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STM32L152 Oscillator Circuit

Question asked by cliveb on Jul 21, 2015
Latest reply on Jul 21, 2015 by Clive One
We are using the L152 processor in a new design. This is the first time we've used the STM32 family and it is mostly working wonderfully. However, on the three prototypes we have, only one crystal oscillator works reliably. The others run fine when driven off an external oscillator.

The layout is tightly run on a 4-layer board with ground plane, there are no close tracks or plane jumps, we have a guard around the crystal, we've scoped out the oscillator using a low-cap FET probe. The crystal is an 8MHz HC-49 type with 20pF load cap and 2 - 20pF caps to ground. We've played around with different output pin series resistors, lower value caps, and different crystals but the oscillator won't budge!

We've read AN2867 and checked the design pin-for-pin against the STM32L152D-EVAL board schematics and everything looks in order. As a test, we've taken the demo code from the STM32L152D-EVAL code and tried to run it verbatim, without any luck. When run off an external oscillator, though, the board runs and executes code flawlessly.

Any thoughts would be appreciated!