ESP32 RMT rmt_symbol_word_t generates incorrect square wave period

[horace99]

the following ESP32 code should generate a 100KHz square wave duty cycle 50% period 10uSec

Code: Select all

#include "Arduino.h"
#include "driver/rmt_tx.h"

void setup() {
	Serial.begin(115200);
	delay(2000);

	rmt_channel_handle_t txChannel {NULL};
	rmt_tx_channel_config_t tx_chan_config = {
			.gpio_num = GPIO_NUM_19,
			.clk_src = RMT_CLK_SRC_DEFAULT, // select source clock
			.resolution_hz = 10000000, // 10MHz resolution,
			.mem_block_symbols = 64,
			.trans_queue_depth = 4, // set the number of transactions that can be pending in the background
			};

	assert(rmt_new_tx_channel(&tx_chan_config, &txChannel)==ESP_OK && "Failed to Create RMT Channel");
	rmt_transmit_config_t transmitConfig = {
			.loop_count = -1
	};
	rmt_encoder_handle_t copyEncoder {NULL};
	rmt_copy_encoder_config_t copyEncoderConfig = {};
	assert(rmt_new_copy_encoder(&copyEncoderConfig, &copyEncoder)==ESP_OK && "Failed to Create Copy Encoder");

	const rmt_symbol_word_t pulsePattern[] = {
			{.duration0 = 50, .level0 = 1, .duration1 = 50, .level1 = 0},
			};

	assert(rmt_enable(txChannel)==ESP_OK && "Failed to Enable txChannel");
	Serial.println("Starting Transmitter");
	assert(rmt_transmit(txChannel, copyEncoder, &pulsePattern, sizeof(pulsePattern), &transmitConfig)==ESP_OK && "Failed to begin transmitting");
	Serial.println("Transmitter Running");
}

void loop() {
}

however, it generates a 99.01KHz square wave period 10.10uSec

F0001tek.jpg (19.39 KiB) Viewed 200 times

the resolution is 10MHz and

Code: Select all

const rmt_symbol_word_t pulsePattern[] = {
			{.duration0 = 50, .level0 = 1, .duration1 = 50, .level1 = 0},
			};

should generate a square wave 5uSec high and 5uSec low

this shows the low is 5.1uSec (high is correct 5Usec)

F0002tek.jpg (20.58 KiB) Viewed 200 times

if I change the rmt_symbol_word_t to

Code: Select all

const rmt_symbol_word_t pulsePattern[] = {
			{.duration0 = 50, .level0 = 1, .duration1 = 49, .level1 = 0},
			};

the generated frequency is then correct

F0003tek.jpg (19.18 KiB) Viewed 200 times

tested on three ESP-WROOM-32 based modules - identical problem

I checked the calibration of the oscilloscope with the built in pulse generator and a separate pulse generator

I implemented this program several months ago and don't remember having this problem at the time

[horace99]

further tests - first test used ESP32 core 3.2.0
tested with ESP32 core 3.1.3 - same problem
tested with ESP32 core 3.0.7 - same problem
tested with ESP32 core 3.0.17 - same problem

tested using a ESP32S3 using core 3.2.0 core same code except for changing output to GPIO18 - works OK - generates 100KHz square wave period 10uSec correctly

[lbernstone]

This is not an Arduino question. You should ask this in the Hardware forum

[ok-home]

hi
for esp32

Note: When enabling the continuous transmission mode by setting RMT_REG_TX_CONTI_MODE, the transmitter
will transmit the data on the channel continuously, that is, from the first byte to the last one, then from the first
to the last again, and so on. In this mode, there will be an idle level lasting one clk_div cycle between N and
N+1 transmissions.

https://www.espressif.com/sites/default ... ual_en.pdf page 414