21. Comparator (COMP)

21.1 COMP introduction

The devices embed two ultra-low-power comparators COMP1, and COMP2.

The comparators can be used for a variety of functions including:

• Wake-up from low-power mode triggered by an analog signal,
• Analog signal conditioning,
• Cycle-by-cycle current control loop when combined with a PWM output from a timer.

21.2 COMP main features

• Each comparator has configurable plus and minus inputs used for flexible voltage selection:
- – Multiplexed I/O pins
- – Internal reference voltage and three submultiple values (1/4, 1/2, 3/4) provided by scaler (buffered voltage divider)
• Programmable hysteresis
• Programmable speed / consumption
• The outputs can be redirected to an I/O or to timer inputs for triggering:
- – Break events for fast PWM shutdowns
• Comparator outputs with blanking source
• The two comparators can be combined in a window comparator
• Each comparator has interrupt generation capability with wake-up from Sleep and Stop modes (through the EXTI controller)

21.3 COMP functional description

21.3.1 COMP block diagram

The block diagram of the comparators is shown in Figure 104 .

Figure 104. Comparator block diagram

Figure 104. Comparator block diagram. The diagram shows a comparator (COMPx) with two inputs: COMPx_INP and COMPx_INM. The COMPx_INP input is selected by a multiplexer (COMPx_INPSEL) from either COMPx_INP I/Os or a reference voltage (V_REFINT, 3/4 V_REFINT, 1/2 V_REFINT, or 1/4 V_REFINT). The COMPx_INM input is selected by a multiplexer (COMPx_INMSEL) from either COMPx_INM I/Os or the same reference voltages. The output of the comparator is COMPx_VALUE, which is inverted by a polarity selection block (COMPx_POL). The output can be connected to a GPIO alternate function or an internal timer input. The diagram is labeled MS47272V1.

21.3.2 COMP pins and internal signals

The I/Os used as comparators inputs must be configured in analog mode in the GPIOs registers.

The comparator output can be connected to the I/Os using the alternate function channel given in “Alternate function mapping” table in the datasheet.

The output can also be internally redirected to a variety of timer input for the following purposes:

• Emergency shut-down of PWM signals, using BKIN and BKIN2 inputs
• Cycle-by-cycle current control, using OCREF_CLR inputs
• Input capture for timing measures

It is possible to have the comparator output simultaneously redirected internally and externally.

Table 108. COMP1 input plus assignment

COMP1_INP	COMP1_INPSEL
PC5 (available only on STM32WB55xx)	00
PB2	01
PA1	10

Table 109. COMP1 input minus assignment

COMP1_INM	COMP1_INMSEL[2:0]	COMP1_INMESEL[1:0]
$\frac{1}{4} V_{REFINT}$	000	N. A. ⁽¹⁾
$\frac{1}{2} V_{REFINT}$	001	N. A. ⁽¹⁾
$\frac{3}{4} V_{REFINT}$	010	N. A. ⁽¹⁾
$V_{REFINT}$	011	N. A. ⁽¹⁾
Reserved	100	N. A. ⁽¹⁾
Reserved	101	N. A. ⁽¹⁾
PA9	110	N. A. ⁽¹⁾
PC4 (available only on STM32WB55xx)	111	00
PA0	111	01
PA4	111	10
PA5	111	11

1. N. A.: not affected.

Table 110. COMP2 input plus assignment

COMP2_INP	COMP2_INPSEL
PB4	00
PB6	01
PA3	10

Table 111. COMP2 input minus assignment

COMP2_INM	COMP2_INMSEL[2:0]	COMP2_INMESEL[1:0]
$\frac{1}{4} V_{REFINT}$	000	N.A. ⁽¹⁾
$\frac{1}{2} V_{REFINT}$	001	N.A. ⁽¹⁾
$\frac{3}{4} V_{REFINT}$	010	N.A. ⁽¹⁾
$V_{REFINT}$	011	N.A. ⁽¹⁾
Reserved	100	N.A. ⁽¹⁾
Reserved	101	N.A. ⁽¹⁾
PB3	110	N.A. ⁽¹⁾
PB7	111	00
PA2	111	01
PA4	111	10
PA5	111	11

1. N. A.: not affected.

21.3.3 COMP reset and clocks

The COMP clock provided by the clock controller is synchronous with the APB2 clock.

