A/D W/O External Circuitry?

SRLM · 2008-12-16 03:58

Is it possible to use the Propeller to measure an analog signal (voltage) without using an external chip? I'm using this sensor to measure some distances, and it outputs between 0 and 5 volts, with 2.5 volts the effective minimum. I'm guessing that I can use a resistor (4.7 kOhm?) just like any other five volt device, but what about getting useful distance information from it?

kwinn · 2008-12-16 04:31

See the info on using 2 pins as a sigma/delta adc.

Mike Green · 2008-12-16 04:59

The best source of info is the application note on the cog counters (AN001) downloadable from the Propeller Downloads webpage. It includes sample code.

SRLM · 2008-12-16 05:55

Wow. A whole object titled ADC: just what I'm looking for! However, I do have a question: I don't want to dedicate a whole cog just to the measurement, so if I can I would like to integrate it into my existing spin application. My guess is that I could simply modify the command

jmp       #:loop                          'wait for next sample period

to jump to a spin method, but then I get confused about spin living in the hub, and assembly in the cog. Is it a viable modification to do this?

What appears to my non-assembly programmer eye to be the main loop:

:loop         waitcnt   asm_cnt,asm_cycles              'wait for next CNT value (timing is determinant after WAITCNT)

              mov       asm_sample,phsa                 'capture PHSA and get difference
              sub       asm_sample,asm_old
              add       asm_old,asm_sample
                                                         
              wrlong    asm_sample, par
    
              jmp       #:loop                          'wait for next sample period

Mike Green · 2008-12-16 06:13

You can't mix Spin and assembly in the same cog. There's a Spin interpreter that's a large assembly program that fills the cog when you're running Spin. Unless you absolutely have to reuse the cog that's used for doing the ADC, you should dedicate a cog to the function and leave it that way. It takes about 100us to start up a cog so you could make a few measurements, then load some other program into the cog, then reload the ADC program as long as you're not in a hurry.

Tubular · 2008-12-16 08:08

hi Srlm,

I'm looking to use the same sensor as a backup to a maxbotix ultrasonic ranger. The curve for that sharp sensor is very flat as the distance ranges out beyond 3 metres. (perhaps 50 mv per meter) So i'll be interested to hear how the sigma delta goes. For my application I'm going straight to an external adc.

tubular

A/D W/O External Circuitry?

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