Clock recovery system (CRS) RM0440
342/2126 RM0440 Rev 4
8.4.4 Frequency error evaluation and automatic trimming
The measured frequency error is evaluated by comparing its value with a set of limits:
– TOLERANCE LIMIT, given directly in the FELIM field of the CRS_CFGR register
– WARNING LIMIT, defined as 3 * FELIM value
– OUTRANGE (error limit), defined as 128 * FELIM value
The result of this comparison is used to generate the status indication and also to control the
automatic trimming which is enabled by setting the AUTOTRIMEN bit in the CRS_CR
register:
• When the frequency error is below the tolerance limit, it means that the actual trimming
value in the TRIM field is the optimal one, hence no trimming action is needed.
– SYNCOK status indicated
– TRIM value not changed in AUTOTRIM mode
• When the frequency error is below the warning limit but above or equal to the tolerance
limit, it means that some trimming action is necessary but that adjustment by one
trimming step is enough to reach the optimal TRIM value.
– SYNCOK status indicated
– TRIM value adjusted by one trimming step in AUTOTRIM mode
• When the frequency error is above or equal to the warning limit but below the error
limit, it means that a stronger trimming action is necessary, and there is a risk that the
optimal TRIM value will not be reached for the next period.
– SYNCWARN status indicated
– TRIM value adjusted by two trimming steps in AUTOTRIM mode
• When the frequency error is above or equal to the error limit, it means that the
frequency is out of the trimming range. This can also happen when the SYNC input is
not clean or when some SYNC pulse is missing (for example when one USB SOF is
corrupted).
– SYNCERR or SYNCMISS status indicated
– TRIM value not changed in AUTOTRIM mode
Note: If the actual value of the TRIM field is so close to its limits that the automatic trimming would
force it to overflow or underflow, then the TRIM value is set just to the limit and the
TRIMOVF status is indicated.
In AUTOTRIM mode (AUTOTRIMEN bit set in the CRS_CR register), the TRIM field of
CRS_CR is adjusted by hardware and is read-only.
8.4.5 CRS initialization and configuration
RELOAD value
The RELOAD value must be selected according to the ratio between the target frequency
and the frequency of the synchronization source after prescaling. It is then decreased by
one to reach the expected synchronization on the zero value. The formula is the following:
RELOAD = (f
TARGET
/ f
SYNC
) - 1
The reset value of the RELOAD field corresponds to a target frequency of 48 MHz and a
synchronization signal frequency of 1 kHz (SOF signal from USB).
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