slave. This is the way SPI is normally used. The master asserts only one chip select at a time.
Pull-up resistors between power source and chip select lines are recommended for systems where the master's chip select pins may default to an undefined state. When separate software routines initialize each chip select and communicate with its slave, pull-up resistors prevent other uninitialized slaves from responding.
Since the MISO pins of the slaves are connected together, they are required to be tri-state pins, where the high-impedance output must be applied when the slave is not selected. Slave devices not supporting tri-state may be used in independent slave configuration by adding a tri-state buffer chip controlled by the chip select signal.

Daisy chain configuration

Some products that implement SPI may be connected in a daisy chain configuration, the first slave output being connected to the second slave input, etc. The SPI port of each slave is designed to send out during the second group of clock pulses an exact copy of the data it received during the first group of clock pulses. The whole chain acts as a communication shift register; daisy chaining is often done with shift registers to provide a bank of inputs or outputs through SPI. Each slave copies input to output in the next clock cycle until active low SS line goes high. Such a feature only requires a single SS line from the master, rather than a separate SS line for each slave.
Other applications that can potentially interoperate with SPI that require a daisy chain configuration include SGPIO, JTAG, and Two Wire Interface.

Valid communications

Some slave devices are designed to ignore any SPI communications in which the number of clock pulses is greater than specified. Others do not care, ignoring extra inputs and continuing to shift the same output bit. It is common for different devices to use SPI communications with different lengths, as, for example, when SPI is used to access the scan chain of a digital IC by issuing a command word of one size and then getting a response of a different size.

Interrupts

SPI devices sometimes use another signal line to send an interrupt signal to a host CPU. Examples include pen-down interrupts from touchscreen sensors, thermal limit alerts from temperature sensors, alarms issued by real time clock chips, SDIO, and headset jack insertions from the sound codec in a cell phone. Interrupts are not covered by the SPI standard; their usage is neither forbidden nor specified by the standard. In other words, interrupts are outside the scope of the SPI standard and are optionally implemented independently from it.

Example of bit-banging the master protocol

Below is an example of bit-banging the SPI protocol as an SPI master with CPOL=0, CPHA=0, and eight bits per transfer. The example is written in the C programming language. Because this is CPOL=0 the clock must be pulled low before the chip select is activated. The chip select line must be activated, which normally means being toggled low, for the peripheral before the start of the transfer, and then deactivated afterward. Most peripherals allow or require several transfers while the select line is low; this routine might be called several times before deselecting the chip.

/*
* Simultaneously transmit and receive a byte on the SPI.
*
* Polarity and phase are assumed to be both 0, i.e.:
* - input data is captured on rising edge of SCLK.
* - output data is propagated on falling edge of SCLK.
*
* Returns the received byte.
*/
uint8_t SPI_transfer_byte

Pros and cons

Advantages

The board real estate savings compared to a parallel I/O bus are significant, and have earned SPI a solid role in embedded systems. That is true for most system-on-a-chip processors, both with higher end 32-bit processors such as those using ARM, MIPS, or PowerPC and with other microcontrollers such as the AVR, PIC, and MSP430. These chips usually include SPI controllers capable of running in either master or slave mode. In-system programmable AVR controllers can be programmed using a SPI interface.
Chip or FPGA based designs sometimes use SPI to communicate between internal components; on-chip real estate can be as costly as its on-board cousin.
The full-duplex capability makes SPI very simple and efficient for single master/single slave applications. Some devices use the full-duplex mode to implement an efficient, swift data stream for applications such as digital audio, digital signal processing, or telecommunications channels, but most off-the-shelf chips stick to half-duplex request/response protocols.
SPI is used to talk to a variety of peripherals, such as
For high-performance systems, FPGAs sometimes use SPI to interface as a slave to a host, as a master to sensors, or for flash memory used to bootstrap if they are SRAM-based.
Although there are some similarities between the SPI bus and the JTAG protocol, they are not interchangeable. The SPI bus is intended for high speed, on board initialization of device peripherals, while the JTAG protocol is intended to provide reliable test access to the I/O pins from an off board controller with less precise signal delay and skew parameters. While not strictly a level sensitive interface, the JTAG protocol supports the recovery of both setup and hold violations between JTAG devices by reducing the clock rate or changing the clock's duty cycles. Consequently, the JTAG interface is not intended to support extremely high data rates.
SGPIO is essentially another application stack for SPI designed for particular backplane management activities. SGPIO uses 3-bit messages.

