imx8m plus Boot mode pins internal pull

[R-J]

Hi,

imx8m plus design has Boot mode pins configured in the same way as the evb, however I am reading 1.8V on pins that have a pull down resistor. It seems the imx has a strong internal pull up on these pins however I do not see anything in the datasheets about this.
As it is, the device is not booting even to serial download, any idea why it may be showing this behaviour compared to the EVB?

Many thanks

[JorgeCas]

Hello,

I hope you are doing well.

Could you please share me your schematic of boot mode pins?

As you said, maybe this could be caused by an internal pull-up resistor on these pins. Could you please confirm it checking the PAD configuration register?

Best regards.

Jorge.

[R-J]

Hi Jorge,

The boot mode pins are brought straight out to test points with no resistor pull on the schematic. I am then connecting them to GND or 1V8 through 100K and 2.2K resistors respectively, the same as the evaluation board.

The data sheet indicates that there should not be a pull on these pins. Measuring on the EVB shows 100K pull down is enough to bring the pins to 0V.
On my board, the pin reads 1V8 even when I change to a 10K pull down. There must be a hard pull up on these Boot mode pins.
I cannot find any short circuit to the pins on the PCB, so I have to assume it is an internal pull.

As it stands, the chip is not powering up its clocks and not booting. I have checked the POR_B signal which is driven correctly from PCA9450C PMIC.

Can you suggest any reason why these Boot mode pins would have a hard pull up?

Many thanks,
Rory

[JorgeCas]

Hello @R-J,

Considering that your schematic uses the same connections as EVK, it should work as expected, there is no additional configuration:

Jorge7u7_0-1687804661031.png

i.MX8MP does not count with a hard internal pull-up resistor, on pad control register you can enable and select a weak pull-down/weak pull-up. Here the information on page 1598 of refence manual.

Jorge7u7_1-1687804668052.png

Jorge7u7_2-1687804680968.png

But considering that you still having 1V8 even when you change to a 10K pull-down, the weak pull-up cannot cause this.

Measuring the EVK I got:

- Off position: 0V.
- On position: 1V6.

Best regards.

[smithcoba]

Jorge,

It is boot mode pin not the common function pin. 

How we can check the PAD configuration? 

It is totaly hardware. 

In i.MX6 datasheet, we can know the pull down and value. such as PD(100K).

In i.MX8MP we only can know pull down and no value.

That information is very important for hardware design of boot. 

i.MX6 datasheet

1.png

2.png

3.png

i.MX6 common function pin:

4.png

i.MX8MP datasheet

3.png