How to add on digital IO pins to raspberry pi hardware using MCP23S17?

Hi,

I would like to add digital I/O pins to the raspberry pi hardware using the SPI of the MCP23S17.

There are already some tutorials for analog input using SPI with the MCP3008 and to add digital I/O Pins by I2C with the Matlab Support Package.

http://de.mathworks.com/help/supportpkg/raspberrypiio/examples/add-digital-i-o-pins-to-raspberry-pi-hardware-using-mcp23017.html

http://de.mathworks.com/help/supportpkg/raspberrypiio/examples/analog-input-using-spi.html

I'm looking forward an answer.

Best regards, Frank

Answer 1

[Greg Buffinton]

OK so after much trial and mostly error! ;)

this is what worked for me...

Registers:

CMD_READ = 65  % Device address + Opcode (ie Device 0 Read)
CMD_WRITE = 64  % Device address + Opcode (ie Device 0 Write)
GPIOA = 18
GPIOB = 19
GPPUB = 13 % Pullups for inputs
IOCON = 10
IODIRA = 0
IODIRB = 1

Commands

pi = raspi;
CE0 = spidev(pi, 'CE0');
writeRead(CE0,[CMD_WRITE IODIRA 0]); % Sets "A" port to outputs
writeRead(CE0,[CMD_WRITE GPIOA bin2dec('10101010')]); % Turns ON LED 7,5,3,1 and Turns OFF LED 6,4,2,0
writeRead(CE0,[CMD_WRITE GPPUB hex2dec('FF')]) % Enable pullups for port B inputs
writeRead(CE0,[CMD_READ GPIOB 0]) % read inputs. Note these may be inverted.
writeRead(CE0,[CMD_WRITE IPOLB bin2dec('11111111')]) % invert polarity in bitmask
writeRead(CE0,[CMD_READ GPIOB 0])

Make sure you send dummy bytes when reading!

Helpful snippets:

MCP23S17 Datasheet

Expanding the ports of a Raspberry PI using a MCP23S17

One thing to remember about SPI is that to receive a byte, you need to send a byte. This may sound a little odd, but consider a device which accepts a command byte, then a data byte, then sends back a result byte… To make this happen, we need to send three bytes to the device, which will send back 2 dummy bytes, followed by the 3rd byte with data in it. For example, a GPIO expander chip (e.g. the MCP23S17 as used in the PiFace) where you send it a command (read register), then the data (the register number), then it sends back a return value (the register contents). So, when you send the command, it will send back a byte which you ignore, you send it the data, it sends back a byte which you also ignore, but to make it send the result, you need to send it a third byte which it will ignore!

And

Briefly, as for the code, mcp is first initialized with bank=0, no auto-increment feature, by the sequence : 0×40 = 01000000 == 0100 (fixed) + 000 (chip addr) + 0 (=write). (nb : the code example does not use the SPI chip addressing feature ) 0×0A = register 0Ah == IOCON register addr 0×20 = 0b00100000 == bank0, SEQOP = OFF then , code sets up Direction of port B : 0×40,0×01,0×00 : register 01h , all outputs (00h) then , the program loops in sending portB latch register (15h) values, from 0 to FFh. Writing to the latch of Port B is : 0×40,0×15,value.

[Walter Roberson]

What an odd I/O system. I wonder why it was thought to be a good idea?