SSI does Microchip Port Expanders!

G’day folks,

it hasn’t even been a week since we published our SSI object, and we already found an application for it. We used the SSI to control Microchip’s MCP23017 16-bit port expander.

For those of you who never tested this, Microchip port expanders are I2C or SPI ICs that give you 8 or 16 I/O lines. Add something like this to our (relatively low-pin count EM1206) and you get 16 additional I/O lines for <1US$!

Without further ado, here is the code that setups an I2C channel, configures all I/O lines of the MCP23017 as outputs, and writes &hAA55 to the output ports:

'------------------------------------------------------------------
  'configure I2C clock as output
  io.num=I2C_SCL 'you need to declare I2C_SCL, I2C_SDA
  io.state=HIGH
  io.enabled=YES

  'setup SSI channel
  ssi.channel=0
  ssi.mode=PL_SSI_MODE_3
  ssi.clkmap=I2C_SCL
  ssi.dimap=I2C_SDA
  ssi.domap=I2C_SDA
  ssi.zmode=PL_SSI_ZMODE_ENABLED_ON_ZERO
  ssi.direction=PL_SSI_DIRECTION_LEFT
  ssi.baudrate=40 'this I2C device is relatively slow
  ssi.enabled=YES

  'setup all I/O lines of the MCP23017 as outputs (see Microchip's documentation)
  i2c_start() 
  ssi.str(chr(&h40)+chr(&h00)+chr(&h00)+chr(&h00),1)
  i2c_stop() 

  'output &hAA55 (see Microchip's documentation)
  i2c_start() 
  ssi.str(chr(&h40)+chr(&h12)+chr(&hAA)+chr(&h55),1)
  i2c_stop() 

'------------------------------------------------------------------
sub i2c_start
  io.lineset(I2C_SCL,HIGH)
  io.num=I2C_SDA 'set SDA to HIGH first so we can have HIGH->LOW transition
  io.state=LOW 'we are manipulating data line through the OE register
  io.enabled=NO
  io.enabled=YES 'this will set the data output to LOW	
end sub

'----------------------------------------------------------------------------
sub i2c_stop
  io.lineset(I2C_SCL,LOW) 'this will remove the ack bit
  io.num=I2C_SDA 'set SDA to LOW first so we can have LOW->HIGH transition			
  io.state=LOW 'we are manipulating data line through the OE register
  io.enabled=YES
  io.lineset(I2C_SCL,HIGH)
  io.enabled=NO 'this will set the data output to HIGH
end sub