7. Clock recovery system (CRS)

This section applies only to STM32L49x/L4Ax devices.

7.1 CRS introduction

The clock recovery system (CRS) is an advanced digital controller acting on the internal fine-granularity trimmable RC oscillator HSI48. The CRS provides powerful means to evaluate the oscillator output frequency, based on comparison with a selectable synchronization signal. The CRS can perform automatic trimming adjustments based on the measured frequency error value, while keeping the possibility of a manual trimming.

The CRS is ideally suited to provide a precise clock to the USB peripheral. In this case, the synchronization signal can be derived from the start-of-frame (SOF) packet signalization on the USB bus, sent by a USB host at 1 ms intervals.

The synchronization signal can also be derived from the LSE oscillator output, or generated by user software.

7.2 CRS main features

• Selectable synchronization source with programmable prescaler and polarity:
- – External pin
- – LSE oscillator output
- – USB SOF packet reception
• Possibility to generate synchronization pulses by software
• Automatic oscillator trimming capability with no need of CPU action
• Manual control option for faster startup convergence
• 16-bit frequency error counter with automatic error value capture and reload
• Programmable limit for automatic frequency error value evaluation and status reporting
• Maskable interrupts/events:
- – Expected synchronization (ESYNC)
- – Synchronization OK (SYNCOK)
- – Synchronization warning (SYNCWARN)
- – Synchronization or trimming error (ERR)

7.3 CRS implementation

Table 35. CRS features

Feature	CRS
TRIM width	6 bits

7.4 CRS functional description

7.4.1 CRS block diagram

Figure 21. CRS block diagram

Figure 21. CRS block diagram. The diagram shows the internal architecture of the Clock Recovery System (CRS). It includes a GPIO pin for CRS_SYNC, an LSE (Low Speed External) oscillator connected to OSC32_IN and OSC32_OUT pins, and a USB block connected to USB_DP and USB_DM pins. The LSE and USB signals are inputs to a multiplexer labeled SYNCSRC. The output of the multiplexer goes to a SYNC divider block with a programmable binary prescaler (/1, /2, /4, ..., /128). The output of the divider is the SYNC signal. A software-generated synchronization event (SWSYNC) is also an input to the SYNC divider. The SYNC signal is fed into a 16-bit counter. The 16-bit counter is also connected to a TRIM block, a FEDIR block, a FECAP block, and a RELOAD block. The TRIM block is connected to the RCC (Reset and Clock Control) block, which contains an RC 48 MHz oscillator. The RC 48 MHz oscillator is connected to the 16-bit counter and also provides the HSI48 clock to the USB and RNG (Random Number Generator) blocks. The FEDIR and FECAP blocks are connected to the 16-bit counter and the TRIM block. The RELOAD block is connected to the 16-bit counter and the TRIM block. The diagram is labeled MSV34708V1.

7.4.2 Synchronization input

The CRS synchronization (SYNC) source, selectable through the CRS_CFGR register, can be the signal from the LSE clock or the USB SOF signal. For better robustness of the SYNC input, a simple digital filter (2 out of 3 majority votes, sampled by the HSI48 clock) is implemented to filter out glitches. This source signal has a configurable polarity, and can be divided by a programmable binary prescaler, to obtain a synchronization signal in a suitable frequency range (usually around 1 kHz).

For more information on the CRS synchronization source configuration, refer to Section 7.7.2 .

It is also possible to generate a synchronization event by software, by setting the SWSYNC bit in the CRS_CR register.

7.4.3 Frequency error measurement

The frequency error counter is a 16-bit down/up counter, reloaded with the RELOAD value on each SYNC event. It starts counting down until it reaches the zero value, where the ESYNC (expected synchronization) event is generated. Then it starts counting up to the OUTRANGE limit, where it eventually stops (if no SYNC event is received), and generates a SYNCMISS event. The OUTRANGE limit is defined as the frequency error limit (FELIM field of the CRS_CFGR register) multiplied by 128.

