This example demonstrates how to use the [Application Level Tracing Library](https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/app_trace.html#) (henceforth referred to as **App Trace**) to log messages to a host via JTAG instead of the normal method of logging via UART.
UART logs are time consuming and can significantly slow down the function that calls it. Therefore, it is generally a bad idea to use UART logs in time-critical functions. Logging to host via JTAG is significantly faster and can be used in time-critical functions. For more details regarding logging to host via JTAG, refer to the [Logging to Host Documentation](https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/app_trace.html#app-trace-logging-to-host).
This example demonstrates JTAG logging to host in the context of polling for a [zero crossing](https://en.wikipedia.org/wiki/Zero_crossing). The ESP32 will continuously sample a 50 to 60 Hz sinusoidal signal (using the ADC) and log the sampled values (via JTAG). Due to the higher speed of JTAG logging, the polling rate of the ADC should be high enough to detect a zero crossing.
This example utilizes the following ESP-IDF features:
* [DAC driver](https://docs.espressif.com/projects/esp-idf/en/latest/api-reference/peripherals/dac.html) to generate the 50 Hz sinusoidal signal.
* [ADC driver](https://docs.espressif.com/projects/esp-idf/en/latest/api-reference/peripherals/adc.html) to sample the sinusoidal signal.
* [Application Level Tracing Library](https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/app_trace.html#) to log ADC samples to host.
* [OpenOCD](https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/jtag-debugging/index.html#setup-of-openocd) to interface with the ESP32 and receive the log output over JTAG.
* [ESP-WROVER-KIT](https://docs.espressif.com/projects/esp-idf/en/latest/hw-reference/modules-and-boards.html#esp-wrover-kit-v4-1) which integrates an on-board JTAG adapter. Ensure that the [required jumpers to enable JTAG are connected](https://docs.espressif.com/projects/esp-idf/en/latest/get-started/get-started-wrover-kit.html#setup-options) on the WROVER-KIT.
* ESP32 core board (e.g. ESP32-DevKitC) can also work as long as you connect it to an external JTAG adapter (e.g. FT2232H, J-LINK).
The sinusoidal signal of 50 to 60 Hz ranging from 0 V ~ 3.1 V should be input into `GPIO34` (`ADC1_CHANNEL_6`). Users may provide this signal themselves, our use the DAC generated signal by bridging GPIO34 with `GPIO25` (`DAC_CHANNEL_1`).
1. Connect the JTAG interface to ESP32 board, and power up both the JTAG and ESP32. For details about how to set up JTAG interface, please see [JTAG Debugging](https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/jtag-debugging/index.html).
2. After connecting JTAG interface, you need to [Run OpenOCD](https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/jtag-debugging/index.html#run-openocd).
* By default, the DAC will generate 130 Hz signal ranging from 0 V ~ 3.1 V. To generate a 50 Hz signal, the RTC 8 MHz clock will need to use a non-standard divider. This is achieved by enabling the `Example Configuration > Set custom RTC 8 MHz clock divider to lower CW frequency` configuration option.
* To enable application tracing, select the `(X) Trace memory` option under `Component config > Application Level Tracing`. This option should have been selected by default.
**Start App Trace:** In the telnet session window, trigger OpenOCD to start App Trace on the ESP32 by entering the command below. This command will collect 9000 bytes of JTAG log data and save them to `adc.log` file in `~/esp/openocd-esp32` folder.
**Note:** For more details on OpenOCD commands regarding App Trace, refer to the [OpenOCD Application Level Tracing Commands](https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/app_trace.html#openocd-application-level-tracing-commands)
The example will continuously sample the ADC for 20ms per iteration, and will alternate between JTAG and UART logging per iteration. However, the JTAG logs should be captured by OpenOCD, thus will not appear in the monitor's output. Therefore, the monitor should only display the iterations where UART logging was used (i.e. every alternate iteration) such as the following:
One likely cause would be an incorrect SPI flash voltage when starting OpenOCD. Suppose an ESP32 board/module with a 3.3 V powered SPI flash is being used, but the `board/esp32-wrover.cfg` configuration file is selected when starting OpenOCD which can set the SPI flash voltage to 1.8 V. In this situation, the SPI flash will not work after OpenOCD connects to the ESP32 as OpenOCD has changed the SPI flash voltage. Therefore, you might not be able to flash ESP32 when OpenOCD is connected.
To work around this issue, users are suggested to use `board/esp32-wrover.cfg` for ESP32 boards/modules operating with an SPI flash voltage of 1.8 V, and `board/esp-wroom-32.cfg` for 3.3 V. Refer to [ESP32 Modules and Boards](https://docs.espressif.com/projects/esp-idf/en/latest/hw-reference/modules-and-boards.html) and [Set SPI Flash Voltage](https://docs.espressif.com/projects/esp-idf/en/latest/api-guides/jtag-debugging/tips-and-quirks.html#why-to-set-spi-flash-voltage-in-openocd-configuration) for more details.
(For any technical queries, please open an [issue](https://github.com/espressif/esp-idf/issues) on GitHub. We will get back to you as soon as possible.)
The following code snippet demonstrates a loop of the sampling and logging the ADC over a 20 ms period in order to capture one full period of a 50 Hz signal.
If `ESP_LOGI()` is routed via UART (occurs by default), the log output produced will likely resemble the output shown below. Notice that due to UART logging is time consuming, thus the ADC is only sampled five times, which is too infrequent to consistently detect a zero crossing (where the zero crossing is `4096/2 = 2048` i.e., the mid point of the 12-bit ADC).
However, by logging via JTAG, the logging is much quicker hence allows a much higher sampling frequency (over 400 times) as shown the the log output below thus would be able to detect a zero crossing more consistently.
This example has demonstrated powerful functionality of logging to host via JTAG interface. With standard UART communication at a baud rate of 115200, printing out a single line log message takes approximately 4 ms. This also means that logged tasks cannot run more frequently than every 4 ms. By providing the same logging over JTAG, logging performance is improved 80 fold.
