Nuts & Volts - May 2004

Stamp

Remember that we don’t have to invert the active-low button inputs, as the MCP23016 has been set up to do that for us (I love this feature). We could save a bit of variable space by aliasing btns to the NIB0 of lcdIO, but I decided not to so that things don’t get mixed up, as lcdIO is used in so many other places.

Back to the button demo code — now that we have the current status, we can display each of them on the second line using LOOKUP again. Here, we’ll use LOOKUP to select the character; as it stands, a dash means that the button is not pressed and an asterisk indicates that a button is pressed. I decided to do this because 0 and 1 are boring — but if that’s what you want to use for one of your apps, you can change the LOOKUP line to this:

Finally, here are a couple of subroutines to update the LED, as required by the demo code:

lcdIO = “0” + btns.LOWBIT(idx)

Alright, we’re almost home — the last thing we need to test is the LED. Just for fun, let’s make it light when all of the buttons are pressed.

Update_LED:
  IF (btns = %1111) THEN
    GOSUB LED_On
  ELSE
    GOSUB LED_Off
  ENDIF

LED_On:
  I2CIN SDA, Rd23016, GP1, [lcdCtrl]
  IF (lcdLED = Is_Off) THEN
    lcdLED = Is_On
    I2COUT SDA, Wr23016, GP1, [lcdCtrl]
  ENDIF
  RETURN

LED_Off:
  I2CIN SDA, Rd23016, GP1, [lcdCtrl]
  IF (lcdLED = Is_On) THEN
    lcdLED = Is_Off
    I2COUT SDA, Wr23016, GP1, [lcdCtrl]
  ENDIF
  RETURN

Set_LED:
  I2CIN SDA, Rd23016, GP1, [lcdCtrl]
  IF (lcdLED <> ledStatus) THEN
    lcdLED = ledStatus
    I2COUT SDA, Wr23016, GP1, [lcdCtrl]
  ENDIF
  RETURN

F o r

E l e c t r o n i c s

NUTS & VOLTS E v e r y t h i n g

All of these routines are quite simple; we read the status of GP1, check the LED control bit, then update it — if required — and send the port data back. While our demo doesn’t actually use the Set_LED routine, it’s included because it will be useful when we want the terminal LED to follow a status bit from elsewhere in our application (that we’ve aliased as ledStatus).

Do note that none of these routines write the LED status bit to the MCP23016 unless a change is actually required.

Okay, we’re done. How about that for a simple — yet abundantly useful — project? As I suggested some time back, I’ve become a very big fan of the I²C buss and the MCP23016 is a great part to use with it.

A couple of final notes: Yes, you can control the MCP23016 with the BS2, BS2e, and BS2sx. You’ll need to use manual I²C code, since those BASIC Stamps don’t have the I2CIN and I2COUT instructions. We did that back in the May 2002 issue (you can find that article online as a PDF on the Parallax website — www.parallax.com). Finally (I promise), the MCP23016 uses the same device address as the PCF8574A — so you can’t mix them on the same SDA buss pin (but you can mix the MCP23016 with the PCF8574AP).