SPI - Simples!

Page last updated 20 Feb 2010, by Martin Spencer. 3 replies 

I was getting very bored of typing spi.write(0xXX); repeatedly along with chip selects and deselects, waits etc etc. so why not bosh a couple of sub routines in and the SPI can be written to like this;
 
Write operation:
write(address,data);
 
Read Operation:
data = read(address);
 
simples! :D
 
Heres the code (obviously labels can be changed to suit) Insert waits as defined in the data sheet
void write(int address, int data) {
    cs=0;                    //Select the deivce
    _spi.write(address);    //write the address of where the data is to be written first
    _spi.write(data);       //then send the data byte directly after it
    cs=1;                   //deselect the device
}
int read(int address) {
    cs=(0);                 //select the device
    _spi.write(address);    //select the register
    _data = _spi.write(0x00);  //send a dummy byte to retrive data
    cs=1;                   //deselect the device
    return _data;           //return the data
}

3 comments

Igor Skochinsky
21 Feb 2010

To read data, you need to use spi.read(). write() will just return whether the write was successful, not the value read.

Martin Spencer
21 Feb 2010 . Edited: 21 Feb 2010

There is no 'spi.read()' function. from the MBed library:

 

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

 

 

to read from SPI you first write the address on MOSI (the slave will return 0xXX)

Then write a dummy variable, on the first clock edge the slave will begin clocking data out on MISO.

 

spi.write(value) writes the value to MOSI and ignores data on MISO

variable = spi.write(value) writes the value to MOSI and returns the data on MISO.

 

using this on a write would indeed return the value just written, as you are addressing that register. Most devices have a 6 bit address and a read/not write bit which when set would return the contents of the register without overwriting it.

Igor Skochinsky
21 Feb 2010

My bad, I confused it with I2C.

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