Standards

The SPI bus is a de facto standard. However, the lack of a formal standard is reflected in a wide variety of protocol options. Different word sizes are common. Every device defines its own protocol, including whether it supports commands at all. Some devices are transmit-only; others are receive-only. Chip selects are sometimes active-high rather than active-low. Some protocols send the least significant bit first.
Some devices even have minor variances from the CPOL/CPHA modes described above. Sending data from slave to master may use the opposite clock edge as master to slave. Devices often require extra clock idle time before the first clock or after the last one, or between a command and its response. Some devices have two clocks, one to read data, and another to transmit it into the device. Many of the read clocks run from the chip select line.
Some devices require an additional flow control signal from slave to master, indicating when data is ready. This leads to a 5-wire protocol instead of the usual 4. Such a ready or enable signal is often active-low, and needs to be enabled at key points such as after commands or between words. Without such a signal, data transfer rates may need to be slowed down significantly, or protocols may need to have dummy bytes inserted, to accommodate the worst case for the slave response time. Examples include initiating an ADC conversion, addressing the right page of flash memory, and processing enough of a command that device firmware can load the first word of the response.
Many SPI chips only support messages that are multiples of 8 bits. Such chips can not interoperate with the JTAG or SGPIO protocols, or any other protocol that requires messages that are not multiples of 8 bits.
There are also hardware-level differences. Some chips combine MOSI and MISO into a single data line ; this is sometimes called 'three-wire' signaling. Another variation of SPI removes the chip select line, managing protocol state machine entry/exit using other methods. Anyone needing an external connector for SPI defines their own: UEXT, JTAG connector, Secure Digital card socket, etc. Signal levels depend entirely on the chips involved.
SafeSPI is an industry standard for SPI in automotive applications. Its main focus is the transmission of sensor data between different devices.

Development tools

When developing or troubleshooting systems using SPI, visibility at the level of hardware signals can be important.

Host adapters

There are a number of USB hardware solutions to provide computers, running Linux, Mac, or Windows, SPI master or slave capabilities. Many of them also provide scripting or programming capabilities.
An SPI host adapter lets the user play the role of a master on an SPI bus directly from a PC. They are used for embedded systems, chips and peripheral testing, programming and debugging.
The key parameters of SPI are: the maximum supported frequency for the serial interface, command-to-command latency and the maximum length for SPI commands. It is possible to find SPI adapters on the market today that support up to 100 MHz serial interfaces, with virtually unlimited access length.
SPI protocol being a de facto standard, some SPI host adapters also have the ability of supporting other protocols beyond the traditional 4-wire SPI.

Protocol analyzers

SPI protocol analyzers are tools which sample an SPI bus and decode the electrical signals to provide a higher-level view of the data being transmitted on a specific bus.

Oscilloscopes

Most oscilloscope vendors offer oscilloscope-based triggering and protocol decoding for SPI. Most support 2-, 3-, and 4-wire SPI. The triggering and decoding capability is typically offered as an optional extra. SPI signals can be accessed via analog oscilloscope channels or with digital MSO channels.

Logic analyzers

When developing or troubleshooting the SPI bus, examination of hardware signals can be very important. Logic analyzers are tools which collect, analyze, decode, and store signals so people can view the high-speed waveforms at their leisure. Logic analyzers display time-stamps of each signal level change, which can help find protocol problems. Most logic analyzers have the capability to decode bus signals into high-level protocol data and show ASCII data.

Related terms

Intelligent SPI controllers

A Queued Serial Peripheral Interface is a type of SPI controller that uses a data queue to transfer data across the SPI bus. It has a wrap-around mode allowing continuous transfers to and from the queue with only intermittent attention from the CPU. Consequently, the peripherals appear to the CPU as memory-mapped parallel devices. This feature is useful in applications such as control of an A/D converter. Other programmable features in QSPI are chip selects and transfer length/delay.
SPI controllers from different vendors support different feature sets; such DMA queues are not uncommon, although they may be associated with separate DMA engines rather than the SPI controller itself, such as used by Multichannel Buffered Serial Port. Most SPI master controllers integrate support for up to four chip selects, although some require chip selects to be managed separately through GPIO lines.

Microwire

Microwire, often spelled μWire, is essentially a predecessor of SPI and a trademark of National Semiconductor. It's a strict subset of SPI: half-duplex, and using SPI mode 0. Microwire chips tend to need slower clock rates than newer SPI versions; perhaps 2 MHz vs. 20 MHz. Some Microwire chips also support a [|three-wire mode].

Microwire/Plus

Microwire/Plus is an enhancement of Microwire and features full-duplex communication and support for SPI modes 0 and 1. There was no specified improvement in serial clock speed.

Three-wire serial buses

As mentioned, one variant of SPI uses single bidirectional data line instead of two unidirectional ones. This variant is restricted to a half duplex mode. It tends to be used for lower performance parts, such as small EEPROMs used only during system startup and certain sensors, and Microwire. Few SPI master controllers support this mode; although it can often be easily bit-banged in software.

