Communication Protocols

 

Universal Asynchronous Receiver/Transmitter (UART)

·         Simple, two-wire protocol for exchanging data and acts as an intermediary between parallel and serial interfaces

·         Asynchronous: no shared clock

o   The same bit / baud rate must be the configured in the transmitter and receiver

·         Can be found as standalone Integrated Circuits (IC) or in microcontrollers – Review documentation to see if UART is available

 

Source: Sparkfun Tutorials

 

 

·         Transmitter appends sync and parity bits to create a data packet

·         Data packet is sent out the Tx line at the defined baud rate

·         Data packet is received on the Rx line and sampled at the defined baud rate

o   Receiving UART picks out the sync bits and delivers the data

 

Source: Sparkfun Tutorials

 

·         Advanced UARTs may place received data into a buffer

o   Buffer: Holding space for data where a microcontroller can retrieve data from

§  Data retrieval is first In, first Out (FIFO)

§  Can be designed to the required size specs

 

·         Software UARTs can be used if a microcontroller doesn’t have a UART or needs more

o   Bit-Banging: Technique for digital communication where software controls GPIO pins to imitate a communication protocol

 

 

Serial Peripheral Interface (SPI)

 

 

 

Master/Controller and Slave/Peripheral are used interchangeably, as are their acronyms

 

 

·         Synchronous serial communication for short-distance wired communication

·         Interface bus commonly used to send data between microcontrollers and small peripherals

·         Synchronous:  Data bus uses separate data transmission and clock lines to keep both sides in sync

o   Clock is an oscillating signal that tells the receiver when to sample bits on a data line

o   Data transmits on the rising or falling edge of the clock signal

§  Data sheet will indicate what edge to use

§  Once the indicated receiver detects the edge, data is read from the next bit on the indicated line

o   Data transmission speed does not have to be defined since SPI transmits data on a clock

 

Source: Sparkfun Tutorials

 

·         The controller generates the clock signal and the peripherals follow

o   1 controller and 1 or many peripherals

·         Data transmits on PICO and POCI lines

o   Controller uses clock to know in advance when and how much data is transmitted along PICO vs. POCI

·         Full Duplex:  Separate send and receive lines means that data can travel along PICO and POCI lines simultaneously

o   The possibility of this capability can be determined from the peripheral’s data sheet

 

Source: Sparkfun Tutorials

 

·         Chip Select (CS) is indicated by the controller and used to select a peripheral for data transmission

·         A single controller can interface with one or many peripherals

·         The CS line is normally held ‘HIGH’ when idle and pulled down when active (Active Low)

o   Controller will pull a peripheral’s line ‘LOW’ when data is to be transmitted, then ‘HIGH’ when transmission is complete

o     A bar over the line label indicates ‘Active Low’ (e.g. )

 

·         Usually each peripheral has its own CS line to avoid multiple peripheral signals transmitting on the same PICO/POCI lines at the same time

o   Binary Decoder Chips can be used to multiply CS outputs if peripherals exceed the available CS lines

 

Source: Sparkfun Tutorials

 

·         Alternatively, a single CS can be used for all peripherals when they are daisy-chained so the output of one peripheral is the input of the next

o   Output-only use-cases that do not require ‘response’

§  ‘Response’ requires loop to close on POCI

·         Data will have to travel across all peripherals to arrive at the controller

·         Send enough receive commands to get the data that’s needed

o   Data overflows from one peripheral to the next, so there must be enough data transmitted to reach all of them

o   First piece of transmitted data will reach the all peripherals

Source: Sparkfun Tutorials

 

Helpful Links

• Sparkfun Tutorials: Wiring and Hardware
• Sparkfun Tutorials: UART
• Sparkfun Tutorials: SPI