PICAXE to PGA2310 Volume Control

[blip1882]

Hi,

I've been thinking about building a PICAXE controlled audio pre-amplifier. For volume control I've been considering using a TI PGA2310 (http://focus.ti.com/docs/prod/folders/print/pga2310.html) digital potentiometer.

I was wondering what the simplest way to interface a PICAXE 18x and the PGA2310 would be? Maybe using the SEROUT command? I'm new to PICAXE so any advice you could give me would be appreciated.

 

[hippy]

It's a three-wire SPI-style bus ( SCK, SDI, SDO ) with an additional chip select, and SEROUT won't help here. You'll need to allocate three output lines to drive the chip ( CS\, SCK, SDI ); SDO is only used for daisy-chaining and doesn't need to be read back to the PICAXE ( Fig 4 ).

You might also want to use two additional output lines to control ZCEN and MUTE\. The chip is 5V so will connect directly to a PICAXE also running at 5V.

With the chip wired up it's a simple matter of ensuring that the three lines are set high and low in the right sequence as indicated by the data sheet ( Fig 2 ).

The documentation on SHIFTOUT in "PICAXE Manual 2 - Basic Commands" is what you will need to get to grips with.

 

[hippy]

SYMBOL CS = pin0 ' Output Pin 0
SYMBOL SCK = pin1 ' Output Pin 1
SYMBOL SDI = pin2 ' Output Pin 2

SYMBOL leftVolume = b0
SYMBOL rightVolume = b1
SYMBOL pan = b2
SYMBOL bitNumber = b3
SYMBOL tmp = b4

StartOfProgram:
CS = 1
:
' Pan Left to Right
FOR pan = 0 TO 255
leftVolume = 255-pan
rightVolume = pan
GOSUB SetVolume
PAUSE 50
NEXT
END

SetVolume:
CS = 0
tmp = rightVolume
FOR bitNumber = 0 TO 7
SDI = tmp AND $80 MAX 1
SCK = 1
SCK = 0
tmp = tmp * 2
NEXT
tmp = leftVolume
FOR bitNumber = 0 TO 7
SDI = tmp AND $80 MAX 1
SCK = 1
SCK = 0
tmp = tmp * 2
NEXT
CS = 1
RETURN

 

[blip1882]

Wow! Thanks for the quick reply. I've just started digging through your code sample with the command reference at hand... Looks like a much simpler implimentation than I had originally imagined.

Thanks again.

 

[blip1882]

Well I think I understand the application of shiftout now. Basically you use shiftout sub-routine and associated symbols... you specify the data length (128 for 8 bit) and then insert the value into the var_out. Seems pretty simple and it would be easy to build an interface for this. (I'm going to have a remote control vary the volume from the -95 dB point to around -8 dB)

I'm having trouble understanding the math behind this, though. I guess I'm struggling with the concept of bitwise &. Does anyone have canned explanation of how the shiftout sub-routine works... Or just a site that will help to give me a bit of education on this topic. Thanks.