topic Re: K60 RTC Crystal not Oscillating in Kinetis Microcontrollers

K60 RTC Crystal not Oscillating

norton256 — Wed, 08 Feb 2017 22:20:16 GMT

I have a copy of a custom design (built over 30 which work) where the RTC crystal stopped oscillating (it did work for a while, and now it is not). I have replaced the crystal to no effect. When I probe the crystal pins, the one connected to XTAL32 is just slightly above ground, and the EXTAL32 pin is around 1V. When we replaced the crystal, the solder balls we inspected on the K60, and looked OK.

Any suggestions on what could be going on?

Re: K60 RTC Crystal not Oscillating

bobpaddock — Thu, 09 Feb 2017 14:13:25 GMT

"it did work for a while, and now it is not"

How long is 'a while'? Seconds or days?

If in longer time lengths it can be a sign of flux under the part that was not removed during the cleaning process after soldering the chip. 'No-Clean' fluxes can still cause problems with high impedance circuits such as this.

The flux builds up contaminants from the environment which lowers the impedance with time causing the osc to stop.

The RTC Osc is high impedance which a scope probe could affect adversely.

What is your actual crystal circuit? Any other capacitors or resistors involved?

Re: K60 RTC Crystal not Oscillating

norton256 — Thu, 09 Feb 2017 14:30:09 GMT

A while is days.

It was assembled with a no clean process, so I will look into that as well.

The circuit is very simple (from a PCB standpoint). Traces are routed on the surface from the RTC pads to the balls of the K60 in a close to a straight line as possible. The traces are only 100mils long. No other components are on the lines.

The crystal part number is: CC4V-T1A 32.768kHz 9pF

I have the 8pF internal load enabled in the RTC module of the K60.

I have been able to probe other boards RTC pins and observed the oscillation.