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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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