There is no clock enable control bit provided in the RCC controller. Reset and clock enable bits are common for COMP and SYSCFG.

Important: The polarity selection logic and the output redirection to the port works independently from the APB2 clock. This allows the comparator to work even in Stop mode.

21.3.4 Comparator LOCK mechanism

The comparators can be used for safety purposes, such as over-current or thermal protection. For applications having specific functional safety requirements, it is necessary to insure that the comparator programming cannot be altered in case of spurious register access or program counter corruption.

For this purpose, the comparator control and status registers can be write-protected (read-only).

Once the programming is completed, the COMPx LOCK bit can be set to 1. This causes the whole register to become read-only, including the COMPx LOCK bit.

The write protection can only be reset by a MCU reset.

21.3.5 Window comparator

The purpose of window comparator is to monitor the analog voltage if it is within specified voltage range defined by lower and upper threshold.

Two embedded comparators can be utilized to create window comparator. The monitored analog voltage is connected to the non-inverting (plus) inputs of comparators connected together and the upper and lower threshold voltages are connected to the inverting (minus) inputs of the comparators. Two non-inverting inputs can be connected internally together by enabling WINMODE bit to save one IO for other purposes.

Figure 105. Window mode

Schematic diagram of window mode for two comparators, COMPx and COMPy. Each comparator has multiplexers for its non-inverting (INP) and inverting (INM) inputs. COMPx_INPSEL selects between COMPx_INP I/Os and internal sources. COMPx_INMSEL selects between COMPx_INM I/Os and internal sources. A WINMODE signal is connected to the non-inverting input of COMPy. The diagram is labeled MSV37667V1.

21.3.6 Hysteresis

The comparator includes a programmable hysteresis to avoid spurious output transitions in case of noisy signals. The hysteresis can be disabled if it is not needed (for instance when exiting from low-power mode) to be able to force the hysteresis value using external components.

Figure 106. Comparator hysteresis

Timing diagram showing hysteresis. The top graph plots INP (non-inverting input) as a sine-like wave and INM (inverting input) as a constant reference voltage. The bottom graph plots COMP_OUT (output) as a digital signal. The output transitions from low to high when INP crosses INM - V_hyst and from high to low when INP crosses INM. Vertical dashed lines align the transitions. The diagram is labeled MS19984V1.

21.3.7 Comparator output blanking function

The purpose of the blanking function is to prevent the current regulation to trip upon short current spikes at the beginning of the PWM period (typically the recovery current in power switches anti parallel diodes). It consists of a selection of a blanking window which is a timer output compare signal. The selection is done by software (refer to the comparator register description for possible blanking signals). Then, the complementary of the blanking signal is ANDed with the comparator output to provide the wanted comparator output. See the example provided in the figure below.

Figure 107. Comparator output blanking

The figure illustrates the comparator output blanking function. The top part is a timing diagram showing the relationship between the PWM signal, the current limit, the actual current, the raw comparator output, the blanking window, and the final comparator output. The bottom part is a logic schematic showing the AND gate that combines the raw comparator output with the inverted blanking signal to produce the final output.

Timing diagram and logic schematic for comparator output blanking. The timing diagram shows five waveforms: PWM (top), Current limit (dashed line), Current (solid line with spikes), Raw comp output, and Final comp output. Below the timing diagram is a logic schematic showing an AND gate with inputs 'Comp out' and 'Blank' (inverted), producing the output 'Comp out (to TIM_BK ...)'.

21.3.8 COMP power and speed modes

COMP1 and COMP2 power consumption versus propagation delay can be adjusted to have the optimum trade-off for a given application.

