soc: nrf53: Fix extraction of the XOSC32MTRIM.SLOPE bitfield

The value in this bitfield is provided in the two's complement form,
so it requires special handling. Previously, it was read as just an
unsigned value and this could result in a wrongly computed CAPVALUE.

Signed-off-by: Andrzej Głąbek <[email protected]>

Author: anangl

soc/arm/nordic_nrf/nrf53/soc.c

@@ -134,16 +134,27 @@ static int nordicsemi_nrf53_init(const struct device *arg)
 #if defined(CONFIG_SOC_HFXO_CAP_INTERNAL)
 	/* This register is only accessible from secure code. */
 	uint32_t xosc32mtrim = soc_secure_read_xosc32mtrim();
+	/* The SLOPE field is in the two's complement form, hence this special
+	 * handling. Ideally, it would result in just one SBFX instruction for
+	 * extracting the slope value, at least gcc is capable of producing such
+	 * output, but since the compiler apparently tries first to optimize
+	 * additions and subtractions, it generates slightly less than optimal
+	 * code.
+	 */
+	uint32_t slope_field = (xosc32mtrim & FICR_XOSC32MTRIM_SLOPE_Msk)
+			       >> FICR_XOSC32MTRIM_SLOPE_Pos;
+	uint32_t slope_mask = FICR_XOSC32MTRIM_SLOPE_Msk
+			      >> FICR_XOSC32MTRIM_SLOPE_Pos;
+	uint32_t slope_sign = (slope_mask - (slope_mask >> 1));
+	int32_t slope = (int32_t)(slope_field ^ slope_sign) - (int32_t)slope_sign;
+	uint32_t offset = (xosc32mtrim & FICR_XOSC32MTRIM_OFFSET_Msk)
+			  >> FICR_XOSC32MTRIM_OFFSET_Pos;
 	/* As specified in the nRF5340 PS:
 	 * CAPVALUE = (((FICR->XOSC32MTRIM.SLOPE+56)*(CAPACITANCE*2-14))
 	 *            +((FICR->XOSC32MTRIM.OFFSET-8)<<4)+32)>>6;
 	 * where CAPACITANCE is the desired capacitor value in pF, holding any
 	 * value between 7.0 pF and 20.0 pF in 0.5 pF steps.
 	 */
-	uint32_t slope = (xosc32mtrim & FICR_XOSC32MTRIM_SLOPE_Msk)
-			 >> FICR_XOSC32MTRIM_SLOPE_Pos;
-	uint32_t offset = (xosc32mtrim & FICR_XOSC32MTRIM_OFFSET_Msk)
-			  >> FICR_XOSC32MTRIM_OFFSET_Pos;
 	uint32_t capvalue =
 		((slope + 56) * (CONFIG_SOC_HFXO_CAP_INT_VALUE_X2 - 14)
 		 + ((offset - 8) << 4) + 32) >> 6;