[Solved] LEDC Highspeed PWM

[BrettG]

Hi, I am trying to supply a clock signal to a external device at a frequency of at least 10 MHz with the LEDC peripheral. After looking through a lot of the questions and answers on the forum it sounds quite possible, but when I gave it a shot the ESP32 seemed unable to provide a square wave at that high of a frequency. Here is a screen shot of my 10 MHz signal:

10MHZ PWM.PNG (137.99 KiB) Viewed 12571 times

Here is the setup code I used for the LEDC module with defined values given in comments:

Code: Select all

esp_err_t image_init_clock(void) {
	ledc_timer_bit_t bit_num = (ledc_timer_bit_t) XVCLK_BIT_ACCURACY;		// 3

	// Enable LEDC PWM peripheral
	periph_module_enable(PERIPH_LEDC_MODULE);

	// Set Duty
	int duty = pow(2, (int) bit_num) / 2;

	// setup the timer
	ledc_timer_config_t xvclk_timer;
	xvclk_timer.bit_num = bit_num;						// 3
	xvclk_timer.freq_hz = XVCLK_FREQ;					// 10000000
	xvclk_timer.speed_mode = LEDC_HIGH_SPEED_MODE;
	xvclk_timer.timer_num = LEDC_TIMER_0;
	ledc_timer_config(&xvclk_timer);

	// setup the pwm channel
	ledc_channel_config_t servo_channel;
	servo_channel.channel = LEDC_CHANNEL_0;
	servo_channel.duty = duty;
	servo_channel.gpio_num = IMG_XVCLK;					// GPIO_NUM_4
	servo_channel.intr_type = LEDC_INTR_DISABLE;
	servo_channel.speed_mode = LEDC_HIGH_SPEED_MODE;
	servo_channel.timer_sel = LEDC_TIMER_0;
	ledc_channel_config(&servo_channel);

	// Set the PWM to the duty specified
	ledc_set_duty(LEDC_HIGH_SPEED_MODE, LEDC_CHANNEL_0, duty);
	ledc_update_duty(LEDC_HIGH_SPEED_MODE, LEDC_CHANNEL_0);
	
	return ESP_OK;
}

Is there anything wrong in my LEDC config? Is LEDC the correct tool to generate this clock signal, or is there a better option?

Thanks,
- Brett

[Hans Dorn]

What kind of load was connected to the pin when you took the measurement?

Looks like the output pin is struggling to charge and discharge a capacitive load.

[BrettG]

When I took that screen shot the only thing that was connected to the pin was a voltage divider (two 1K resistors to step the signal down to ~ 1.8V). Just to check I removed the voltage divider so nothing is on the pin and it still produced the same waveform.

[BrettG]

I took it a step further, the previous tests were on a new PCB design, so I decided to take that out of the equation and use my ESP_WROOM_32 DevKit. Same result, exact same waveform. Either my setup/config is incorrect, or the device is not capable of this high of a frequency.

[Hans Dorn]

Well, assuming the ESP32 pin will drive @10mA, I'm getting at something in the neighborhood of 150pF to match your waveform.

Are you using a 10:1 probe?


I might be able to borrow a scope over the weekend and have a look at some output waveforms myself...

[BrettG]

I think I got it. Since you mentioned the 10x option I looked into the probes specs and found:
1x DC-6MHz
10x DC-100MHz

I had it on 1x which was out of spec for the probe.

The new signal with the correct probe settings looks much better:

10MHZ PWM 10x.PNG (161.34 KiB) Viewed 12554 times

It is still a bit ugly, but a lot of that can be attributed to the sampling rate. The analog discovery is a useful tool but it is no replacement for a real lab oscilloscope.

Anyways this should be more than good enough for my uses.

Thanks a lot for the help.
- Brett

[Hans Dorn]

You're welcome

I've been bitten by that one too. An 1x probe is too much of a capacitive load for fast signals.


Cheers