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RM0351 Clock recovery system (CRS) (only valid for STM32L496xx/4A6xx devices)
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FELIM value
The selection of the FELIM value is closely coupled with the HSI48 oscillator characteristics
and its typical trimming step size. The optimal value corresponds to half of the trimming step
size, expressed as a number of HSI48 oscillator clock ticks. The following formula can be
used:
FELIM = (f
TARGET
/ f
SYNC
) * STEP[%] / 100% / 2
The result should be always rounded up to the nearest integer value in order to obtain the
best trimming response. If frequent trimming actions are not wanted in the application, the
trimming hysteresis can be increased by increasing slightly the FELIM value.
The reset value of the FELIM field corresponds to (f
TARGET
/ f
SYNC
) = 48000 and to a typical
trimming step size of 0.14%.
Caution: There is no hardware protection from a wrong configuration of the RELOAD and FELIM
fields which can lead to an erratic trimming response. The expected operational mode
requires proper setup of the RELOAD value (according to the synchronization source
frequency), which is also greater than 128 * FELIM value (OUTRANGE limit).
7.4 CRS low-power modes
7.5 CRS interrupts
Table 35. Effect of low-power modes on CRS
Mode Description
Sleep
No effect.
CRS interrupts cause the device to exit the Sleep mode.
Stop CRS registers are frozen.
The CRS stops operating until the Stop or Standby mode is exited and the HSI48 oscillator
restarted.
Standby
Table 36. Interrupt control bits
Interrupt event Event flag
Enable
control bit
Clear
flag bit
Expected synchronization ESYNCF ESYNCIE ESYNCC
Synchronization OK SYNCOKF SYNCOKIE SYNCOKC
Synchronization warning SYNCWARNF SYNCWARNIE SYNCWARNC
Synchronization or trimming error
(TRIMOVF, SYNCMISS, SYNCERR)
ERRF ERRIE ERRC