When the SYNC event is detected, the actual value of the frequency error counter and its counting direction are stored in the FECAP (frequency error capture) field and in the FEDIR (frequency error direction) bit of the CRS_ISR register. When the SYNC event is detected during the down-counting phase (before reaching the zero value), it means that the actual frequency is lower than the target (the TRIM value must be incremented). When it is detected during the up-counting phase, it means that the actual frequency is higher (the TRIM value must be decremented).

Figure 22. CRS counter behavior

The graph illustrates the CRS counter behavior. The y-axis is labeled 'CRS counter value' and has tick marks for 'RELOAD', 'OUTRANGE (128 x FELIM)', 'WARNING LIMIT (3 x FELIM)', and 'TOLERANCE LIMIT (FELIM)'. The x-axis is labeled with 'Trimming action' values (0, +2, +1, 0, -1, -2, 0) and 'CRS event' labels (SYNCEERR, SYNCWARN, SYNCOK, SYNCWARN, SYNCMISS). The counter starts at the RELOAD value and counts down (labeled 'Down') through the warning and tolerance limits, crossing zero at the ESYNC event. It then counts up (labeled 'Up') through the tolerance and warning limits, eventually reaching the OUTRANGE limit where the counter stops, labeled 'Frequency error counter stopped'. The graph also includes a break symbol on the y-axis and a small code 'MSv32122V1' in the bottom right corner.

Figure 22. CRS counter behavior. A graph showing the CRS counter value over time. The y-axis represents the CRS counter value, with markers for RELOAD, OUTRANGE (128 x FELIM), WARNING LIMIT (3 x FELIM), and TOLERANCE LIMIT (FELIM). The x-axis represents time, with markers for Trimming action (0, +2, +1, 0, -1, -2, 0) and CRS events (SYNCEERR, SYNCWARN, SYNCOK, SYNCWARN, SYNCMISS). The counter starts at RELOAD, counts down (Down) through the warning and tolerance limits, crosses zero at ESYNC, counts up (Up) through the tolerance and warning limits, and finally reaches the OUTRANGE limit where it stops (Frequency error counter stopped).

7.4.4 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 $3 \times \text{FELIM}$ value
• OUTRANGE (error limit), defined as $128 \times \text{FELIM}$ 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, a stronger trimming action is necessary, and there is a risk that the optimal TRIM value is not 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, 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 close to its limits and the automatic trimming can force it to overflow or underflow, the TRIM value is set 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.

7.4.5 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. This value is decreased by 1, to reach the expected synchronization on the zero value. The formula is the following:

\text{RELOAD} = (f_{\text{TARGET}} / f_{\text{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).

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 oscillator clock ticks. The following formula can be used:

\text{FELIM} = (f_{\text{TARGET}} / f_{\text{SYNC}}) * \text{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 $(f_{\text{TARGET}} / f_{\text{SYNC}}) = 48000$ , and to a typical trimming step size of 0.14%.

Note: The trimming step size depends upon the product, check the datasheet for accurate setting.

Caution: There is no hardware protection from a wrong configuration of the RELOAD and FELIM fields, this 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 $128 * \text{FELIM}$ value (OUTRANGE limit).

7.5 CRS low-power modes

Table 36. 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 is restarted.
Standby	The CRS peripheral is powered down and must be reinitialized after exiting Standby mode.
Shutdown	The CRS peripheral is powered down and must be reinitialized after exiting Shutdown mode.

7.6 CRS interrupts

Table 37. 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

7.7 CRS registers

Refer to Section 1.2 on page 69 for a list of abbreviations used in register descriptions.

The peripheral registers can be accessed only by words (32-bit).

