RM0444 Rev 5 225/1390
RM0444 Clock recovery system (CRS)
231
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 must be always rounded up to the nearest integer value to obtain the best
trimming response. If frequent trimming actions are not needed in the application, the
hysteresis can be increased by slightly increasing 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).
6.5 CRS low-power modes
6.6 CRS interrupts
Table 39. 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 mode is exited and the
HSI48 oscillator restarted.
Standby The CRS peripheral is powered down and must be reinitialized after exiting Standby mode.
Table 40. 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
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