User`s manual

XP8500
40 s Software Reference
 int adc4
_
set( unsigned int board
_
adr,
int chan )
Sets the A/D converter chip to the specified channel (chan).
PARAMETERS: board
_
adr is the physical address of the XP8500
board, defined as 0000 1100 ppxy.
chan ranges from 0 to 10, corresponding to the boards 11 A/D
channels. Passing chan = 11 will return (VREF+  VREF)/2,
passing chan = 12 will return VREF, passing chan = 13 will return
VREF+, and passing chan = 14 will put the boards A/D chip, a
TLC2543, into software power-down mode. All data defaults to 12
bits unipolar mode with MSB first. The returned datas nominal zero
point is 4095 for unipolar conversion and 2047 for bipolar conversion.
RETURN VALUE: whole number from 0 to 4095 from the last A/D
conversion (caller should be aware of which A/D channel was set
previously); 1 if the specified XP8500 board cannot be found.
Because the A/D converter chip is hardwired to return a
converted value while accepting new settings, adc4
_
set
returns a value converted with the chips previous settings.
Therefore, subsequent calls to adc4
_
set using the same
arguments will return conversions using the new settings.
The key to understanding the difference between adc4
_
read
and adc4
_
set is the pipelined nature of the A/D converter.
By design, shifting a command into the A/D converter simulta-
neously shifts a reading out. However, the A/D converter
made this shifted-out reading according to the previous
commands setup.
So to return a correct reading for a single function call, the
adc4
_
read command shifts a command into the A/D con-
verter, discards the resulting reading, and makes a second read
from the now properly set up A/D converter. The faster
adc4
_
set
function simply returns the first reading. Succeed-
ing adc4
_
set calls will return proper readings with the same
arguments.
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