[PATCH] thermal: imx: update formula for thermal sensor

[Philipp Zabel]

Hi,

On Thu, Feb 13, 2014 at 2:02 PM, Shawn Guo <shawn.guo at linaro.org> wrote:
>
> Copy LAKML and pengutronix folks in case they have comments.
>
> On Wed, Feb 12, 2014 at 06:06:35PM +0800, Anson Huang wrote:
> > Thermal sensor used to need two calibration points which are
> > in fuse map to get a slope for converting thermal sensor's raw
> > data to real temperature in degree C. Due to the chip calibration
> > limitation, hardware team provides an universal formula to get
> > real temperature from internal thermal sensor raw data:
> >
> > Slope = 0.4297157 - (0.0015976 * 25C fuse);

I have an i.MX6Q 1.2 with 1417 counts at 25°C and 1296 counts at 105°C fused.
Assuming that those values are fused correctly, with the old formula I get:

t1=25, n1=1417, t2=105, n2=1296
c1=1000*(t1-t2)/(n1-n2)=-662, c2=1000*t2-c1*n2=962952
T_meas=c2+c1*N_meas=962952-662*N_meas

N_meas=1417 --> T_meas = 24898
N_meas=1296 --> T_meas = 105000

With the new formula, on the other hand, I would get:

t1=25, n1=1417
F0=10000000, F1=15976, 4297157
c1=F0*1000/(F1*n1-F2)=545, c2=n1*c1+1000*25=797265
T_meas=c2-N_meas*c1=797265-N_meas*545

N_meas=1417 --> T_meas = 25000
N_meas=1296 --> T_meas = 90945

That's off by over 14°C!

The default passive cooling trip point is at 85°C,
which translated to 1326 counts before:
alarm_value=(alarm_temp-c2)/c1=(85000-962952)/(-662)=1326

With the new formula the trip count is only
alarm_value=(c2-alarm_temp)/c1=(797265-85000)/545=1306
That translates to 98.4°C according to the old formula. I think
that is awfully close to the critical trip point at 100°C, which,
according to the old formula, corresponded to 1303 counts.

The new critical trip point for 100°C is at 1279 counts. According
to the old formula this corresponds to 116.2°C, well over the
rated maximum of 105°C.

Do I need to be afraid for my old silicon?

> > Update the formula, as there will be no hot point calibration
> > data in fuse map from now on.

I wonder if it would be better to keep using the high temperature
calibration point if it is fused at all.

regards
Philipp