[nrf fromlist] soc: nrf54l15: Fix HFXO INTCAP code calculation rounding error

The implementation of formula, to calculate HFXO INTCAP code,
had a rounding error. That may lead so small deviation of HFXO
clock accuracy.

The IPS formula uses capacitance values in piko Fartd units.
That requires use of floating point data types. To avoid that
implementation of the formula uses femto Farats (1000 smaller unit).

In the former implementation conversion from femto Farad to piko Farad
was done just after reading of the desired capacitance from DTS.

To make sure the calculations are correct the change of unit must be
done at very end. Also rounding must be applied.

Also the formula was split and more comments were added to make the
implementation clear.

The commit fixes the implementation of the IPS formula.

Upstream PR: zephyrproject-rtos/zephyr#74668

Signed-off-by: Piotr Pryga <[email protected]>

Author: ppryga-nordic

soc/nordic/nrf54l/soc.c

@@ -119,11 +119,28 @@ static int nordicsemi_nrf54l_init(void)
 	 * where CAPACITANCE is the desired total load capacitance value in pF,
 	 * holding any value between 4.0 pF and 17.0 pF in 0.25 pF steps.
 	 */
-	uint32_t capvalue =
-		(((((DT_PROP(HFXO_NODE, load_capacitance_femtofarad) * 4UL) / 1000UL) - 22UL) *
-		  (uint32_t)(slope_m + 791) / 4UL) + (offset_m << 2UL)) >> 8UL;
 
-	nrf_oscillators_hfxo_cap_set(NRF_OSCILLATORS, true, capvalue);
+	/* NOTE 1: Requested HFXO internal capacitance in femto Faradas is used directly in formula
+	 *         to calculate INTCAP code. That is different than in case of LFXO.
+	 *
+	 * NOTE 2: PS formula uses piko Farads, the implementation of the formula uses femto Farads
+	 *         to avoid use of floating point data type.
+	 */
+	uint32_t cap_val_femto_f = DT_PROP(HFXO_NODE, load_capacitance_femtofarad);
+
+	uint32_t mid_val_intcap = (((cap_val_femto_f - 5500UL) * (uint32_t)(slope_m + 791UL))
+		 + (offset_m << 2UL) * 1000UL) >> 8UL;
+
+	/* Convert the calculated value to piko Farads */
+	uint32_t hfxo_intcap = mid_val_intcap / 1000;
+
+	/* Round based on fractional part */
+	if (mid_val_intcap % 1000 >= 500) {
+		hfxo_intcap++;
+	}
+
+	nrf_oscillators_hfxo_cap_set(NRF_OSCILLATORS, true, hfxo_intcap);
+
 #elif DT_ENUM_HAS_VALUE(HFXO_NODE, load_capacitors, external)
 	nrf_oscillators_hfxo_cap_set(NRF_OSCILLATORS, false, 0);
 #endif