diff options
| author | Ian Abbott <abbotti@mev.co.uk> | 2015-06-05 20:30:34 +0300 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2015-06-13 03:06:42 +0300 |
| commit | 34cce4628b8509dc894735cdc5de8b10db21873a (patch) | |
| tree | ee417940929e15198ae0bcd69828156c8175f170 /drivers | |
| parent | 3919c3d5891b079b5d03c03b4df4e3fca9572eca (diff) | |
| download | linux-34cce4628b8509dc894735cdc5de8b10db21873a.tar.xz | |
staging: comedi: das08: clarify sign-magnitude conversion
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 <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'drivers')
| -rw-r--r-- | drivers/staging/comedi/drivers/das08.c | 21 |
1 files changed, 18 insertions, 3 deletions
diff --git a/drivers/staging/comedi/drivers/das08.c b/drivers/staging/comedi/drivers/das08.c index f74c4b009d8b..6c9bd10243b7 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; |