diff options
Diffstat (limited to 'drivers/thermal/imx_thermal.c')
| -rw-r--r-- | drivers/thermal/imx_thermal.c | 42 |
1 files changed, 19 insertions, 23 deletions
diff --git a/drivers/thermal/imx_thermal.c b/drivers/thermal/imx_thermal.c index 21b8c4c4da3c..c08883dff2cb 100644 --- a/drivers/thermal/imx_thermal.c +++ b/drivers/thermal/imx_thermal.c @@ -359,32 +359,28 @@ static int imx_init_calib(struct platform_device *pdev, u32 ocotp_ana1) } /* - * Sensor data layout: - * [31:20] - sensor value @ 25C - * Use universal formula now and only need sensor value @ 25C - * slope = 0.4297157 - (0.0015976 * 25C fuse) + * The sensor is calibrated at 25 °C (aka T1) and the value measured + * (aka N1) at this temperature is provided in bits [31:20] in the + * i.MX's OCOTP value ANA1. + * To find the actual temperature T, the following formula has to be used + * when reading value n from the sensor: + * + * T = T1 + (N - N1) / (0.4297157 - 0.0015976 * N1) °C + * = [T1 - N1 / (0.4297157 - 0.0015976 * N1) °C] + N / (0.4297157 - 0.0015976 * N1) °C + * = [T1 + N1 / (0.0015976 * N1 - 0.4297157) °C] - N / (0.0015976 * N1 - 0.4297157) °C + * = c2 - c1 * N + * + * with + * + * c1 = 1 / (0.0015976 * N1 - 0.4297157) °C + * c2 = T1 + N1 / (0.0015976 * N1 - 0.4297157) °C + * = T1 + N1 * C1 */ n1 = ocotp_ana1 >> 20; - t1 = 25; /* t1 always 25C */ + t1 = 25; /* °C */ - /* - * Derived from linear interpolation: - * slope = 0.4297157 - (0.0015976 * 25C fuse) - * slope = (FACTOR2 - FACTOR1 * n1) / FACTOR0 - * (Nmeas - n1) / (Tmeas - t1) = slope - * We want to reduce this down to the minimum computation necessary - * for each temperature read. Also, we want Tmeas in millicelsius - * and we don't want to lose precision from integer division. So... - * Tmeas = (Nmeas - n1) / slope + t1 - * milli_Tmeas = 1000 * (Nmeas - n1) / slope + 1000 * t1 - * milli_Tmeas = -1000 * (n1 - Nmeas) / slope + 1000 * t1 - * Let constant c1 = (-1000 / slope) - * milli_Tmeas = (n1 - Nmeas) * c1 + 1000 * t1 - * Let constant c2 = n1 *c1 + 1000 * t1 - * milli_Tmeas = c2 - Nmeas * c1 - */ - temp64 = FACTOR0; - temp64 *= 1000; + temp64 = FACTOR0; /* 10^7 for FACTOR1 and FACTOR2 */ + temp64 *= 1000; /* to get result in °mC */ do_div(temp64, FACTOR1 * n1 - FACTOR2); data->c1 = temp64; data->c2 = n1 * data->c1 + 1000 * t1; |