For DAS08/JR/16 and DAS08/JR/AO-16 boards, the 16-bit analog input
readings are assumed to be in a sign-magnitude format and need
converting to the COMEDI unsigned sample format. The expressions to do
the conversion look a little messy. Use a local variable `magnitude` to
make it easier to follow.
Also, there seems to be some discrepancy between the manual for these
boards and the COMEDI code. The manual implies that 0 is full-scale
negative and 65535 is full-scale positive. However, the COMEDI code has
used the sign-magnitude conversion for these boards since these two
boards were initially supported by a patch from an external contributor
to the COMEDI project back in 2001. Assume the code is correct for now,
but add a comment to mention the discrepancy.
Signed-off-by: Ian Abbott <abbo...@mev.co.uk>
---
drivers/staging/comedi/drivers/das08.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/drivers/staging/comedi/drivers/das08.c
b/drivers/staging/comedi/drivers/das08.c
index f74c4b0..6c9bd10 100644
--- a/drivers/staging/comedi/drivers/das08.c
+++ b/drivers/staging/comedi/drivers/das08.c
@@ -228,11 +228,26 @@ static int das08_ai_insn_read(struct comedi_device *dev,
} else if (thisboard->ai_encoding == das08_pcm_encode12) {
data[n] = (msb << 8) + lsb;
} else if (thisboard->ai_encoding == das08_encode16) {
- /* FPOS 16-bit boards are sign-magnitude */
+ /*
+ * "JR" 16-bit boards are sign-magnitude.
+ *
+ * XXX The manual seems to imply that 0 is full-scale
+ * negative and 65535 is full-scale positive, but the
+ * original COMEDI patch to add support for the
+ * DAS08/JR/16 and DAS08/JR/16-AO boards have it
+ * encoded as sign-magnitude. Assume the original
+ * COMEDI code is correct for now.
+ */
+ unsigned int magnitude = lsb | ((msb & 0x7f) << 8);
+
+ /*
+ * MSB bit 7 is 0 for negative, 1 for positive voltage.
+ * COMEDI 16-bit bipolar data value for 0V is 0x8000.
+ */
if (msb & 0x80)
- data[n] = (1 << 15) | lsb | ((msb & 0x7f) << 8);
+ data[n] = (1 << 15) + magnitude;
else
- data[n] = (1 << 15) - (lsb | (msb & 0x7f) << 8);
+ data[n] = (1 << 15) - magnitude;
} else {
dev_err(dev->class_dev, "bug! unknown ai encoding\n");
return -1;
--
2.1.4
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