Connecting Prop to a ADC

SSkalko

I am working on a program to connect the Prop to a ADC (ADS7818P-ND http://focus.ti.com/lit/ds/symlink/ads7818.pdf ) not much luck so far. I am wondering if I need to use pull-up or pull-down resistors on the DATA, Clock and Enable connections between the IC and the Prop. I have a feeling I make need to invest in something like the USB oscilloscope that Parallax sells to make some head way on this.

Leon

It uses a clocked serial interface, you don't need any pull-up or pull-down resistors. It's a 5V device so you will need series resistors to prevent damage to the Propeller inputs.

CannibalRobotics

A scope will save huge time and pain later - do it!

ChrisGadd

I've had a bit of success with that chip. The timing is very finicky; the Prop seems to miss counts when it's clocked above about 4MHz, limiting the sampling rate to about 250KHz for the full 12-bit resolution.

I use one counter to output a continuous clock signal, use waitpeq/waitpne instructions to sync the reads, test the data line, rotate the carry flag into my result register, and store the full result in hub memory. See the attachment for more info.

DAT   org
ADC                  
          mov       dira,#$0000_0005               ' P0-Clk, P1-Data, P2-Conv
          mov       frqa,CtrFreq                           
          mov       ctra,CtrCfg
          mov       ADC_Add,#@Hub_Memory_1         ' Store relative address
          add       ADC_Add,PAR                    ' Add relative address to absolute address of ADC routine
:reset
          mov       ADC_Bit_Counter,#3
          mov       ADC_Data,#0                    ' Clear ADC_Data register
:reset_loop
          waitpne   CLK_Mask,CLK_Mask              ' Wait for clock to go low
          waitpeq   CLK_Mask,CLK_Mask              ' Wait for clock to go high
          djnz      ADC_Bit_Counter,#:reset_loop   ' Wait for three clocks
          andn      outa,Conv_Mask                 ' Clear CONV to begin data transmit
          mov       ADC_Bit_Counter,#13            ' Initialize to rotate 12 bits into ADC_Data
:loop                                              ' Number of instructions in loop determines maximum clock rate, loop takes a minimum of four instructions
'         waitpne   CLK_Mask,CLK_Mask              ' This line only necessary for slow clocks (already low at 4MHz)
          waitpeq   CLK_Mask,CLK_Mask              ' Wait for clock to go high
          test      Data_Mask,ina         wc       ' Test Data pin, set C if high
          rcl       ADC_Data,#1                    ' Rotate C into ADC_Data
          djnz      ADC_Bit_Counter,#:loop         ' Loop if 12 bits not rotated in
          or        outa,Conv_Mask                 ' Raise CONV to begin sampling mode
          wrword    ADC_Data,ADC_Add               ' Store ADC_Data word at calculated absolute address of Hub_Memory_1
          jmp       #:reset
                        
'--------------------------------------------------------------------------

CtrCfg           long      %0_00100_000_00000000_000000_000_000000       ' NCO, P0
CtrFreq          long      214_748_364     ' 4MHz
CLK_Mask         long      $0000_0001      ' CLK on P0
Data_Mask        long      $0000_0002      ' Data on P1
Conv_Mask        long      $0000_0004      ' CONV on P2
ADC_Bit_Counter  long      13              ' Count 13 clocks
ADC_Data         res       1               ' Working register for received data
ADC_Add          res       1               ' Address in hub memory to store valid data

groggory

CannibalRobotics wrote: »

A scope will save huge time and pain later - do it!

My saelig logic analyzer is money for this sorta stuff.

SSkalko

Thanks for the help, I bought the propscope today, so that should be a big help and I will give that code a try tonight. Thanks again.