7.7.1 CRS control register (CRS_CR)

Address offset: 0x00

Reset value: 0x0000 2000

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16
Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.
15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Res.	Res.	TRIM[5:0]						SW SYNC	AUTO TRIMEN	CEN	Res.	ESYNC IE	ERR IE	SYNC WARNIE	SYNC OKIE
		rw	rw	rw	rw	rw	rw	rl_w1	rw	rw		rw	rw	rw	rw

Bits 31:14 Reserved, must be kept at reset value.

Bits 13:8 TRIM[5:0] : HSI48 oscillator smooth trimming

These bits provide a user-programmable trimming value to the HSI48 oscillator. They can be programmed to adjust to variations in voltage and temperature that influence the oscillator frequency.

The default value is 32, corresponding to the middle of the trimming interval. The trimming step is specified in the product datasheet. A higher TRIM value corresponds to a higher output frequency.

When the AUTOTRIMEN bit is set, this field is controlled by hardware and is read-only.

Bit 7 SWSYNC : Generate software SYNC event

This bit is set by software in order to generate a software SYNC event. It is automatically cleared by hardware.

0: No action

1: A software SYNC event is generated.

Bit 6 AUTOTRIMEN : Automatic trimming enable

This bit enables the automatic hardware adjustment of TRIM bits according to the measured frequency error between two SYNC events. If this bit is set, the TRIM bits are read-only. The TRIM value can be adjusted by hardware by one or two steps at a time, depending on the measured frequency error value. Refer to Section 7.4.4 for more details.

0: Automatic trimming disabled, TRIM bits can be adjusted by the user.

1: Automatic trimming enabled, TRIM bits are read-only and under hardware control.

Bit 5 CEN : Frequency error counter enable

This bit enables the oscillator clock for the frequency error counter.

0: Frequency error counter disabled

1: Frequency error counter enabled

When this bit is set, the CRS_CFGR register is write-protected and cannot be modified.

Bit 4 Reserved, must be kept at reset value.

Bit 3 ESYNCIE : Expected SYNC interrupt enable

0: Expected SYNC (ESYNCF) interrupt disabled

1: Expected SYNC (ESYNCF) interrupt enabled

Bit 2 ERRIE : Synchronization or trimming error interrupt enable

0: Synchronization or trimming error (ERRF) interrupt disabled

1: Synchronization or trimming error (ERRF) interrupt enabled

Bit 1 SYNCWARNIE : SYNC warning interrupt enable

0: SYNC warning (SYNCWARNF) interrupt disabled

1: SYNC warning (SYNCWARNF) interrupt enabled

Bit 0 SYNCOKIE : SYNC event OK interrupt enable

0: SYNC event OK (SYNCOKF) interrupt disabled

1: SYNC event OK (SYNCOKF) interrupt enabled

7.7.2 CRS configuration register (CRS_CFGR)

This register can be written only when the frequency error counter is disabled (the CEN bit is cleared in CRS_CR). When the counter is enabled, this register is write-protected.

Address offset: 0x04

Reset value: 0x2022 BB7F

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16
SYNCPOL	Res.	SYNCSRC[1:0]		Res.	SYNCDIV[2:0]			FELIM[7:0]
rw		rw	rw		rw	rw	rw	rw	rw	rw	rw	rw	rw	rw	rw
15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
RELOAD[15:0]
rw	rw	rw	rw	rw	rw	rw	rw	rw	rw	rw	rw	rw	rw	rw	rw

Bit 31 SYNCPOL : SYNC polarity selection

This bit is set and cleared by software to select the input polarity for the SYNC signal source.

0: SYNC active on rising edge (default)

1: SYNC active on falling edge

Bit 30 Reserved, must be kept at reset value.

Bits 29:28 SYNCSRC[1:0] : SYNC signal source selection

These bits are set and cleared by software to select the SYNC signal source.

00: GPIO selected as SYNC signal source

01: LSE selected as SYNC signal source

10: USB SOF selected as SYNC signal source (default).

11: Reserved

Note: When using USB LPM (link power management) and the device is in Sleep mode, the periodic USB SOF is not generated by the host. No SYNC signal is therefore provided to the CRS to calibrate the HSI48 oscillator on the run. To guarantee the required clock precision after waking up from Sleep mode, the LSE or reference clock on the GPIOs should be used as SYNC signal.

