AN3248

Pulse width measurement

Doc ID 17758 Rev 3

9/17

3

Pulse width measurement

In STM32L1 devices, the COMP2 output can be redirected to the input capture of the

embedded timers: TIM2, TIM3, TIM4, and TIM10. Redirecting the COMP2 output allows a

signal width or frequency with specific low and high levels (for example, a shifted signal) to

be measured. Figure 7 displays all the possible output redirections of the COMP2 output.

The input signal, whose signal width should be measured, is connected to any I/O of analog

switches group 6 (PB4 or PB5). The reference signal can be powered by:

●

●

the built-in DAC (channel 1 or channel 2)

●

an external pin through PB3

The COMP2 output redirection is achieved through the OUTSEL[2:0] bits.

The timer input capture channel is configured to save the timer counter at both rising and

falling edges. When the input signal goes above the reference voltage, COMP2 output is at

a high level generating a rising edge on the timer input capture. When the input signal goes

under the reference voltage, COMP2 output is at low level generating a falling edge. The

elapsed time between the two consecutive events (falling then rising edge or rising then

falling edge) represents the pulse width. Hence, the pulse width measurement is performed

by simple subtraction of the counter values. Figure 8 gives an overview of the pulse width

measurement as measured by COMP2.

Figure 7.

COMP2 with output redirection feature

1.

Legend for Figure 7

DAC_OUT1: DAC channel 1 output

DAC_OUT2: DAC channel 2 output

CMP2OUT: Comparator 2 output (internal output)

TIMx ICy: Timer x input capture channel y

TIMx OCREFCLR: Timer x output compare reference clear

Note:

In High-density devices, pins PB6 and PB7 can be used also as COMP2 non-inverting

inputs.

PB5

-

+

COMP2

PB4

GR6

GR6

CMP2OUT

Wakeup

EXTI line 22

PB3

DAC_OUT1

DAC_OUT2

V

REFINT

3/4 V

REFINT

1/2 V

REFINT

1/4 V

REFINT

Input voltage

TIM2 IC4

TIM2 OCRECLR

TIM3 IC4

TIM3 OCREFCLR

TIM4 IC4

TIM4 OCRECLR

TIM10 IC1

OUTSEL[2:0]

ai18706

Threshold2:

multiple sources

-1

-2