Correct voltage to nRESET pin on nRF9160

[michaelatom]

Hi,

We have a board with an nRF52840 working as the primary MCU which controls an nRF9160 via AT commands.

P109 from nRF52840 is connected to the nRESET of nRF9160. To control the resets, P109 is configured as output high (3.3V) for most of the time and only when we need to reset the nRF9160 it's set to low for a short time.

Is it ok to have the nRESET pin at 3.3V, since the product spec mentions that it is already pulled to 2.2V, or is it better to configure P109 as input high-z and change it to output low when needing to reset the nRF9160?

Thanks

[Håkon Alseth]

Hi,

 

Is it ok to have the nRESET pin at 3.3V, since the product spec mentions that it is already pulled to 2.2V, or is it better to configure P109 as input high-z and change it to output low when needing to reset the nRF9160?

VDD_IO is configurable for the nRF9160. If both devices are operating with the same GPIO voltage, it should be fine.

You can have a voltage of VDD + 0.3V to not violate the specification, as per the maximum ratings chapter in each respective product specification.

 

If the nRF9160 has the same or a lower VDD_IO voltage than the nRF52-device, you can configure the nRF52 device gpio to be in open-drain (S0D1 in PIN_CNF[] register: https://infocenter.nordicsemi.com/topic/ps_nrf52840/gpio.html?cp=4_0_0_5_8_1_9#register.PIN_CNF-0-31). This way you can only pull the pin low.

 

Kind regards,

Håkon

  

[michaelatom]

Thanks for the info.

nRF9160 VDD is also 3.3 so it should be fine, but I will take into consideration the open-drain suggestion.

On one of my boards, I noticed that the nRF52840's P109 was stuck at 0.6V.
When I checked resistance on that pin, I saw 20MΩ to gnd and around 2.5MΩ to VCC. The pin is not connected to anything else, do you have any idea of what could be the reason or any other tests I can do to try to understand what happened to it?

[Håkon Alseth]

Hi,

 

michaelatom said:

On one of my boards, I noticed that the nRF52840's P109 was stuck at 0.6V.

 Sounds like a soldering issue, especially if this is only found on one board. Given the impedance, I would guess that its not properly connected to the nRF9160's nRESET pin.

 

Try measuring current consumption on both nRF-devices and see if you can spot any differences there.

 

Kind regards,

Håkon

[michaelatom]

Hi Håkon,

Haven't been able to follow up on the possible disconnected part.

I'm having stability issues on my system from turning the modem ON.

Whenever I power the modem there is a significant voltage drop that causes the whole board to reset.

The Enable pin is connected to VDD via a 10k resistor and VDD_GPIO is directly connected to VDD
I managed to reduce by half the drop if I power the reset line before providing power to the VDD pins.

The following pictures show the power section from the nRF9160 module.

VMOD is the external power which comes from a 3.3V boost (powered from 2AA Alkaline batteries)

Is there a modification or specific booting sequence that I should follow to mitigate the drops?

[Håkon Alseth]

Hi,

 

michaelatom said:

Whenever I power the modem there is a significant voltage drop that causes the whole board to reset.

It sounds like your batteries / boost converter isn't able to handle the peaks of the modem. These can be quite high, in the 500 mA range. Could you scope the VDD and battery voltage to see how it behaves? What boost converter ic are you using?

If you dip underneath 3.0V, the nRF9160 will do a brown out reset.

 

Kind regards,

Håkon