Add fixed-point temp calculation as backup option.

We prefer to use floating-point version but we might need fixed-point
version in the future if we don't have FPU support. If CONFIG_FPU flag
is not set, fixed-point object temperature calculation would be used.

Signed-off-by: Vic Yang <victoryang@chromium.org>

BUG=chrome-os-partner:7801
TEST=none

Change-Id: I69364b10bedf1351206e52266d669b4c566bd6f6

1 files changed, 46 insertions, 0 deletions

diff --git a/common/temp_sensor.c b/common/temp_sensor.c
index 80b3166e32..9313623213 100644
--- a/common/temp_sensor.c
+++ b/common/temp_sensor.c
@@ -53,6 +53,7 @@ int temp_sensor_tmp006_read_die_temp(const struct temp_sensor_t* sensor)
  */
 int temp_sensor_tmp006_calculate_object_temp(int Tdie_i, int Vobj_i, int S0_i)
 {
+#ifdef CONFIG_FPU
 	float Tdie, Vobj, S0;
 	float Tx, S, Vos, Vx, fv, Tobj, T4;
 	int Tobj_i;
@@ -80,6 +81,51 @@ int temp_sensor_tmp006_calculate_object_temp(int Tdie_i, int Vobj_i, int S0_i)
 	disable_fpu(Tobj_i);
 
 	return Tobj_i;
+#else
+	/* This is the fixed-point version of object temperature calculation.
+	 * Should be accurate but it is hard to prevent and debug
+	 * overflow/underflow problem. Only use this version if there is no
+	 * FPU support.
+	 * Division is delayed when possible to preserve precision, but should
+	 * not cause overflow.
+	 * Assuming Tdie is between 200K and 400K, and S0 between 3e-14 and
+	 * 9e-14, the maximum value during the calculation should be less than
+	 * (1 << 30), which fits in int32_t.
+	 */
+	int32_t Tx, S19, Vos, Vx, fv9, ub, lb;
+
+	Tx = Tdie - 29815;
+	/* S19 is the sensitivity multipled by 1e19 */
+	S19 = S0 * (100000 + 175 * Tx / 100 -
+			1678 * Tx / 100 * Tx / 100000) / 1000;
+	/* Vos is the offset voltage in nV */
+	Vos = -29400 - 570 * Tx / 100 + 463 * Tx / 100 * Tx / 10000;
+	Vx = Vobj - Vos;
+	/* fv9 is Seebeck coefficient f(Vobj) multipled by 1e9 */
+	fv9 = Vx + 134 * Vx / 100000 * Vx / 100000;
+
+	/* The last step in the calculation involves square root, so we use
+	 * binary search.
+	 * Assuming the object temperature is between 200K and 400K, the search
+	 * should take at most 14 iterations.
+	 */
+	ub = 40000;
+	lb = 20000;
+	while (lb != ub) {
+		int32_t t, rhs, lhs;
+
+		t = (ub + lb) / 2;
+		lhs = t / 100 * t / 10000 * t / 10000 * (S19/100) / 1000 * t;
+		rhs = Tdie / 100 * Tdie / 10000 * Tdie / 10000 * (S19/100) /
+			1000 * Tdie + fv9 * 1000;
+		if (lhs > rhs)
+			ub = t;
+		else
+			lb = t + 1;
+	}
+
+	return ub;
+#endif
 }
 
 int temp_sensor_tmp006_read_object_temp(const struct temp_sensor_t* sensor)