Bit 27 Reserved, must be kept at reset value.

Bits 26:24 SYNCDIV[2:0] : SYNC divider

These bits are set and cleared by software to control the division factor of the SYNC signal.

000: SYNC not divided (default)

001: SYNC divided by 2

010: SYNC divided by 4

011: SYNC divided by 8

100: SYNC divided by 16

101: SYNC divided by 32

110: SYNC divided by 64

111: SYNC divided by 128

Bits 23:16 FELIM[7:0] : Frequency error limit

FELIM contains the value to be used to evaluate the captured frequency error value latched in the FECAP[15:0] bits of the CRS_ISR register. Refer to Section 7.4.4 for more details about FECAP evaluation.

Bits 15:0 RELOAD[15:0] : Counter reload value

RELOAD is the value to be loaded in the frequency error counter with each SYNC event. Refer to Section 7.4.3 for more details about counter behavior.

7.7.3 CRS interrupt and status register (CRS_ISR)

Address offset: 0x08

Reset value: 0x0000 0000

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16
FECAP[15:0]
r	r	r	r	r	r	r	r	r	r	r	r	r	r	r	r
15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
FEDIR	Res.	Res.	Res.	Res.	TRIM OVF	SYNC MISS	SYNC ERR	Res.	Res.	Res.	Res.	ESYNCF	ERRF	SYNC WARNF	SYNC OKF
r					r	r	r					r	r	r	r

Bits 31:16 FECAP[15:0] : Frequency error capture

FECAP is the frequency error counter value latched in the time of the last SYNC event. Refer to Section 7.4.4 for more details about FECAP usage.

Bit 15 FEDIR : Frequency error direction

FEDIR is the counting direction of the frequency error counter latched in the time of the last SYNC event. It shows whether the actual frequency is below or above the target.

0: Up-counting direction, the actual frequency is above the target

1: Down-counting direction, the actual frequency is below the target

Bits 14:11 Reserved, must be kept at reset value.

Bit 10 TRIMOVF : Trimming overflow or underflow

This flag is set by hardware when the automatic trimming tries to over- or under-flow the TRIM value. An interrupt is generated if the ERRRIE bit is set in the CRS_CR register. It is cleared by software by setting the ERRRC bit in the CRS_ICR register.

0: No trimming error signaled

1: Trimming error signaled

Bit 9 SYNCMISS: SYNC missed

This flag is set by hardware when the frequency error counter reaches value $$ FELIM * 128 $$ and no SYNC is detected, meaning either that a SYNC pulse was missed, or the frequency error is too big (internal frequency too high) to be compensated by adjusting the TRIM value, hence some other action must be taken. At this point, the frequency error counter is stopped (waiting for a next SYNC), and an interrupt is generated if the ERRIE bit is set in the CRS_CR register. It is cleared by software by setting the ERRC bit in the CRS_ICR register.

0: No SYNC missed error signaled

1: SYNC missed error signaled

Bit 8 SYNCERR: SYNC error

This flag is set by hardware when the SYNC pulse arrives before the ESYNC event and the measured frequency error is greater than or equal to $$ FELIM * 128 $$ . This means that the frequency error is too big (internal frequency too low) to be compensated by adjusting the TRIM value, and that some other action has to be taken. An interrupt is generated if the ERRIE bit is set in the CRS_CR register. It is cleared by software by setting the ERRC bit in the CRS_ICR register.

0: No SYNC error signaled

1: SYNC error signaled

Bits 7:4 Reserved, must be kept at reset value.

Bit 3 ESYNCF: Expected SYNC flag

This flag is set by hardware when the frequency error counter reached a zero value. An interrupt is generated if the ESYNCIE bit is set in the CRS_CR register. It is cleared by software by setting the ESYNCC bit in the CRS_ICR register.

