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Supported Targets	ESP32	ESP32-S2

Application Level Tracing Example (Logging to Host)

(See the README.md file in the upper level 'examples' directory for more information about examples.)

This example demonstrates how to use the Application Level Tracing Library (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.

This example demonstrates JTAG logging to host in the context of polling for a zero crossing. The example app 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 to generate the 50 Hz sinusoidal signal.
ADC driver to sample the sinusoidal signal.
Application Level Tracing Library to log ADC samples to host.
OpenOCD to interface with the target and receive the log output over JTAG.

How to use example

Hardware Required

To run this example, you need a supported target dev board connected to a JTAG adapter, which can come in the following forms:

ESP-WROVER-KIT which integrates an on-board JTAG adapter. Ensure that the required jumpers to enable JTAG are connected on the WROVER-KIT.
ESP32 or ESP32-S2 core board (e.g. ESP32-DevKitC, ESP32-S2-Saola-1) can also work as long as you connect it to an external JTAG adapter (e.g. FT2232H, J-LINK).

This example will assume that that an ESP-WROVER-KIT is used.

Pin Assignment:

The sinusoidal signal of 50 to 60 Hz ranging from 0 V ~ 3.1 V should be input into ADC1_CHANNEL_6. Users may provide this signal themselves, or use the example-generated signal in DAC_CHANNEL_1. Listed below are the corresponding DAC/ADC channel pins for supported targets.

Target	DAC Output	ADC Input
ESP32	Channel 1 (GPIO25)	Channel 6 (GPIO34)
ESP32S2	Channel 1 (GPIO17)	Channel 6 (GPIO7)

Extra Connections:

Connect the JTAG interface to the target board. For details about how to set up JTAG interface, please see JTAG Debugging. Power up both the JTAG debugger and target board.
After connecting JTAG interface, you need to Run OpenOCD.
Open a separate terminal window and run telnet by entering the command below. The telnet terminal window is used to feed commands to OpenOCD:
```
telnet localhost 4444
```

Configure the project

idf.py menuconfig

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.

Build, Flash, and Run

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

idf.py -p PORT flash monitor

(Replace PORT with the name of the serial port to use.)

Start App Trace: In the telnet session window, trigger OpenOCD to start App Trace on the target by entering the command below. This command will collect 9000 bytes of JTAG log data and save them to the file file://adc.log (note file:// depends on where OpenOCD was started). Assuming that OpenOCD was started in this example's directory, adc.log will be saved here as well.

esp apptrace start file://adc.log 0 9000 5 0 0

Note: For more details on OpenOCD commands regarding App Trace, refer to the OpenOCD Application Level Tracing Commands

(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

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:

I (4289) example: Sampling ADC and sending data to the host...
I (4309) example: Collected 427 samples in 20 ms.

I (4309) example: Sampling ADC and sending data to the UART...
I (4309) example: Sample:1, Value:2224
I (4309) example: Sample:2, Value:840
I (4309) example: Sample:3, Value:3503
I (4319) example: Sample:4, Value:27
I (4319) example: Sample:5, Value:4095
I (4329) example: Collected 5 samples in 20 ms.

Note: The UART log above was produced with the CPU running at 240 MHz.

To access the JTAG logs, the adc.log file should be decoded. This can be done by using the logtrace_proc.py script as such:

$IDF_PATH/tools/esp_app_trace/logtrace_proc.py adc.log build/app_trace_to_host.elf

The logtrace_proc.py script should produce the following output when decoding:

Parse trace file '/user-home/esp/openocd-esp32/adc.log'...
Unprocessed 7 bytes of log record args!
Parsing completed.
====================================================================
I (59369) example: Sample:1, Value:3717
I (59369) example: Sample:2, Value:3647
I (59369) example: Sample:3, Value:3575
I (59369) example: Sample:4, Value:3491
...

I (59379) example: Sample:398, Value:78
I (59379) example: Sample:399, Value:58
I (59379) example: Sample:400, Value:22
I (59379) example: Sample:401, Value:14
I (59379) example: Sample:402, Value:0
I (59379) example: Sample:403, Value:0
I (59379) example: Sample:404, Value:0
I (59379) example: Sample:405, Value:0
I (59379) example: Sample:406, Value:0
I (59379) example: Sample:407, Value:0
I (59379) example: Sample:408, Value:0
I (59379) example: Sample:409, Value:0
I (59379) example: Sample:410, Value:0
I (59379) example: Sample:411, Value:0
I (59379) example: Sample:412, Value:0
I (59379) example: Sample:413, Value:0
I (59379) example: Sample:414, Value:16
I (59379) example: Sample:415, Value:32
I (59379) example: Sample:416, Value:40
I (59379) example: Sample:417, Value:74
I (59379) example: Sample:418, Value:89
I (59379) example: Sample:419, Value:113
I (59379) example: Sample:420, Value:160
I (59379) example: Sample:421, Value:192
I (59379) example: Sample:422, Value:221
I (59379) example: Sample:423, Value:256
I (59379) example: Sample:424, Value:298
I (59379) example: Sample:425, Value:345
I (59379) example: Sample:426, Value:386
I (59379) example: Sample:427, Value:432
I (61409) example: Sample:1, Value:2653

====================================================================

Log records count: 428

Troubleshooting

Unable to flash when OpenOCD is connected to the target

On ESP32 boards, one likely cause would be an incorrect SPI flash voltage when starting OpenOCD. Suppose a target board/module with a 3.3 V powered SPI flash is being used, but the configuration file (ex. board/esp32-wrover.cfg for ESP32) 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 target as OpenOCD has changed the SPI flash voltage. Therefore, you might not be able to flash to the target 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 and Set SPI Flash Voltage for more details.

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

The log should be identical to those printed via UART (complete with timestamps), but almost two orders of magnitude faster.

Example Breakdown

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.

int sampling_period = 20;
int i = 0;
uint32_t sampling_start =  esp_log_timestamp();  //this clock counts milliseconds
do {
    ESP_LOGI(TAG, "Sample:%d, Value:%d", ++i, adc1_get_raw(ADC1_TEST_CHANNEL));
} while (esp_log_timestamp() - sampling_start < sampling_period);

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).

I (4309) example: Sample:1, Value:2224
I (4309) example: Sample:2, Value:840
I (4309) example: Sample:3, Value:3503
I (4319) example: Sample:4, Value:27
I (4319) example: Sample:5, Value:4095
I (4329) example: Collected 5 samples in 20 ms.

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.

esp_log_set_vprintf(esp_apptrace_vprintf);

...

I (59379) example: Sample:423, Value:256
I (59379) example: Sample:424, Value:298
I (59379) example: Sample:425, Value:345
I (59379) example: Sample:426, Value:386
I (59379) example: Sample:427, Value:432
I (61409) example: Sample:1, Value:2653

====================================================================

Log records count: 428

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.

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