On 2016-12-10 23:50, Vesa Jääskeläinen wrote:
Texas Instrument's TPS65910 has support for compensating RTC crystal
inaccuracies. When enabled every hour RTC counter value will be compensated
with two's complement value.
Vesa Jääskeläinen (2):
drivers: rtc: rtc-tps65910: Add RTC calibration support
drivers: rtc: rtc-tps65910: use 'unsigned int' instead of 'unsigned'
in arguments
drivers/rtc/rtc-tps65910.c | 135 ++++++++++++++++++++++++++++++++++++++++++-
include/linux/mfd/tps65910.h | 1 +
2 files changed, 135 insertions(+), 1 deletion(-)
This version now uses 'offset' sysfs file for calibration purposes.
It supports rounding so if one calculates absolute 'offset' compensation
value then it should give "optimal" calibration.
I believe one should calculate compensation value like:
offset = ((measured_freq/32768)-1)*10^9
So if meaured_freq == 32771.2768, then matching offset == 100000.
Or if mesured_freq == 32764.7232, then matching offset == -100000.
'offset' compensation value is following existing rtc.txt definition
meaning positive values causes time to slowdown and negative values to
fasten up. Which I believe should be other way around to be more
logical. This is easy to swap if I just understood it wrongly.
'offset' value calculation assumes "common" understanding how ppm values
are used, eg. it is negated compensation value (expect now positive).
Eg. if RTC crystal is off by +100ppm then 'offset' value must be 100ppm
(write 100000 to 'offset').
I believe offset documentation in rtc.txt should be clarified on what
value means.
'offset' value has limits due to RTC chip itself, if out-of-limits value
is given -ERANGE is returned.
Due to RTC chip's implementation -- it loses RTC compensation enable bit
on reset, so compensation value must be written on every boot at least
once to be active.
When compensation is not enabled "cat offset" returns value. There is
currently no way to know from users space (without i2cget -f) whether
compensation is enabled or not. If this should not be the case it can be
easily fixed, -EINVAL might do the trick.
Thanks,
Vesa Jääskeläinen
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