SPI interface Tutorial. The SPI Interface (Serial Peripheral Interface) bus is a high speed, 3-wire, serial communications protocol (4 if you include SSn - see below). Its primiary purpose is to reduce on-PCB wire routing by replacing the traditional parallel bus with a serial interface. (You can just about manage an 8 bit bus routing it through a several layer PCB but when you get to 16, 32 bits and more it gets far more difficult). The connections are:. MOSI (Master Out Slave In) SDO. MISO (Master In Slave Out) SDI.

• How To Program Spi Flash

Serial Peripheral Interface (SPI) SPI “Gotchas” “Now my board won’t program.” SPI shares SCK with programming interface. If it won’t program anymore.

SCK (Slave Clock) SCK. SSn (Slave Select). - denotes PIC nomenclature Note: The last signal SS or slave select is separate from the protocol and is usually implemented as an enabling control pin from the microcontroller. It is included here for completeness. The SPI PIC interface allows connection of peripherals using a high speed serial interface. SPI Flash Memory and SPI SRAM can easily be added to any system. Other types of device include:.

ADC. Accellerometers. Temperature & Humidity sensors.and many more. Its only other real competition is the I2C bus which is why you often see these interfaces both available on processors and microcontrollers.

The SPI interface was designed in the 1970s by Motorola, who used it in their 68000 processor, and it was quickly adopted by many other manufacturers as a defacto standard. It is intended for transmission of data from a master device to one or more slave devices over short distances and at high speeds (MHz). How it Works It works by transferring data one bit at a time between two devices with the master device sending the clock signal (SCK). The clock controls the timing of the data transfer.

Data (MOSI SDO )is sent out of a shift register in the Master SPI device along with a clock signal (SCK) while at the same time another shift register receives data from the slave (MISO, SDI). The Master is always in control and initiates data transfer using the clock signal. Slave devices are selected using a separate slave select signal that is software controlled i.e.

Those signals are separate from the SPI hardware module. Note: SPI defines a single master system. The alternative protocol, IIC, allows multiple bus master operation.

SPI interface Clock Definition The SPI clock is not defined at all - meaning that different slave devices can assume different clock operation; One slave may require an idle clock that is high, while another may require an idle state of low. Others will react to the rising edge while others react to the falling edge!

The SPI interface protocl copes with this by letting you program these details to the SPI hardware module. The following image shows the PIC interface for SPI (all SPI hardware modules in other microcontrollers will allow the same operation just using different registers).

SPI Interface PIC signals (extract from PIC datasheet DS39582B). With this scheme you control each slave device using its chip select line (usually active low- red arrows show control lines). When disabled the Data output from the slave goes into a high impedance state so it does not interfere with the currently selected slave and the slave's data input is ignored (check datasheet).

The advantage of this scheme is that you can consider (control) each device separately when you compare it with the daisy chain method - allowing connection of SPI devices that require different clock schemes. If you have SPI slaves that operate using different clocks (edges/idle states) you can re-programme the master SPI hardware module before enabling a specific CS so each slave has the correct signals sent to it. If you use the Daisy chain method then you need to make sure all the chips use the same clock edge and idle clock state. SPI Interface: Daisy chaining With this scheme all data sent by the master is shifted into all devices and all data sent from each device is shifted out to the next (shown by red dotted arrow). For this scheme to work you have to make sure that each slave uses the clock in the same way and you have to get the right number of bits, so there is more work to do in software. SPI Daisy Chain Example Use several HC595 chips, daisy-chain linked, giving an easy increase in the number of outputs available without using many microcontroller pins.

The limits of operation are the speed of HC, the speed of the SPI output. Also design consideration e.g. If driving a set of LEDs that must be updated every 20ms. Parallel Versus Serial SPI The trade off between using a parallel interface and the SPI interface is speed e.g.

If you read a parallel 12bit ADC at 200ksps then you could read the device at a 200kHz rate but if you want to get the same data rate using SPI then you need a serial speed of 200kHz x 12 = 2.4MHz. So the actual trade off is speed and the consequential noise introduced into the circuit. Summary The SPI interface defines a very popular protocol that works at high speed.

The main problem is that slave devices are not bound by any particular clocking scheme so they may operate differently to each other and that can make controlling them difficult i.e. The clock idle and edge must be set correctly for each device. Advantages:. High speed.

Exremely simple interface that is easy to replicate in software. Disadvantages:. Slave chips are free to adopt any clocking scheme they require.

Reference SPI library This library allows you to communicate with SPI devices, with the Arduino as the master device. A Brief Introduction to the Serial Peripheral Interface (SPI) 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.

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 second SPISettings 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. 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 SPIMODE0 0 0 Falling Rising SPIMODE1 0 1 Rising Falling SPIMODE2 1 0 Rising Falling SPIMODE3 1 1 Falling Rising Once you have your SPI parameters, use SPI.beginTransaction to begin using the SPI port. The SPI port will be configured with your 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, SPIMODE0)); 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 doesn't change SPI settings. Unless you, or some library, calls beginTransaction a second time, the setting are 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 SPI.endTransaction. For more on SPI, see.

Note about Slave Select (SS) pin on AVR based boards All AVR based boards have an SS pin that is useful when they act as a slave controlled by an external master. Since this library supports only master mode, this pin should be set always as OUTPUT otherwise the SPI interface could be put automatically into slave mode by hardware, rendering the library inoperative. It is, however, possible to use any pin as the Slave Select (SS) for the devices. For example, the Arduino Ethernet shield uses pin 4 to control the SPI connection to the on-board SD card, and pin 10 to control the connection to the Ethernet controller. Examples.: Read air pressure and temperature from a sensor using the SPI protocol.: Control a AD5206 digital potentiometer using the SPI protocol. Functions. See also.

How To Program Spi Flash

Corrections, suggestions, and new documentation should be posted to the. The text of the Arduino reference is licensed under a. Code samples in the reference are released into the public domain.