By default, on most of the Arduinos, SS is defined to be on pin 10, and on the Arduino Mega2560, it is on pin 53. The ICSP interface however does not expose the SS line. This is one of the reasons that some of the Arduino shields designed for Arduino Uno does not work with Arduino Leonardo when SPI communication is involved. This however is not the case for Arduino Leonardo and Arduino Micro which are based on ATMega32U4, for those boards, the SPI is only available on ICSP header.
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Uno, Nano and Pro Mini), it has pin 11, 12 and 13 as SPI's MOSI, MISO and SCLK signals, but at the same time these signals are also available on those board's In-Circuit Serial Programming (ICSP) header pin 4, 1 and 3, except for Pro Mini which does not have a dedicated ICSP header. For ATMega328p-based Arduino boards (i.e. The SPI bus on Arduino varies depend on the model of Arduino board.
This bit-wise exchange of data will continue until there is no more data to be exchanged or an interrupt occurs, at which point the master will stop sending clock signals to the slave. The SPI module works in Full-Duplex mode, meaning that during each clock cycle, the SPI module is simultaneously transmitting via MOSI and receiving data via MISO from the activated slave. This allows multiple peripherals to be connected on the same bus that share the lines of MOSI, MISO and SCLK, and each peripheral has its own SS line, and only the peripheral that has its SS been pull low to listen to the communication sent by the master.ĭata transfers between master and slave devices on the SPI bus are handled by separate transmit and receive FIFO (First-In-First-Out) buffers. The slave devices on the SPI bus will always receive the SPI clock signal, but will not respond to the SPI master until their respective Slave Select line has been activated (i.e. SS: Slave Select to select the peripheral device/component.SCLK: A Serial Clock tat regulates the speed of the communication.MISO: Master-In-Slave-Out for master to receive data from a slave.MOSI: Master-Out-Slave-In for master to send data to a slave.Each SPI bus requires 4 wires between master and slave. This allows one master to communicate with one or more slaves. SPI is more than just a serial communication protocol, it is a bus with master and slaves connecting on the same communication bus. This allows SPI to be run at higher data speed and easy to use once it is setup. the Master) and therefore does not need the checksum bits. SPI is gnerally designed for high-speed, shorter range of communication at a clock speed set by the data initiator (i.e. In order to ensure the integrity of the data, simple parity check can be introduced into the communication stream as well.
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UART uses two wires for transmission and reception (Tx and Rx), and requires a clock on both sides to be on the same speed, UART is designed for long range communication, both hardware and software flow control can be added if necessary. SPI is simpler in design compared to UART. The name imply that it is a Serial communication protocol and interface, but it is different from serial communication protocol like UART in many different aspects. It is important to understand how SPI (Serial Peripheral Interface) works in the embedded world because SPI is widely used deep inside embedded systems, ranging from sensor connection, to SD card interface, to even between the flash memory and its MCU.
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