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SPI

SPI class hierarchy

The SPI Interface provides a Serial Peripheral Interface Master.

You can use this interface for communication with SPI slave devices, such as FLASH memory, LCD screens and other modules or integrated circuits.

Interface

A pinout map.

The default settings of the SPI interface are 1MHz, 8-bit, Mode 0.

You can use the SPI interface to write data words out of the SPI port, returning the data received back from the SPI slave. You can also configure the SPI clock frequency and format. The format is set to data word length 8 to 16 bits, and the mode as per the table below:

Mode Polarity Phase
0 0 0
1 0 1
2 1 0
3 1 1

The SPI master generates a clock to synchronously drive a serial bit stream slave. The slave returns a bit stream, also synchronous to the clock. To communicate with multiple slave devices connected over the same SPI peripheral, you must use multiple SPI objects, one for each slave, but instantiate them with a different Slave select (SSEL) and configuration corresponding to each slave. Note that the SPI object automatically configures the SPI peripheral with the current object's configuration when the application code invokes the interfaces on the SPI object.

SPI class reference

Public Member Functions
 SPI (PinName mosi, PinName miso, PinName sclk, PinName ssel=NC)
 Create a SPI master connected to the specified pins. More...
 SPI (PinName mosi, PinName miso, PinName sclk, PinName ssel, use_gpio_ssel_t)
 Create a SPI master connected to the specified pins. More...
void format (int bits, int mode=0)
 Configure the data transmission format. More...
void frequency (int hz=1000000)
 Set the SPI bus clock frequency. More...
virtual int write (int value)
 Write to the SPI Slave and return the response. More...
virtual int write (const char *tx_buffer, int tx_length, char *rx_buffer, int rx_length)
 Write to the SPI Slave and obtain the response. More...
virtual void lock (void)
 Acquire exclusive access to this SPI bus. More...
virtual void unlock (void)
 Release exclusive access to this SPI bus. More...
void select (void)
 Assert the Slave Select line, acquiring exclusive access to this SPI bus. More...
void deselect (void)
 Deassert the Slave Select line, releasing exclusive access to this SPI bus. More...
void set_default_write_value (char data)
 Set default write data. More...
template<typename Type >
int transfer (const Type *tx_buffer, int tx_length, Type *rx_buffer, int rx_length, const event_callback_t &callback, int event=SPI_EVENT_COMPLETE)
 Start non-blocking SPI transfer using 8bit buffers. More...
void abort_transfer ()
 Abort the on-going SPI transfer, and continue with transfers in the queue, if any. More...
void clear_transfer_buffer ()
 Clear the queue of transfers. More...
void abort_all_transfers ()
 Clear the queue of transfers and abort the on-going transfer. More...
int set_dma_usage (DMAUsage usage)
 Configure DMA usage suggestion for non-blocking transfers. More...

SPI hello, world

The following example uses the WHOAMI register. The WHOAMI register is an ID register of the slave device. In other words, it's just an example that you can write to a slave's register. In this example, Mbed OS acts as the SPI master, and DigitalOut acts as the slave chip select (cs).

/* mbed Example Program
 * Copyright (c) 2006-2014 ARM Limited
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#include "mbed.h"
 
SPI spi(D11, D12, D13); // mosi, miso, sclk
DigitalOut cs(D0);
 
int main() {
    // Chip must be deselected
    cs = 1;

    // Setup the spi for 8 bit data, high steady state clock,
    // second edge capture, with a 1MHz clock rate
    spi.format(8,3);
    spi.frequency(1000000);
 
    // Select the device by seting chip select low
    cs = 0;
 
    // Send 0x8f, the command to read the WHOAMI register
    spi.write(0x8F);
 
    // Send a dummy byte to receive the contents of the WHOAMI register
    int whoami = spi.write(0x00);
    printf("WHOAMI register = 0x%X\n", whoami);
 
    // Deselect the device
    cs = 1;
}
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