The bits PWRMODE[1:0] in COMPx_CSR registers can be programmed as follows:

00: High speed / full power
01 or 10: Medium speed / medium power
11: Low speed / ultra-low-power

21.4 COMP low-power modes

Table 112. Comparator behavior in the low power modes

Mode	Description
Sleep	No effect on the comparators. Comparator interrupts cause the device to exit the Sleep mode.
Low-power run	No effect.
Low-power sleep	No effect. COMP interrupts cause the device to exit the Low-power sleep mode.
Stop 0	No effect on the comparators. Comparator interrupts cause the device to exit the Stop mode.
Stop 1
Stop 2
Standby	The COMP registers are powered down and must be reinitialized after exiting Standby or Shutdown mode.
Shutdown

21.5 COMP interrupts

The comparator outputs are internally connected to the Extended interrupts and events controller. Each comparator has its own EXTI line and can generate either interrupts or events. The same mechanism is used to exit from low-power modes.

Refer to Interrupt and events section for more details.

To enable COMPx interrupt, it is required to follow this sequence:

1. Configure and enable the EXTI line corresponding to the COMPx output event in interrupt mode and select the rising, falling or both edges sensitivity
2. Configure and enable the NVIC IRQ channel mapped to the corresponding EXTI lines
3. Enable COMPx.

Table 113. Interrupt control bits

Interrupt event	Event flag	Enable control bit	Exit from Sleep mode	Exit from Stop modes	Exit from Standby mode
COMP1 output	VALUE in COMP1_CSR	Through EXTI	Yes	Yes	N/A
COMP2 output	VALUE in COMP2_CSR	Through EXTI	Yes	Yes	N/A

21.6 COMP registers

21.6.1 Comparator 1 control and status register (COMP1_CSR)

The COMP1_CSR is the Comparator 1 control/status register. It contains all the bits /flags related to comparator1.

Address offset: 0x00

System reset value: 0x0000 0000

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16
LOCK	VALUE	Res.	Res.	Res.	INMESEL[1:0]		Res.	SCAL EN	BRG EN	Res.	BLANKING[2:0]			HYST[1:0]
rs	r				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
POLARITY	Res.	Res.	Res.	Res.	Res.	Res.	INP SEL[1:0]		INMSEL[2:0]			PWRMODE[1:0]		Res.	EN
rw							rw	rw	rw	rw	rw	rw	rw		rw

Bit 31 LOCK : COMP1_CSR register lock bit

This bit is set by software and cleared by a hardware system reset. It locks the whole content of the comparator 1 control register, COMP1_CSR[31:0].

0: COMP1_CSR[31:0] for comparator 1 are read/write

1: COMP1_CSR[31:0] for comparator 1 are read-only

Bit 30 VALUE : Comparator 1 output status bit

This bit is read-only. It reflects the current comparator 1 output taking into account POLARITY bit effect.

Bits 29:27 Reserved, must be kept at reset value.

Bits 26:25 INMESEL[1:0] : Comparator 1 input minus extended selection bits.

These bits are set and cleared by software (only if LOCK is not set). They select which extended GPIO input is connected to the input minus of comparator if INMSEL = 111.

00: PC4 (available only on STM32WB55xx)

01: PA0

10: PA4

11: PA5

Bit 24 Reserved, must be kept at reset value.

Bit 23 SCALEN : Voltage scaler enable bit

This bit is set and cleared by software. This bit enable the outputs of the V _REFINTdivider available on the minus input of the Comparator 1.

0: Bandgap scaler disable (if SCALEN bit of COMP2_CSR register is also reset)

1: Bandgap scaler enable

Bit 22 BRGEN : Scaler bridge enable

This bit is set and cleared by software (only if LOCK not set). This bit enable the bridge of the scaler.

0: Scaler resistor bridge disable (if BRGEN bit of COMP2_CSR register is also reset)

1: Scaler resistor bridge enable

If SCALEN is set and BRGEN is reset, BG voltage reference is available but not 1/4 BGAP, 1/2 BGAP, 3/4 BGAP. BGAP value is sent instead of 1/4 BGAP, 1/2 BGAP, 3/4 BGAP.

If SCALEN and BRGEN are set, 1/4 BGAP 1/2 BGAP 3/4 BGAP and BGAP voltage references are available.

Bit 21 Reserved, must be kept at reset value.

