Has anyone had any difficulties with the crystal used for the USB peripheral interface on the TMS320DM6446 not starting up? The leading production board failure that our board supplier reports is that this particular crystal circuit is not staring, sometimes replacing the crystal 'fixes' the board, sometimes not. Based on what we've seen so far it doesn't appear to be a generic 'crystal not starting problem', but seems to be quite specific to the TI part.
I was just wondering if anyone here has happened to encountered this problem. Details for those interested are listed below.
=============== Details of the design:
- Crystal circuit layout is reasonable, the crystal and caps are located close to the 6446, short traces.
- The 6446 has two crystal input circuits, one to drive the processor core, the other is strictly for USB. We're not observing any problems with the crystal used to drive the processor core, only the USB one.
- There are a total of four crystals on the board, they're all the same supplier (different frequencies only). The other three crystals do not have any reported startup issues.
- Per the TI spec, both the 'processor core crystal' and 'USB crystal' I/O pins are powered from the same supply voltage (DVdd18). There is nothing indicating that the DVdd18 supplies are in any way segregated for the USB pins, so it would seem that since one crystal circuit works and the other one doesn't that it wouldn't likely be a power supply issue (speculating).
- The TI part in question is TMS320DM6446ZWT. Pins 'F18' (name M24XI) and 'F19' (M24XO) are the 'USB crystal' pins in question.
- The USB crystal that won't start is CITIZEN AMERICA CORP. CS10-24.000MABJ-UT
- The crystal capacitor load is spec'ed at 18pF which is implemented on the board with two 33pF caps, allowing for ~3pF of stray capacitance.
Some other observations:
- Most boards do not exhibit this problem. However, of the boards that end up in the 'not working' pile, the crystal not starting is the most frequently observed problem. On the couple of boards that we've investigated, the odds of the circuit starting up properly are ~20% or so.
- Touching with a scope probe, will sometimes get the circuit started, though many times it does not.
- Cycling power will sometimes get the circuit started.
- Waiting ~30 seconds will sometimes get the circuit started.
- Measuring the frequency error that results from using the two 33 pF caps and comparing that with the frequency error that results from instead using two 18 pF caps indicates that the ideal value to use on the board is ~30pF which suggests that the load capacitance is about right.
- The odds of the circuit starting up is the same whether using the33pF or 18pF capacitors which again suggests that the crystal load capacitance is not the issue.
- When the circuit does not start, both crystal I/O pins sit at 0 Volts. Since the 6446 should be implementing an inverter function, this seems to suggest that maybe that inverter is either not biased into it's linear operating region, or the gain of that inverter circuit is too low or the inverter is not being powered.
Based on all of the above, the root cause of the problem would seem to be
- Defective crystals (going to try swapping the two physical crystals to see if that is the case)
- Defective 6446 (looking for input from someone who might have seen this)
- Unidentified design error (any other suggestions on what that error could be?)