Application examples

AN4232

10/17

DocID024134 Rev 2

2.2

Frequency and pulse width measurement

In STM32F30x and STM32F37x devices, the comparator output can be redirected to the

input capture of the embedded timers. This feature allows a pulse width and/or frequency

measurement. The input signal, whose signal width/frequency has to be measured, is

connected to the non-inverting input of the analog comparator. The threshold (reference)

can be powered by:

•

•

the embedded digital-to-analog converters

•

an external pin

The comparator output is redirected internally to the input capture. In this mode, the timer

counter is captured at each effective edge. When the input signal goes higher than the

reference voltage, the comparator output is set to a high level generating a rising edge on

the timer input capture, and the timer counter is captured in the internal register. When the

input signal goes lower than the reference voltage, the comparator output is set to a low

level generating a falling edge and the timer counter is captured a second time in the

internal register. The time elapsed between the two consecutive captures represents the

pulse width. Hence, the pulse width measurement is performed by a simple subtraction of

the second and the first capture.

2.3

Analog voltage monitoring

STM32F30x and STM32F37x devices embed a 12-bit analog-to-digital converter (ADC)

which is very fast with a sampling rate in the order of several mega samples per second.

However, with a typical consumption of 1 mA, it may not be suitable for battery-powered

applications if it is left powered-on continuously. It is therefore useful to use an analog

comparator in application cases where an analog voltage (sensor output) needs to be

measured only when a predefined threshold is exceeded.

In STM32F30x and STM32F37x devices, the analog comparators are designed to work

even in low-power modes (Sleep mode and Stop mode). They are still powered-on and so

they are able to wake-up the MCU from low-power modes. In fact, the comparator output is

connected to the EXTI controller which also remains powered-on in lower-power modes.

Figure 5. Comparator output capability in low-power mode

Moreover, the comparator input is an ADC channel so PCB designer doesn't need to make

an external connection between the comparator input and the ADC channel.

In an analog voltage monitoring application where the sensor output voltage is lower than

the threshold, the MCU remains in Stop mode thereby saving power. As soon as the sensor

output exceeds the threshold, the analog comparator wakes up the MCU, the ADC is