Bits 20:18 BLANKING[2:0] : Comparator 1 blanking source selection bits

These bits select which timer output controls the comparator 1 output blanking.

000: No blanking

001: TIM1 OC5 selected as blanking source

010: TIM2 OC3 selected as blanking source

All other values: reserved

Bits 17:16 HYST[1:0] : Comparator 1 hysteresis selection bits

These bits are set and cleared by software (only if LOCK not set). They select the hysteresis voltage of the comparator 1.

00: No hysteresis

01: Low hysteresis

10: Medium hysteresis

11: High hysteresis

Bit 15 POLARITY : Comparator 1 polarity selection bit

This bit is set and cleared by software (only if LOCK not set). It inverts Comparator 1 polarity.

0: Comparator 1 output value not inverted

1: Comparator 1 output value inverted

Bits 14:9 Reserved, must be kept at reset value.

Bits 8:7 INPSEL[1:0] : Comparator1 input plus selection bit

This bit is set and cleared by software (only if LOCK not set).

00: External I/O - PC5 (available only on STM32WB55xx)

01: PB2

10: PA1

11: Reserved

Bits 6:4 INMSEL[2:0] : Comparator 1 input minus selection bits

These bits are set and cleared by software (only if LOCK not set). They select which input is connected to the input minus of comparator 1.

000 = 1/4 V _REFINT

001 = 1/2 V _REFINT

010 = 3/4 V _REFINT

011 = V _REFINT

100 = Reserved

101 = Reserved

110 = PA9

111 = GPIOx selected by INMESEL bits

Bits 3:2 PWRMODE[1:0] : Power Mode of the comparator 1

These bits are set and cleared by software (only if LOCK not set). They control the power/speed of the Comparator 1.

00: High speed
01 or 10: Medium speed
11: Ultra low power

Bit 1 Reserved, must be kept at reset value.

Bit 0 EN : Comparator 1 enable bit

This bit is set and cleared by software (only if LOCK not set). It switches on Comparator1.

0: Comparator 1 switched OFF
1: Comparator 1 switched ON

21.6.2 Comparator 2 control and status register (COMP2_CSR)

The COMP2_CSR is the Comparator 2 control/status register. It contains all the bits /flags related to comparator 2.

Address offset: 0x04

System reset value: 0x0000 0000

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16
LOCK	VALUE	Res.	Res.	Res.	INMESEL[1:0]		Res.	SCAL EN	BRG EN	Res.	BLANKING[2:0]			HYST[1:0]
rs	r				rw			rw	rw		rw			rw

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
POLARITY	Res.	Res.	Res.	Res.	Res.	WIN MODE	INPSEL[1:0]		INMSEL[2:0]			PWRMODE[1:0]		Res.	EN
rw						rw	rw		rw			rw			rw

Bit 31 LOCK : CSR register lock bit

This bit is set by software and cleared by a hardware system reset. It locks the whole content of the comparator 2 control register, COMP2_CSR[31:0].

0: COMP2_CSR[31:0] for comparator 2 are read/write
1: COMP2_CSR[31:0] for comparator 2 are read-only

Bit 30 VALUE : Comparator 2 output status bit

This bit is read-only. It reflects the current comparator 2 output taking into account POLARITY bit effect.

Bits 29:27 Reserved, must be kept at reset value.

Bits 26:25 INMESEL[1:0] : comparator 2 input minus extended selection bits.

These bits are set and cleared by software (only if LOCK is not set). They select which extended GPIO input is connected to the input minus of comparator if INMSEL = 111.

00: PB7
01: PA2
10: PA4
11: PA5

Bit 24 Reserved, must be kept at reset value.

Bit 23 SCALEN : Voltage scaler enable bit

This bit is set and cleared by software. This bit enable the outputs of the $V_{REFINT}$ divider available on the minus input of the Comparator 2.

0: Bandgap scaler disable (if SCALEN bit of COMP1_CSR register is also reset)

1: Bandgap scaler enable

Bit 22 BRGEN : Scaler bridge enable

This bit is set and cleared by software (only if LOCK not set). This bit enable the bridge of the scaler.

