Pin behaviour before U-Boot (chip internal bootloader)

Technical Support
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1

Hello everyone!

We use the colibri-imx6ull SoM with a Display connected. Also we use PWM_B and PWM_C for status LEDs. While booting, BL_ON and PWM_B switch to high, effectively turning on the Backlight and one status LED.
As a workaround I set the BL_ON and the GPIO for PWM_B to 0 in U-Boot, but they still switch to high for about half a second. It seems like this happens before U-Boot is even loaded. The result is a “flashing” Display and LED. When the Button for the special boot-mode (e.g. to load the easy installer via USB) is pressed while booting, PWM_B stays low, the display backlight still turns on.

Is it possible to flash a custom internal bootloader that does not turn on the mentioned pins?

Sincerely,
Georg

2

Could you please describe how did you change BL_ON and the GPIO for PWM_B to 0 in U-Boot?
By default almost all GPIO are set to Hi-Z.
Then U-Boot Init required peripherals including PWM and display related. So if you you modify U-Boot code accordingly you can avoid “flashing”.

3

I just changed

- gpio_direction_output(GPIO_BL_ON, 1);
+ gpio_direction_output(GPIO_BL_ON, 0);

and to disable the PWM and

static iomux_v3_cfg_t const pwm_pads[] = {
	/* PWM<B> (multiplexed pin) */
	MX6_PAD_NAND_DQS__PWM5_OUT		| MUX_PAD_CTRL(NO_PAD_CTRL),
	/* PWM<C> (multiplexed pin) */
	MX6_PAD_ENET1_TX_EN__PWM6_OUT		| MUX_PAD_CTRL(NO_PAD_CTRL),
	/* PWM<D> (multiplexed pin) */
	/*MX6_PAD_ENET1_TX_CLK__PWM7_OUT	| MUX_PAD_CTRL(NO_PAD_CTRL),*/
};

#define GPIO_PWM_B IMX_GPIO_NR(4, 16)
#define GPIO_PWM_C IMX_GPIO_NR(2,  5)

static int setup_pwm_pins(void)
{
	imx_iomux_v3_setup_multiple_pads(pwm_pads, ARRAY_SIZE(pwm_pads));

	/* Set PWM<B> off */
	gpio_request(GPIO_PWM_B, "PWM<B>");
	gpio_direction_output(GPIO_PWM_B, 0);

	/* Set PWM<A> off */
	gpio_request(GPIO_PWM_C, "PWM<C>");
	gpio_direction_output(GPIO_PWM_C, 0);

	return 0;
}

which is exactly like it is done with BL_ON and PWM. I added a call to “setup_pwm_pins” right before “setup_lcd” is called. This solutions works, but it only sets the named pins to low, when u-boot is actually loaded. Even if i clear the flash-memory with the Easy Installer and reboot, no (visible) thing happens (no u-boot output on the serial console) but the display still turns on and PWM becomes high.

4

Most likely the only thing you can do is to fix your hardware. The proper way to avoid such glitches is to gate any such signal with a power good signal which has a know reset state and can be transitioned in a controlled fashion.