Each time we need to read a sample (from the sysfs interface, since the
driver supports only it) the driver writes the configuration register
with the proper settings needed to perform the said read, then it runs
another xfer to actually read the resulting value. Most notably the
configuration register is updated to set the ADC internal MUX depending by
which channel the read targets.
Unfortunately this seems not enough to ensure correct operation because
the ADC works in a pipelined-like fashion and the new configuration isn't
applied in time.
The ADC alternates two phases: acquisition and conversion. During the
acquisition phase the ADC samples the analog signal in an internal
capacitor; in the conversion phase the ADC performs the actual analog to
digital conversion of the stored voltage. Note that of course the MUX
needs to be set to the proper channel when the acquisition phase is
performed.
Once the conversion phase has been completed, the device automatically
switches back to a new acquisition; on the other hand the device switches
from acquisition to conversion on the rising edge of SPI cs signal (that
is when the xfer finishes).
Only after both two phases have been completed (with the proper settings
already written in the configuration register since the beginning) it is
possible to read the outcome from SPI bus.
With the current driver implementation, we end up in the following
situation:
_______ 1st xfer ____________ 2nd xfer ___________________
SPI cs.. \_________/ \_________/
SPI rd.. idle |(val N-2)+ idle | val N-1 + idle ...
SPI wr.. idle | cfg N + idle | (X) + idle ...
------------------------ + -------------------- + ------------------
AD .. acq N-1 + cnv N-1 | acq N + cnv N | acq N+1
As shown in the diagram above, the value we read in the Nth read belongs
to configuration setting N-1.
In case the configuration is not changed (config[N] == config[N-1]), then
we still get correct data, but in case the configuration changes (i.e.
switching the MUX on another channel), we get wrong data (data from the
previously selected channel).
This patch fixes this by performing one more "dummy" transfer in order to
ending up in reading the data when it's really ready, as per the following
timing diagram.
_______ 1st xfer ____________ 2nd xfer ___________ 3rd xfer ___
SPI cs.. \_________/ \_________/ \_________/
SPI rd.. idle |(val N-2)+ idle |(val N-1)+ idle | val N + ..
SPI wr.. idle | cfg N + idle | (X) + idle | (X) + ..
------------------------ + -------------------- + ------------------- + --
AD .. acq N-1 + cnv N-1 | acq N + cnv N | acq N+1 | ..
NOTE: in the latter case (cfg changes), the acquisition phase for the
value to be read begins after the 1st xfer, that is after the read request
has been issued on sysfs. On the other hand, if the cfg doesn't change,
then we can refer to the fist diagram assuming N == (N - 1); the
acquisition phase _begins_ before the 1st xfer (potentially a lot of time
before the read has been issued via sysfs, but it _ends_ after the 1st
xfer, that is _after_ the read has started. This should guarantee a
reasonably fresh data, which value represents the voltage that the sampled
signal has after the read start or maybe just around it.
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Charles-Antoine Couret <charles-antoine.couret@essensium.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
This driver supports 14-bits and 16-bits devices. All of them have a 14-bit
configuration registers. All SPI trasfers, for reading AD conversion
results and for writing the configuration register, fit in two bytes.
The driver always uses 4-bytes xfers which seems at least pointless (maybe
even harmful). This patch trims the SPI xfer len and the buffer size to
two bytes.
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
The device could be configured to spit out also the configuration word
while reading the AD result value (in the same SPI xfer) - this is called
"readback" in the device datasheet.
The driver checks if readback is enabled and it eventually adjusts the SPI
xfer length and it applies proper shifts to still get the data, discarding
the configuration word.
The readback option is actually never enabled (the driver disables it), so
the said checks do not serve for any purpose.
Since enabling the readback option seems not to provide any advantage (the
driver entirely sets the configuration word without relying on any default
value), just kill the said, unused, code.
Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Compatible with AD7682 and AD7689 chips.
It is a Analog Devices ADC driver 14/16 bits 4/8 channels
with SPI protocol
Datasheet of the device:
http://www.analog.com/media/en/technical-documentation/data-sheets/AD7949.pdf
Signed-off-by: Charles-Antoine Couret <charles-antoine.couret@essensium.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Andrea Merello