0: No expected SYNC signaled

1: Expected SYNC signaled

Bit 2 ERRF: Error flag

This flag is set by hardware in case of any synchronization or trimming error. It is the logical OR of the TRIMOVF, SYNCMISS and SYNCERR bits. An interrupt is generated if the ERRIE bit is set in the CRS_CR register. It is cleared by software in reaction to setting the ERRC bit in the CRS_ICR register, which clears the TRIMOVF, SYNCMISS and SYNCERR bits.

0: No synchronization or trimming error signaled

1: Synchronization or trimming error signaled

Bit 1 SYNCWARNF: SYNC warning flag

This flag is set by hardware when the measured frequency error is greater than or equal to $$ FELIM * 3 $$ , but smaller than $$ FELIM * 128 $$ . This means that to compensate the frequency error, the TRIM value must be adjusted by two steps or more. An interrupt is generated if the SYNCWARNIE bit is set in the CRS_CR register. It is cleared by software by setting the SYNCWARNC bit in the CRS_ICR register.

0: No SYNC warning signaled

1: SYNC warning signaled

Bit 0 SYNCOKF: SYNC event OK flag

This flag is set by hardware when the measured frequency error is smaller than $$ FELIM * 3 $$ . This means that either no adjustment of the TRIM value is needed or that an adjustment by one trimming step is enough to compensate the frequency error. An interrupt is generated if the SYNCOKIE bit is set in the CRS_CR register. It is cleared by software by setting the SYNCOKC bit in the CRS_ICR register.

0: No SYNC event OK signaled

1: SYNC event OK signaled

7.7.4 CRS interrupt flag clear register (CRS_ICR)

Address offset: 0x0C

Reset value: 0x0000 0000

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16
Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.
15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	Res.	ESYNCC	ERRC	SYNCWARNC	SYNCOKC
												rw	rw	rw	rw

Bits 31:4 Reserved, must be kept at reset value.

Bit 3 ESYNCC : Expected SYNC clear flag

Writing 1 to this bit clears the ESYNCF flag in the CRS_ISR register.

Bit 2 ERRC : Error clear flag

Writing 1 to this bit clears TRIMOVF, SYNCMISS, and SYNCERR bits and consequently also the ERRF flag in the CRS_ISR register.

Bit 1 SYNCWARNC : SYNC warning clear flag

Writing 1 to this bit clears the SYNCWARNF flag in the CRS_ISR register.

Bit 0 SYNCOKC : SYNC event OK clear flag

Writing 1 to this bit clears the SYNCOKF flag in the CRS_ISR register.

7.7.5 CRS register map

Table 38. CRS register map and reset values

Offset	Register name Reset value	31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Offset	Register name Reset value	0x00	CRS_CR	Res.																		TRIM[5:0]						SWSYNC	AUTOTRIMEN	CEN	Res.	ESYNCIE	ERRIE	SYNCWARNIE	SYNCOKIE
Reset value																			1	0	0	0	0	0	0	0	0		0	0	0	0
0x04	CRS_CFGR	0x00	SYNCPOL	Res.	SYNC SRC [1:0]		Res.	SYNC DIV [2:0]			FELIM[7:0]								RELOAD[15:0]
0x04	Reset value	0		1	0		0	0	0	0	0	1	0	0	0	1	0	1	0	1	1	1	0	1	1	0	1	1	1	1	1	1	1
0x08	CRS_ISR	FECAP[15:0]																FEDIR	Res.				TRIMOVF	SYNCMISS	SYNCERR	Res.				ESYNCF	ERRF	SYNCWARNF	SYNCOKF
0x08	Reset value	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0					0	0	0					0	0	0	0
0x0C	CRS_ICR	Res.																												ESYNCC	ERRC	SYNCWARNC	SYNCOKC
0x0C	Reset value																													0	0	0	0

Refer to Section 2.2 on page 75 for the register boundary addresses.