16. Comparator (COMP)

16.1 COMP introduction

The devices embed one ultra-low-power comparator, COMP1.

The comparator 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.

16.2 COMP main features

• • The 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)

16.3 COMP functional description

16.3.1 COMP block diagram

The block diagram of the comparators is shown in Figure 56 .

16.3.2 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 76. COMP1 input plus assignment

Table 77. COMP1 input minus assignment

1. N. A.: not affected.

16.3.3 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.

16.3.4 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.

16.3.5 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 57. Window mode

16.3.6 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 58. Comparator hysteresis

16.3.7 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 59. Comparator output blanking

16.3.8 COMP power and speed modes

COMP1 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

16.4 COMP low-power modes

Table 78. Comparator behavior in the low power modes

16.5 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. 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. 2. Configure and enable the NVIC IRQ channel mapped to the corresponding EXTI lines
3. 3. Enable COMPx.

Table 79. Interrupt control bits

16.6 COMP registers

16.6.1 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

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: Reserved

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 _REFINT divider available on the minus input of the Comparator 1.

0: Bandgap scaler disable

1: Bandgap scaler 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

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.

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

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

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.

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

00: Reserved

01: PB2

10: PA2

11: Reserved

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

16.6.2 COMP register map

The following table summarizes the comparator registers.

Table 80. COMP register map and reset values

Refer to Section 2.2 on page 55 for the register boundary addresses.