Does Sigma-Delta ADC always require its own cog?

LarryG

I have found several Sigma-Delta objects. But it seems each one requires its own cog. I just need a real simple way to read an analog voltage from an accelerometer. I just need to grab a reading about once a second or so, do not need continuous reading. Range is about 1v to 3v. I am out of cogs. Any ideas?

Thank you.

-Larry

Phil Pilgrim (PhiPi)

You can do the sigma-delta conversion in Spin if you have a counter to spare. The integration time will probably be longer than it would have been in assembly, but that shouldn't be a problem.

-Phil

LarryG

Thanks Phil. I was reading "Programming, Customizing..." book page 162. It mentions an object called "SigmaDeltaADC(SPIN)". I checked the sticky-thread in the forum for the PE Kit Labs, but the object is nowhere to be found. I think it falls under the section titles "coming soon".

If anyone has the object SigmaDeltaADC(SPIN) I would appreciate a copy.

Thanks.

-Larry

hover1

@Larry

The files are available from the ftp site at the lower right side of this page:

·http://www.parallax.com/Store/Books/Propeller/tabid/171/CategoryID/45/List/0/SortField/0/Level/a/ProductID/637/Default.aspx

I also attached the file.

Jim

LarryG

Wow, thanks Jim! I searched, Googled, etc. for about an hour for that.

ErNa

This may help:

Dat
mADCIdx      long  0     ' Gemultiplexter ADC: von ADCInPin bis ADCPutPin-1
mADC_MuxO                ' Number of counts to have zero result for grnd voltage on analog input
mADC_MuxO0   long  537   ' Offset  first  input    
mADC_MuxO1   long  514   ' Offset  second input  
mADC_MuxO2   long  569   ' Offset  third  input 
mADC_MuxO3   long  541   ' Offset  forth  input 
mADC_MuxV
mADC_MuxV0   long  0     ' counts  first  input  
mADC_MuxV1   long  0     ' counts  second input  
mADC_MuxV2   long  0     ' counts  third  input  
mADC_MuxV3   long  0     ' counts  forth  input  
mADC_Val
mADC_Val0    long  0     ' normalized input value
mADC_Val1    long  0     ' 
mADC_Val2    long  0     ' 
mADC_Val3    long  0     '
 
ADC_Offs     long  ADC_def_Offs     ' ADC offset
ADC_Valu     long  0     ' ADC counts 
ADC_Norm     long  0     ' ADC value, normalized

ADCRunTim    long  0     ' Runtime of last convertion

con
  mADCOutPin = 26      ' Multiplexed ADC: compensation pulses
  mADCInPin  = 22      ' Pin 22 up to ADCOutPin: analog inputs
  mADC_Nmbr  =  4      ' Number of channels to scan  
  ADCOutPin  = 16      ' single measurement: compensation pulses
  ADCInPin   = 17      ' single measurement: analog inputs
  ScalF_ADC  = 1000    ' Scaling factor ADC
PUB Main
  ' Enable muxed ADC
  DIRA[noparse][[/noparse]mADCOutPin]~~             ' Pin to output compensation pulses
  CTRA := %01001_000 << 23  +  mADCOutPin << 9  + mADCInPin + mADCIdx
  FRQA := 1                     ' mode: ADC
  ' Enable single ADC    
  DIRA[noparse][[/noparse]ADCOutPin]~~             ' Pin to output compensation pulses     
  CTRB := %01001_000 << 23  +  ADCOutPin << 9  + ADCInPin
  FRQB := 1                     ' mode: ADC

  ADCRunTim := cnt  ' First convertion
  ADC_Valu  := phsb 

  repeat
     MuxADC


PRI MuxADC  'Multiplexed ADC and "normal" ADC
{
ADC is scanned 20 ms. Value from 0 - 4095 full scale
This ADC measures non time critical voltages

  'MUX-AD-Converter: feedback Control Output mADCOutPin, Inputs: starting from mADCInPin


  These ADC's are handled in the main routine in poll-mode

  MuxADC is called, checks for elapsed time and reads next channel.
  The output value is counts/time, normalized to 12 bits


  ADCOutPin Output compensation pulses
  ADCInPin  Input, actively controlling output
  ADC_Nmbr  Number of multiplexed channels
  ADCIdx    actual index from 0 to ADC_Nmbr-1
  ADCMuxO   offset, the value determined at input voltage 0 (gnd)
}
  if  cnt - ADCRunTim >  clkfreq/500 '   measure if more than 2 ms elapsed  ( if longer time: take care of shift operation: overflow!)

     ADC_Valu           := phsb - ADC_Valu
     mADC_MuxV[noparse][[/noparse]mADCIdx] := phsa - mADC_MuxV[noparse][[/noparse]mADCIdx]  ' number of counts during last sequence
     ADCRunTim          := cnt  - ADCRunTim           ' time elapsed

     ADC_Morm := ((ADC_Valu<<12 / ADCRunTim - ADC_Offs) * ScaF_ADC) ~>12    ' shift: possible overflow!
     
     mADC_Val[noparse][[/noparse]mADCIdx]  := ((mADC_MuxV[noparse][[/noparse]mADCIdx]<<12 / ADCRunTim ) - mADC_MuxO[noparse][[/noparse]mADCIdx]) <<12 / (4096- mADC_MuxO[noparse][[/noparse]mADCIdx])

     mADCIdx            := (mADCIdx+1) // ADC_Nmbr                      ' next index
     CTRA := %01001_000 << 23  +  mADCOutPin << 9  + mADCInPin+mADCIdx  ' Activate new control input
     ' optional: wait 100 µs to increase precision 
     mADC_Valu          := phsb 
     mADC_MuxV[noparse][[/noparse]mADCIdx] := phsa
     ADCRunTim          := cnt

As the integration time influences the result, the tick count has to be normalized to the elapsed time. At full scale, the ticks are equal to clock cycles and therefor, to have a result >1 , the ticks have to be shifted. Also take into account, that 0V doesn't give zero counts.
It is also shown, how to measure more inputs with a single counter. Just connect the compensation output to different compensation networks and dynamically switch the compensation input of the counter. It works quite well, determine the offset by grounding the input. The result is not depending on the actual cycle time, therefor the routine can be called more or less periodically, if only you take care of the overflow. As full input gives an overflow after 50 seconds, measuring less than 100 hz will create an overflow if the shift value is 12 bits.-

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
cmapspublic3.ihmc.us:80/servlet/SBReadResourceServlet?rid=1181572927203_421963583_5511&partName=htmltext
Hello Rest Of The World
Hello Debris
Install a propeller and blow them away

LarryG

Thanks ErNa. I never would have thought to be able to essentially MUX for multiple ADC readings. This could be quite useful for my project. Looks like I will be adding yet more features to my "Ultimate Bike Computer".

-Larry