On Tue, Jun 02, 2015 at 01:48:27PM -0500, Chris Lesiak wrote:
> The function get_ohm_of_thermistor has both the measured voltage and the
> pullup voltage available in microvolts. But it was promptly converting
> both to millivolts before using them to calculate the thermistor
> resistance. That conversion unnecessarily hurt the precision of the
> calculation.
>
> For example, take the ncpXXwb473 connected to 5000 mV and pulled down
> through a 47000 ohm resistor. At 25 C, the resistance of the thermistor
> is 47000 ohms. The measured voltage will be 2500 mV. If we measure
> instead 2501 mV, then the calculated resistance will be 46962 ohms --
> a difference of 38 ohms. So the precision of the resistance estimate
> could be increased by 38X by doing the calculations in microvolts.
>
> Signed-off-by: Chris Lesiak <chris.lesiak@xxxxxxxxx>
> ---
> drivers/hwmon/ntc_thermistor.c | 23 ++++++++++-------------
> 1 file changed, 10 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/hwmon/ntc_thermistor.c b/drivers/hwmon/ntc_thermistor.c
> index e08ed47..e112814 100644
> --- a/drivers/hwmon/ntc_thermistor.c
> +++ b/drivers/hwmon/ntc_thermistor.c
> @@ -310,30 +310,27 @@ static inline u64 div64_u64_safe(u64 dividend, u64 divisor)
> static int get_ohm_of_thermistor(struct ntc_data *data, unsigned int uv)
> {
> struct ntc_thermistor_platform_data *pdata = data->pdata;
> - u64 mv = uv / 1000;
> - u64 pmv = pdata->pullup_uv / 1000;
> + u32 puv = pdata->pullup_uv;
> u64 n, puo, pdo;
> puo = pdata->pullup_ohm;
> pdo = pdata->pulldown_ohm;
>
> - if (mv == 0) {
> - if (pdata->connect == NTC_CONNECTED_POSITIVE)
> - return INT_MAX;
> - return 0;
> - }
> - if (mv >= pmv)
> + if (uv <= 0)
Hi Chris,
nitpick, but like mv, uv is unsigned and can not be < 0.
Thanks,
Guenter
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