Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link standard, named by Motorola, that operates in full duplex mode. Devices communicate in master/slave mode where the master device initiates the data frame.
- The serial hardware with minimal fuss and minimal code. At the end I’ll use the UART and I2C interfaces in a small RTC project. 2) SERIAL PERIPHERAL INTERFACE (SPI) At its core, the SPI algorithm is very straightforward: Put a data bit on the serial data line. Pulse the clock line.
- SPIE (SPI Interrupt Enable) bit: Set SPIE to one if you want the SPI interrupt to be executed when a serial transfer is completed. SPE (SPI Enable) bit: If you want to use the SPI, you must set this bit. DORD (Data Order) bit: You can choose in which order the data will be transmitted. Set DORD to one to send the least significant bit (LSB) first.
MEAM.Design - ATmega32 Programming - Serial Peripheral Interface (SPI)
Overview
The ATmega32U4 has a number of serial communications modules, including one that conforms to the SPI standard. This port has full-duplex communication over three wires, can be used as either a master or a slave, has a configurable bit rate, and received packets can generate interrupts.
Serial Program Ing (spi) Enable
Configuration
To configure the SPI module, you need to disable power reduction, configure a few I/O pins, set a clock prescaler, select the SPI mode, (optionally) enable interrupts, and finally enable the SPI subsystem. It is also possible to modify many of the details of SPI operation, including the order in which bytes will be shifted between the devices, the clock polarity, and the clock phase - see the full datasheet for more information. Once enabled, the steps that you will take will depend upon whether you are configuring the device as a master or a slave. Herein we are going to assume that the ATmega32U4 is operating as the master device.
1. Disable Power Reduction
If the device has been put into a low-power mode, you must bring it back to full power to allow use of the SPI module. This is done by setting the PRSPI bit in PRR0 register.
I2c To Spi Converter
2. Configure I/O Pins
The SPI lines consist of SCLK (Serial CLock), MOSI (Master-Out, Slave-In), MISO (Master-In, Slave-Out), and the optional SS (Slave Select). These lines are multiplexed onto the lower half of Port B. To configure as the master device, the data direction registers should be set as:
Serial Program Ing (spi) Enabled
| SPI function | Port pin | direction |
| MISO | B3 | input |
| MOSI | B2 | output |
| SCLK | B1 | output |
| SS | B0 | output (input for slave) |
3. Set the clock prescaler
The SPI clock is prescaled by the SPR1 and SPR0 bits in SPCR register, where
| SPCR: SPR1 | SPCR: SPR0 | |
| 0 | 0 | /4 |
| 0 | 1 | /16 |
| 1 | 0 | /64 |
| 1 | 1 | /128 |
4. Select either master or slave mode
To place the SPI module into Master mode, the MSTR bit in the SPCR register should be set to 1.
Parallel To Serial Spi
(Note - If SS is configured as an input and is driven low while MSTR is set, MSTR will be cleared, and SPIF in SPSR will become set. The user will then have to set MSTR to re-enable SPI Master mode.)
5. (Optionally) Enable interrupts
To enable interrupts from the SPI module, set the SPIE bit in the SPCR register to 1, and be sure to enable global interrupts, as explained here. Interrupts will point to SPI_STC_vect when a serial transfer is complete.
6. Enable the module
To enable SPI, set the SPE bit in the SPCR register to 1.
Usage
The SPI system operates by shifting data between two registers, one on the master and one on the slave.
1. To begin a new serial transfer, you must first pull the SS line low using clear(PORTB,0);
2. With SS low, write 8-bits of data into the SPDR register. The SPI module will recognize that you've written new data, and it will shift the packet over the slave, while simultaneously shifting in a packet from the slave.
3. Once the 8-bit transfer is complete, the SPIF bit in SPSR will go high (and if enabled, the SPI interrupt will be generated). The SPIF interrupt flag can be cleared be either executing the ISR or by first reading the SPIF bit then by either reading from or writing to SPDR.
4. If you are interested in the data returned from the slave, you can read it rom the SPDR register.
5. To transfer additional bytes, repeat steps 2-4.
6. To end transmission, return the SS line high using set(PORTB,0);
Serial Program Ing (spi) Enable Command
Sample Code
Spi Bus Specification Pdf
This sample code should enable the SPI module, transfer 0xAA to the slave, and read whatever is returned by the slave.
Spi Bus Protocol
char outgoing = 0xAA;char incoming;//disable power reduction to allow SPI:clear(PRR0,PRSPI);// MOSI, SCLK, and SS as output:DDRB |= (1<<DDRB2) | (1<<DDRB1) | (1<<DDRB0);// Enable SPI, Master mode, prescaler = /128:SPCR = (1<<SPE) | (1<<MSTR) | (1<<SPR1) | (1<<SPR0);// send SS low to begin transmissionclear(PORTB,0);// send a byte to the slaveSPDR = outgoing;// wait for transmission to finishwhile(!check(SPSR,SPIF));// read the byte returned from the slaveincoming = SPDR;// send SS high to end transmissionset(PORTB,0);