The RTC/LCD clock can be derived from HSE (prescaled). In this case, the prescaler must be chosen appropriately so as to get an RTC/LCD frequency within the admissible range.
Using CubeMX, when the limits are exceeded, the tooltip says:
* (tooltip over the HSE-to-HSE_RTC prescaler): frequency must be in [0, 1] MHz
* (tooltip over RTC): frequency must be in [0, 1000] kHz
* (tooltip over LCD): frequency must be 1000 kHz
The reference manual RM0367 says something different, though:
* Section 7.2 (Clocks):
The RTC/LCD clock which is derived from the following clock sources:
– LSE clock,
– LSI clock,
– 4 MHz HSE_RTC (HSE divided by a programmable prescaler).
* Section 7.2.12 (RTC and LCD clock):
The RTC and LCD have the same clock source which can be either the LSE, the LSI, or the HSE 4 MHz clock (HSE divided by a programmable prescaler).
* Section 7.2.15 (Internal/external clock measurement using TIM21):
The HSE_RTC frequency (HSE divided by a programmable prescaler) being relatively high (4 MHz), the relative frequency measurement is not very accurate.
* Section 7.3.1 (Clock control register (RCC_CR)):
RTCPRE[1:0] ... These bits are set and reset by software to obtain a 4 MHz clock from HSE.
* Section 17.4.1 (LCD functional description):
The frequency generator allows you to achieve various LCD frame rates starting from an LCD input clock frequency (LCDCLK) which can vary from 32 kHz up to 1 MHz.
3 different clock sources can be used to provide the LCD clock (LCDCLK/RTCCLK):
• 32 kHz Low speed external RC (LSE)
• 32 kHz Low speed internal RC (LSI)
• High speed external (HSE) divided by 2, 4, 8 or 16 to obtain a 1 MHz clock
* Table 181 (Revision history):
14-Nov-2016: HSE RTC clock source frequency changed to 4 MHz.
What is the proper frequency for LCD clock, when derived from HSE?
