Component: SPI Master (Comms: Interface)
| Author | Matrix Ltd |
| Version | 2.0 (Release) |
| Category | Comms: Interface |
Contents
SPI Master component
Low level routines for controlling or interacting with an SPI interface. SPI or Serial Peripheral Interface is a bus used for board level communications between devices. A target microcontroller will usually have at least one hardware SPI peripheral built in. If the hardware SPI pins are in use or more SPI channels are required then there is also a software mode available.
Detailed description
Overview
The SPI bus is a high speed communications bus which is usually best suited for talking between devices situated on the same circuit board. Due to the high frequency digital nature of the bus care should be taken to keep tracks as short as possible and as far away as possible from other sources of noise. A typical SPI bus consists of four signals, chip select, data out, data in and clock. The SPI bus usually consists of a single master device and then one or more slave devices.
As the data out and data in signals are subjective in terms of who is sending and receiving the data there are alternate names to help clarify the connection.
Data out (SDO) is also known as MOSI which stands for Master Out Slave In.
Data in (SDI) is also known as MISO which stands for Master In Slave Out.
Each SPI transaction consists of a byte made up of 8 clock cycles allowing the 8-bits of the byte to be transferred.
Chip Select CS / Slave Select SS
Each device connected to the SPI bus connects in parallel to the main three SPI pins: data out, data in and clock. The fourth pin the CS pin must be unique for every SPI device present on the bus allowing only one device to be addressed at any one time. As every slave device on the SPI bus requires a individual CS signal from the SPI Master a good way of adding more devices without consuming more of the pins of the SPI master is to use things like 3 to 8 pin multiplexer ICs. The CS signal activates the slave device when pulled low and disables the slave device when pulled high.
Adding a pull up resistor of around 10K - 100K between the CS signal and VCC allows the SPI slave device to be disabled whenever the Microcontroller is not ready to perform the functionality of the SPI master e.g. during reprogramming or when the device is held in reset.
Three Wire SPI
Three wire SPI is a version of SPI where the master can send data to the slave device but cannot read any data back for example a DAC or graphical LCD.
Four Wire SPI
Four wire SPI is the complete version of SPI where the master can sent data to the device and receive data back from the device. A single SPI operation simultaneously transfers a byte from the master to the slave via the MOSI signal and also a byte from the slave to the master via the MISO signal. Normally only one of these bytes is meaningful so it's down to the protocol of the slave device as to how to perform writes and reads.
Examples
More information on SPI can be found here,
Matrix Flowcode Blog: Simplified communications I2C and SPI
Downloadable macro reference
SendString
Sends a string of bytes to the SPI bus
Parameters
- <- STRING Str
- This parameter may be returned back to the caller
Return value
- This call does not return a value
GetString
Attempts to read a string of bytes from the SPI bus.
Parameters
- BYTE NumBytes
- BYTE Ch
Return value
SendChar
General purpose SPI send byte macro
Parameters
- BYTE Char
Return value
- This call does not return a value
Transaction
Allows simultaneous two way communications, send and receive,
Parameters
- BYTE DataOut
Return value
Disable_CS
Outputs a high signal to the chip select pin to deselect the remote slave device.
Parameters
- This macro has no parameters
Return value
- This call does not return a value
GetChar
General purpose SPI get byte macro
Parameters
- This macro has no parameters
Return value
UnInitialise
Deactivates the SPI peripheral leaving the I/O pins in a state where they can be used for general purpose I/O.
Parameters
- This macro has no parameters
Return value
- This call does not return a value
Enable_CS
Outputs a low signal to the chip select pin to select the remote slave device.
Parameters
- This macro has no parameters
Return value
- This call does not return a value
Initialise
Activates the SPI peripheral and claims control over the I/O pins.
Parameters
- This macro has no parameters
Return value
- This call does not return a value
Simulation macro reference
This component does not contain any simulation macros
Property reference
Channel
This property is of type Fixed list of ints and can be referenced with the variable name cal_spi::CHANNEL.
SPI Channel selector
MOSI
This property is of type Single digital pin and can be referenced with the variable name cal_spi::MOSI.
SPI Data Out Pin SDO - Also Known as Master Out Slave In (MOSI) when used in Master mode.
MISO
This property is of type Single digital pin and can be referenced with the variable name cal_spi::MISO.
SPI Data In Pin SDI - Also Known as Master In Slave Out (MISO) when used in Master mode.
CLK
This property is of type Single digital pin and can be referenced with the variable name cal_spi::CLK.
SPI Clock Pin CLK - The Clock signal is driven by the SPI master.
Use Auto CS
This property is of type True or false and can be referenced with the variable name cal_spi::UseAutoCS.
Allow the SPI component to manage the CS pin for us in Master mode.
On initialise the CS pin is automatically set high to disable the remote SPI device.
CS / SS
This property is of type Single digital pin and can be referenced with the variable name cal_spi::SS.
Chip Select / Slave Select Pin
Master Mode: General purpose output pin used to select the remote SPI device.
Slave Mode: Hardware chip select pin input used to select the SPI device.
Prescale
This property is of type Fixed list of ints and can be referenced with the variable name cal_spi::PR_SCALE.
Prescale option selector
Clock Phase
This property is of type Fixed list of ints and can be referenced with the variable name cal_spi::CKE.
Clock Phase (data change edge) selection
Clock Polarity
This property is of type Fixed list of ints and can be referenced with the variable name cal_spi::CKP.
Clock Polarity setting, inactive (idle) state
Sample Point
This property is of type Fixed list of ints and can be referenced with the variable name cal_spi::SMP.
Data bit read sample point
Label
This property is of type Line of text and can be referenced with the variable name label.
Label used to identify the component on the panel.
Scope Traces
This property is of type True or false and can be referenced with the variable name cal_spi::ScopeTraces.
Selects if the scope traces are automatically generated or not
Console Data
This property is of type True or false and can be referenced with the variable name cal_spi::ConsoleData.
Selects if the console data is automatically generated or not
Injector
This property is of type Fixed list of ints and can be referenced with the variable name cal_spi::Injector.
No additional information