Dual SPI

For instances where the full-duplex nature of SPI is not used, an extension uses both data pins in a half-duplex configuration to send two bits per clock cycle. Typically a command byte is sent requesting a response in dual mode, after which the MOSI line becomes SIO0 and carries even bits, while the MISO line becomes SIO1 and carries odd bits. Data is still transmitted msbit-first, but SIO1 carries bits 7, 5, 3 and 1 of each byte, while SIO0 carries bits 6, 4, 2 and 0.
This is particularly popular among SPI ROMs, which have to send a large amount of data, and comes in two variants:
While dual SPI re-uses the existing serial I/O lines, quad SPI adds two more I/O lines and sends 4 data bits per clock cycle. Again, it is requested by special commands, which enable quad mode after the command itself is sent in single mode.
SQI Type 1: Commands sent on single line but addresses and data sent on four lines
SQI Type 2: Commands and addresses sent on a single line but data sent/received on four lines

QPI/SQI

Further extending quad SPI, some devices support a "quad everything" mode where all communication takes place over 4 data lines, including commands. This is variously called "QPI" or "serial quad I/O"
This requires programming a configuration bit in the device and requires care after reset to establish communication.

Double data rate

In addition to using multiple lines for I/O, some devices increase the transfer rate by using double data rate transmission.

Intel Enhanced Serial Peripheral Interface Bus

Intel has developed a successor to its Low Pin Count bus that it calls the Enhanced Serial Peripheral Interface Bus, or eSPI for short. Intel aims to allow the reduction in the number of pins required on motherboards compared to systems using LPC, have more available throughput than LPC, reduce the working voltage to 1.8 volts to facilitate smaller chip manufacturing processes, allow eSPI peripherals to share SPI flash devices with the host, tunnel previous out-of-band pins through the eSPI bus, and allow system designers to trade off cost and performance.
The eSPI bus can either be shared with SPI devices to save pins or be separate from the SPI bus to allow more performance, especially when eSPI devices need to use SPI flash devices.
This standard defines an Alert# signal that is used by an eSPI slave to request service from the master. In a performance-oriented design or a design with only one eSPI slave, each eSPI slave will have its Alert# pin connected to an Alert# pin on the eSPI master that is dedicated to each slave, allowing the eSPI master to grant low-latency service because the eSPI master will know which eSPI slave needs service and will not need to poll all of the slaves to determine which device needs service. In a budget design with more than one eSPI slave, all of the Alert# pins of the slaves are connected to one Alert# pin on the eSPI master in a wired-OR connection, which will require the master to poll all the slaves to determine which ones need service when the Alert# signal is pulled low by one or more peripherals that need service. Only after all of the devices are serviced will the Alert# signal be pulled high due to none of the eSPI slaves needing service and therefore pulling the Alert# signal low.
This standard allows designers to use 1-bit, 2-bit, or 4-bit communications at speeds from 20 to 66 MHz to further allow designers to trade off performance and cost.
All communications that were out-of-band of the LPC bus like general-purpose input/output and System Management Bus are tunneled through the eSPI bus via virtual wire cycles and out-of-band message cycles respectively in order to remove those pins from motherboard designs using eSPI.
This standard supports standard memory cycles with lengths of 1 byte to 4 kilobytes of data, short memory cycles with lengths of 1, 2, or 4 bytes that have much less overhead compared to standard memory cycles, and I/O cycles with lengths of 1, 2, or 4 bytes of data which are low overhead as well. This significantly reduces overhead compared to the LPC bus, where all cycles except for the 128-byte firmware hub read cycle spends more than one-half of all of the bus's throughput and time in overhead. The standard memory cycle allows a length of anywhere from 1 byte to 4 kilobytes in order to allow its larger overhead to be amortised over a large transaction. eSPI slaves are allowed to initiate bus master versions of all of the memory cycles. Bus master I/O cycles, which were introduced by the LPC bus specification, and ISA-style DMA including the 32-bit variant introduced by the LPC bus specification, are not present in eSPI. Therefore, bus master memory cycles are the only allowed DMA in this standard.
eSPI slaves are allowed to use the eSPI master as a proxy to perform flash operations on a standard SPI flash memory slave on behalf of the requesting eSPI slave.
64-bit memory addressing is also added, but is only permitted when there is no equivalent 32-bit address.
The Intel Z170 chipset can be configured to implement either this bus or a variant of the LPC bus that is missing its ISA-style DMA capability and is underclocked to 24 MHz instead of the standard 33 MHz.