Commit 34cce462 authored Jun 05, 2015 by Ian Abbott Committed by Greg Kroah-Hartman Jun 12, 2015

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>

parent 3919c3d5

drivers/staging/comedi/drivers/das08.c

+18 −3

Original line number	Diff line number	Diff line
		@@ -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;