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authorUwe Kleine-König <u.kleine-koenig@pengutronix.de>2017-11-30 10:17:36 +0100
committerEduardo Valentin <edubezval@gmail.com>2018-01-01 11:45:55 -0800
commitc5bbdb4ba30977a30f485e66c8af9b4c44f3798e (patch)
treed7bdf167a1b7267b4507d05ec9416ec4185b631f /drivers/thermal
parente4bb2240d4db6188f2e88610a68bda12e2bc98e1 (diff)
thermal: imx: improve comments describing algorithm for temp calculation
The description of the implemented algorithm is hardly understandable without having the right application note side-by-side to the code. Fix this by using shorter and more intuitive variable names, describe their meaning and transform a single formula instead of first talking about slope and then about "milli_Tmeas". There are no code changes. Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
Diffstat (limited to 'drivers/thermal')
-rw-r--r--drivers/thermal/imx_thermal.c42
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;

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