Possible to use nrf51822 without antenna (like simple MCU)?

[the_summer]

Hi,

I had a few nrf51822s lying around and as I find the chip and the SDK quite convenient to use, I thought about using them in a small MCU project which does not need any radio communication. This is also the first custom nrf51822 board I designed therefore I stayed very close to the reference schematic of the product specification (LDO setup). 

However, after assembling a test circuit I could not get openocd to connect to the chip (it works with a bought module). After reading several other threads here I am not sure if the unconnected ANT1/2 pins could prevent the the chip from initializing? They are currently free floating. Should I ground them or are there any components necessary for proper operation?

Best regards,

    Jan

[AmbystomaLabs]

Normally Ant1 and Ant2 DC float and are connected to VDD_PA across the output balun.  It shouldn't be required to provide power to the final stage of the radio for the MCU to fire up.  Leaving these floating should be fine.

Likely something else is wrong, such as insufficient local capacitance, too low of a supply voltage for SWD to operate, etc.

[the_summer]

Thank you, good to know. I use 3.3 V for the power supply and the SWD. I guess that should be sufficient.

I used the following schematic based on the reference circuits of the product specification. Does anyone see grave mistakes?

[AmbystomaLabs]

Schematically it looks fine(except pin 49, see comment below).  But how does the layout look?

People often make mistakes by putting all the caps on one side of the board instead of near the pins they need to be next to, also I have seen where the ground slug under the nRF wasn't placed and they assumed one or two pins for ground is good enough, it's not.

Also make sure you didn't rotate your crystal.  Occasionally people rotate it 90 degrees. Functionally it doesn't need the external crystal. It can work on HF_RC, but if it is rotated the crystal shorts out the driver pins.

[AmbystomaLabs]

In fact I notice, pin 49 EPad is not connected.  That's a big mistake.  You need to have one stable ground for the entire device even when you are not using the radio.

So, that is your problem, but you may have other mistakes.  I recommend you have someone look over your layout before you have the pcb made again.

[the_summer]

Thanks you for your help. The pin 49 is actually connected to GND. The wire must have gone missing when I rotated schematic for a better screenshot.

I will check out your suggestions from your other answers.