From the mbed microcontroller Handbook.

SPI

/media/uploads/mbedofficial/spi_interfaces.png

The SPI Interface provides a "Serial Peripheral Interface" Master.

This interface can be used for communication with SPI slave devices, such as FLASH memory, LCD screens and other modules or integrated circuits.

Hello World!

Reading the WHOAMI register of an LIS302 SPI accelerometer

#include "mbed.h"

SPI spi(p5, p6, p7); // mosi, miso, sclk
DigitalOut cs(p8);

Serial pc(USBTX, USBRX); // tx, rx

int main() {
    // 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);
    pc.printf("WHOAMI register = 0x%X\n", whoami);

    // Deselect the device
    cs = 1;
}

API

SPI	A SPI Master, used for communicating with SPI slave devices
Functions
SPI	Create a SPI master connected to the specified pins
format	Configure the data transmission format
frequency	Set the spi bus clock frequency
write	Write to the SPI Slave and return the response

class SPI : public Base

A SPI Master, used for communicating with SPI slave devices

SPI( PinName mosi,
PinName miso,
PinName sclk,
const char * name = NULL )

Create a SPI master connected to the specified pins

void format( int bits,
int mode = 0 )

Configure the data transmission format

void frequency( int hz = 1000000 )

Set the spi bus clock frequency

virtual int write( int value )

Write to the SPI Slave and return the response

class DigitalOut : public Base

A digital output, used for setting the state of a pin

Details

The SPI Interface can be used on mbed pins p5/p6/p7 and p11/p12/p13

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

The SPI Interface can be used to write data words out of the SPI port, returning the data received back from the SPI slave. The SPI clock frequency and format can also be configured. 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.

Reference

SPI on Wikipedia

Page history

Last modified 27 Oct 2010, by Simon Ford

No tags | 16 comments

16 comments on SPI:

david wyland

# 30 Nov 2010

I am trying to use the SPI interface. When I instantiate using "SPI spi(p9, p10, p11); mosi, miso, sclk" and ONLY that line, the 4 blue LEDs at the bottom of the MBED 1768 flash in an alternating sequence (2 outside, 2 inside, repeat). I have no other LED code active. What is going on? Should I worry? Also, it looks like this means I cannot use those 4 LEDs with the SPI code.

david wyland

# 30 Nov 2010

Oops. Apparently a typo in above comment. Should be: I am trying to use the SPI interface. When I instantiate using "SPI spi(p9, p10, p11); mosi, miso, sclk" and ONLY that line, the 4 blue LEDs at the bottom of the MBED 1768 flash in an alternating sequence (2 outside, 2 inside, repeat). I have no other LED code active. What is going on? Should I worry? Also, it looks like this means I cannot use those 4 LEDs with the SPI code.

david wyland

# 30 Nov 2010

Hmmm. Should be a double backslash after "... p11);" The comment processor apparently swallows the double slash for line comments. Happened both times, even though it showed OK in preview.

Simon Ford

# 05 Dec 2010

Hi David,

The flashing lights are the Blue Lights of Death, which tend to mean there is some runtime error. In your case, it is that you are trying to create a SPI interface on pins that don't support it.

The SPI Interface can be used on mbed pins p5/p6/p7 and p11/p12/p13

david wyland

# 06 Dec 2010

"Well, that explains a lot!" I had slipped a cog and was using p9/p10/p11 instead of p11/p12/p13 - oops! Thanks a lot for the information!

Mark Watt

# 27 Dec 2010

Can anyone let me know the maximum frequency the SPI interfaces will run at. Thanks

Simon Ford

# 28 Dec 2010

Hi Mark,

It should be 48MHz I think; SSP prescaler divides the pclk (96MHz) by a minimum of 2 for master mode. But will confirm this and any other restrictions (and update the documentation) when I'm next at a computer.

Simon

Awesome Panda

# 04 Feb 2011

the max frequency is definitely between 25MHz and 20MHZ, too lazy to pinpoint it though

jay jay

# 28 Feb 2011

There is an important option missing in this. Some devices operate in LSBFIRST mode (i.e. Futaba GP9002A01A Display). So I need to reverse the bits before calling write. That should obviously be handled in the class itself as it is much less efficient i.e. SPI::setBitOrder(BITORDER); and BITORDER as an enum or whatever like LSBFIRST MSBFIRST

Jason Johnson

# 28 Feb 2011

One nice thing about the mbed SPI is you can opt to not use the MISO pin if you are talking to a "write only" device (like a 74HC595):

Code

SPI sinkDriver (p5, NC, p7);

This frees up p6 as another usable pin for general purpose digital I/O. The same thing applies to the 2nd SPI interface on p11, p12, and p13.

Mark Howell

# 12 May 2011

Jason Johnson wrote:

One nice thing about the mbed SPI is you can opt to not use the MISO pin if you are talking to a "write only" device (like a 74HC595):

Code

SPI sinkDriver (p5, NC, p7);

This frees up p6 as another usable pin for general purpose digital I/O. The same thing applies to the 2nd SPI interface on p11, p12, and p13.

Does the same apply for "read only" devices, i.e., SPI reader (NC, p6, p7); mosi, miso, sclk ?

Thanks!

AquaMW SW

# 01 Jul 2011

we have interfaced ADC (AD7924) with Mbed via SPI interface

coding written as follows

include "mbed.h"

SPI spi(p5, p6, p7); mosi, miso, sclk DigitalOut cs(p10); Serial pc(USBTX, USBRX); tx, rx Serial pc(p28, p27); int adc;

int main()

{ pc.baud(115200); pc.format(8,Serial::None,1); spi.format(16,3); spi.frequency(1000000); pc.printf("Hello world\r\n"); cs=0; spi.write(0x8C3); int adc = spi.write(0x000);

pc.printf("%d\r\n", adc); cs=1; }

we probed scl and MOSI and CS its working fine we are not getting MISO

spi.write(0x8C3); address to read channel 4

can plz help Is my sample coding is correct?

Hugo Vincent

# 25 Jan 2012

If you're hitting the bit order problem Jay Jay mentioned above, and need a quick way of reversing bits, the following works nicely (for 8-bit byte oriented data only):

Code

// From: http://graphics.stanford.edu/~seander/bithacks.html#ReverseByteWith32Bits - by Sean Anderson, July 13, 2001
unsigned char reverse_bits(unsigned char b)
{
    return ((b * 0x0802LU & 0x22110LU) | (b * 0x8020LU & 0x88440LU)) * 0x10101LU >> 16; 
}

... and if you're wondering how it works, "magic".

Gaurav Patel

# 24 Feb 2012

IF I WANT TO VISUALIZE SPI DATA ON LCD DISPLAY FROM at93C46(SPI_EEPROM)

hiroyuki kajimoto

# 1 week, 2 days ago

I am using two SPIs for two ICs(AD5452:12bit DA and AD7276:12bit AD). When running each separately, it works fine, but when running both alternatively, it suddenly becomes very slow. Does anyone have idea if the switching from one SPI module to another require overhead time? (about 20us!)

Hamid Naseri

# 1 week, 1 day ago

Hello,

The example shows how to read a register on spi bus. I need to program my a2d chip by writeing to its 3 control registers. Could you please let me know how do I wirte to a register?

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