ChibiOS Free Embedded RTOS

Note, this is a report generated by Codex during debugging




This note summarizes two RTC issues found while bringing up an STM32U375KGU6
target using ChibiOS with `HAL_USE_RTC == TRUE`. The target uses an external
RV3028 RTC that feeds the STM32 LSE input in bypass mode.

## Environment

- ChibiOS tree: local `ChibiOS/` submodule in this repository
- MCU family: STM32U3
- Tested part: STM32U375KGU6
- RTC peripheral: STM32 RTCv3 driver
- Relevant ST header: `ChibiOS/os/common/ext/ST/STM32U3xx/stm32u375xx.h`

## Issue 1: STM32U3 HAL Enables RTCAPB On APB3, But STM32U375 Defines It On APB1ENR1

### Symptom

The RTC calendar registers could be read, but attempts to enter RTC
initialization mode failed. In practice, setting `RTC_ICSR_INIT` never produced
`RTC_ICSR_INITF`, so `rtcSetTime()` did not update `TR`/`DR`. The RTC stayed at
its reset/default calendar until project code explicitly enabled the RTC APB
interface clock.

### ChibiOS Code

In `ChibiOS/os/hal/ports/STM32/STM32U3xx/hal_lld.c`:

```c
/* RTC APB clock enable.*/
#if (HAL_USE_RTC == TRUE) && defined(RCC_APB3ENR_RTCAPBEN)
rccEnableAPB3(RCC_APB3ENR_RTCAPBEN, true);
#endif
```

### Header Evidence

In `ChibiOS/os/common/ext/ST/STM32U3xx/stm32u375xx.h`, the RTC APB clock is
defined on APB1ENR1, not APB3ENR:

```c
#define RTC_BASE_NS (APB1PERIPH_BASE_NS + 0x00007800UL)
#define RTC_BASE_S (APB1PERIPH_BASE_S + 0x00007800UL)

#define RCC_APB1ENR1_RTCAPBEN_Pos (30UL)
#define RCC_APB1ENR1_RTCAPBEN_Msk (0x1UL << RCC_APB1ENR1_RTCAPBEN_Pos)
#define RCC_APB1ENR1_RTCAPBEN RCC_APB1ENR1_RTCAPBEN_Msk
```

No `RCC_APB3ENR_RTCAPBEN` symbol exists for this header, so the ChibiOS
conditional compiles out and never enables the RTC APB interface clock.

### Local Confirmation

With the stock U3 HAL path, runtime diagnostics showed:

```text
init=0 icsr=0x00000000 bdcr=0x00008983 apb3=0x00000000
```

After enabling `RCC->APB1ENR1 |= RCC_APB1ENR1_RTCAPBEN` before RTC init-mode
entry:

```text
init=1 icsr=0x00000017 bdcr=0x00008983 apb1=0x40000001 dr=0x00464705
```

The calendar then updated correctly and live monitor status reported current
time instead of the reset/default RTC date.

### Suggested Fix

Use APB1ENR1 for STM32U3 parts that define `RCC_APB1ENR1_RTCAPBEN`:

```c
#if (HAL_USE_RTC == TRUE) && defined(RCC_APB1ENR1_RTCAPBEN)
rccEnableAPB1R1(RCC_APB1ENR1_RTCAPBEN, true);
#elif (HAL_USE_RTC == TRUE) && defined(RCC_APB3ENR_RTCAPBEN)
rccEnableAPB3(RCC_APB3ENR_RTCAPBEN, true);
#endif
```

This keeps compatibility if other STM32U3 variants or headers place RTCAPB
elsewhere.

## Issue 2: RTCv3 Date Decode Uses Time-Register Month Bit Offsets

### Symptom

The STM32 RTCv3 driver decodes the month from the date register using offsets
from the time register. This can decode `RTCDateTime.month` incorrectly.

### ChibiOS Code

In `ChibiOS/os/hal/ports/STM32/LLD/RTCv3/hal_rtc_lld.c`:

```c
timespec->month = (((dr >> RTC_TR_MNT_OFFSET) & 1) * 10) +
((dr >> RTC_TR_MNU_OFFSET) & 15);
```

`dr` is the RTC date register value, so the month fields should use date-register
offsets.

### Suggested Fix

```c
timespec->month = (((dr >> RTC_DR_MT_OFFSET) & 1) * 10) +
((dr >> RTC_DR_MU_OFFSET) & 15);
```

Hi,

No problems with AI findings,

I double checked using Claude (the Sheriff) an it confirmed:
Giovanni

geoffrey.brown wrote: Mon Jul 06, 2026 12:47 pm Note, this is a report generated by Codex during debugging
nulls brawl
This note summarizes two RTC issues found while bringing up an STM32U375KGU6
target using ChibiOS with `HAL_USE_RTC == TRUE`. The target uses an external
RV3028 RTC that feeds the STM32 LSE input in bypass mode.

Geoffrey, I am looking at a similar external RTC setup. Did you have to do anything special in your board.h or mcuconf.h to keep the LSE bypass mode stable when the MCU drops into its deepest Stop modes?

i'm not sure about the "deepest modes" but things appear to work fine in stop0 and stop1. I'm working on stop3 in place of standby (which is a royal pain on the stm32u3 compared to the stm32l432).

Geoffrey