This example shows how to use a GPTimer periodic ETM event to trigger the analog comparator scan task. The analog comparator uses the internal 50% VDD reference, and the comparator positive and negative crossing events set or clear a monitor GPIO through ETM. With an external sine wave connected to the source channel, the monitor GPIO becomes a square wave representation of the sampled input.

Realization

This example builds the following ETM chain:

GPTimer alarm event -> GPTimer enable-alarm task
GPTimer alarm event -> Analog comparator start task
Analog comparator positive cross event -> GPIO set task
Analog comparator negative cross event -> GPIO clear task

The steady-state signal path runs without CPU intervention. The CPU is only used during one-time initialization.

How to Use Example

Hardware Requirement

A development board with a supported Espressif SOC chip (see Supported Targets table above)
A USB cable for power supply and programming
A signal generator for generating the source sine wave
An oscilloscope or logic analyzer to observe the source input and monitor GPIO

Example Connection

The example uses a configurable comparator source input GPIO. The shipped default value matches the comparator pad0 GPIO for each supported target, and the example logs the actual source GPIO number at startup.

     +--------------+                +--------------+
     |   ESP Board  |                |  Signal Gen  |
     |              |  source signal |              |
+----+GPIO    Src In|<----+----------+OUT           |
|    |              |     |          |              |
|    |           GND+-----+----+-----+GND           |
|    |              |     |    |     |              |
|    +--------------+     |    |     +--------------+
|                         |    |
|    +--------------+     |    |
|    | Oscilloscope |     |    |
|    |              |     |    |
+--->|Probe1  Probe2|<----+    |
     |              |          |
     |           GND+----------+
     |              |
     +--------------+

Probe the source sine wave on the comparator source GPIO and probe the monitor GPIO at the same time.

Configure the Project

Open the project configuration menu:

idf.py menuconfig

Under Example Configuration, you can configure:

Source GPIO number
Monitor GPIO number
Comparator scan period (us)

The comparator reference voltage is fixed to the internal 50% VDD reference in this example. The shipped default source GPIO value matches comparator pad0 on each supported target.

Build and Flash

Build the project and flash it to the board, then run the monitor tool to view serial output:

idf.py -p PORT build flash monitor

(To exit the serial monitor, type Ctrl-].)

See the Getting Started Guide for full steps to configure and use ESP-IDF to build projects.

Example Output

I (252) main_task: Started on CPU0
I (262) main_task: Calling app_main()
I (262) example: Monitor GPIO 4
I (262) example: Analog comparator source GPIO 37
I (262) example: Analog comparator internal reference 50% VDD
I (272) example: GPTimer scan period 50 us
I (282) example: Periodic ETM-driven comparator scan started
I (282) main_task: Returned from app_main()

The exact source GPIO number depends on the target and package.

Expected Result On Hardware

Feed a sine wave into the comparator source channel. Because the comparator reference is fixed at 50% VDD, the monitor GPIO stays high while the sampled source voltage is above the threshold and low while it is below the threshold.

On an oscilloscope, the monitor GPIO appears as a square wave derived from the sampled sine wave.

Troubleshooting

This example only works on targets that support analog comparator channel scan and the ETM scan-task path.
If the monitor GPIO does not change, reduce the input frequency or shorten the scan period.
If the square wave looks unstable, confirm the input sine wave amplitude crosses the 50% VDD threshold.

For any technical queries, please open an issue on GitHub. We will get back to you soon.