0: Scaler resistor bridge disable (if BRGEN bit of COMP1_CSR register is also reset)

1: Scaler resistor bridge enable

If SCALEN is set and BRGEN is reset, BG voltage reference is available but not 1/4 BGAP, 1/2 BGAP, 3/4 BGAP. BGAP value is sent instead of 1/4 BGAP, 1/2 BGAP, 3/4 BGAP.

If SCALEN and BRGEN are set, 1/4 BGAP 1/2 BGAP 3/4 BGAP and BGAP voltage references are available.

Bit 21 Reserved, must be kept at reset value.

Bits 20:18 BLANKING[2:0] : Comparator 2 blanking source selection bits

These bits select which timer output controls the comparator 2 output blanking.

000: No blanking
001: TIM1 OC5 selected as blanking source
010: TIM2 OC3 selected as blanking source
All other values: reserved

Bits 17:16 HYST[1:0] : Comparator 2 hysteresis selection bits

These bits are set and cleared by software (only if LOCK not set). Select the hysteresis voltage of the comparator 2.

00: No hysteresis
01: Low hysteresis
10: Medium hysteresis
11: High hysteresis

Bit 15 POLARITY : Comparator 2 polarity selection bit

This bit is set and cleared by software (only if LOCK not set). It inverts Comparator 2 polarity.

0: Comparator 2 output value not inverted
1: Comparator 2 output value inverted

Bits 14:10 Reserved, must be kept at reset value.

Bit 9 WINMODE : Windows mode selection bit

This bit is set and cleared by software (only if LOCK not set). This bit selects the window mode of the comparators. If set, both positive inputs of comparators are connected together.

0: Input plus of Comparator 2 is not connected to Comparator 1
1: Input plus of Comparator 2 is connected with input plus of Comparator 1

Bits 8:7 INPSEL[1:0] : Comparator 2 input plus selection bit

This bit is set and cleared by software (only if LOCK not set).

00: PB4
01: PB6
10: PA3
11: Reserved

Bits 6:4 INMSEL[2:0]: Comparator 2 input minus selection bits

These bits are set and cleared by software (only if LOCK not set). They select which input is connected to the input minus of comparator 2.

000 = 1/4 V _REFINT

001 = 1/2 V _REFINT

010 = 3/4 V _REFINT

011 = V _REFINT

100 = Reserved

101 = Reserved

110 = PB3

111 = GPIOx selected by INMSEL bits

Bits 3:2 PWRMODE[1:0]: Power Mode of the comparator 2

These bits are set and cleared by software (only if LOCK not set). They control the power/speed of the Comparator 2.

00: High speed

01 or 10: Medium speed

11: Ultra low power

Bit 1 Reserved, must be kept at reset value.

Bit 0 EN: Comparator 2 enable bit

This bit is set and cleared by software (only if LOCK not set). It switches on comparator 2.

0: Comparator 2 switched OFF

1: Comparator 2 switched ON

21.6.3 COMP register map

The following table summarizes the comparator registers.

Table 114. COMP register map and reset values

Offset	Register	31	30	29	28	27	26	25	24	23	22	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0x00	COMP1_CSR	LOCK	VALUE	Res.	Res.	Res.	INMESEL	Res.	SCALEN	BRGEN	Res.	BLANKING			HYST		POLARITY.	Res.	Res.	Res.	Res.	Res.	Res.	INPSEL			INMSEL			PWRMODE	Res.	EN
0x00	Reset value	0	0				0	0	0	0		0	0	0	0	0	0							0	0	0	0	0	0	0	0	0
0x04	COMP2_CSR	LOCK	VALUE	Res.	Res.	Res.	INMESEL	Res.	SCALEN	BRGEN	Res.	BLANKING			HYST		POLARITY.	Res.	Res.	Res.	Res.	Res.	WINMODE	INPSEL			INMSEL			PWRMODE	Res.	EN
0x04	Reset value	0	0				0	0	0	0		0	0	0	0	0	0						0	0	0	0	0	0	0	0	0	0

Refer to Section 2.2 on page 66 for the register boundary addresses.