16-Bit SPI I/O Expander

Hardware

If you need a fast and cheap way to add more (much more!) I/Os to your controller, here it is. For this little project I used the Microchip MCP23S17 SPI I/O-Expander on the hardware side of things.

The chip is available as PDIP, SOIC and SSOP package. I chose the SOIC version and designed a simple (one sided) PCB with EAGLE (download here).

The circuit diagram for this PCB

The etched, drilled and assembled board, but without mounted DIL-Switches for selecting the device adress (didn't have one).

With the MCP23S17 you can cascade up to eight of these devives on one SPI-CS line, you just have to set different device adresses for each MCP23S17 via the dip switch, in this way you can get up to 8*16 = 128 additional I/Os for your controller!

Makingthings software

We need two main functions to talk to the MCP23S17, one for writing values (MCP23S17_Write()) and one to read data from the chip via SPI (MCP23S17_Read()). But first we need to initialize the SPI subsystem (setting the SPI-Clock speed). After that we can start with setting up the MCP23S17 (define which IOs are inputs/ outputs and if and under what conditions to trigger a interrupt) this is done in SPI_portexpanderinit() ).

In case you want to use the interrupt functionality of the MCP23S17, keep in mind that you need to connect the interrupt output pin(s) to your controller and write a interrupt service routine. This is not implemented in this demo project (download here or below).

Now we can start using the I/O-Expander by reading values:

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gpioa_value = MCP23S17_Read(MCP23S17, GPIOA); // or
gpiob_value = MCP23S17_Read(MCP23S17, GPIOB);

Or by writing values:

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MCP23S17_Write(MCP23S17,GPIOA,0x00);  // clear all outputs of GPIOA
MCP23S17_Write(MCP23S17,GPIOB,0xff);   // set all outputs of GPIOB

Or if you want to set or reset only one specific GPIO pin:

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setMCP23S17_GPIO(GPIOA, CMG_BKLGHT, 1, MCP23S17, &portA_status);  // set
setMCP23S17_GPIO(GPIOA, CMG_BKLGHT, 0, MCP23S17, &portA_status);  // reset

Reading a specific portpin:

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getMCP23S17_GPIO(GPIOA, CMG_BKLGHT, MCP23S17);

The makingthings mcbuilder example project can be downloaded below.

Links:

MCP23S17 Datasheet

MCP23S17 Eagle Project Files

MCP23S17 Makingthings Project Files