Problem: I have 50% of my CC3200 first pre-production batch of 20 PCBs, have intermittent 32.768KHz Crystal (do not oscillate). It seems like is intermittently starting up with Pin 51 in external 32KHz input mode (as if Pin 52 is held high to powering up in external oscillator mode).
Circuit Conditions:
1) Has the normal 32KHz circuit is the crystal and two 10pF caps, as per the Launchpad reference design. Crystal is 32.768kHz, 4pin, CL: 12.5pF, 50KR ESR, Freq Tol@25°C: ±20ppm, Abracon Corporation: ABS25-32.768KHZ-T, Digikey 535-9166-2-ND - Chosen for "hysterical" reasons - I know it is physically large. I have also tried placing a simple through hole common, round-can, 32KHz crystal (Abacorn: AB38T-32.768KHZ, 32KR ESR, 12.5pF, Digikey: 535-9034-ND) with the same results. Also tried pulling the 32KHz crystal off the launch pad and placing it on my PCB, and it still intermittently works (same fault).
2) The nReset pin has only the 100k 100nF cap as per the reference design.
3) The power supply is rock solid VIN 2.5V (pin 10, 37, 39, 44, 54) , DC-DC Converter with a super-cap (1F) on VIN pins (output of DC-DC converter) to provide the high 700mA calibration pulse. I have tried it also without the 1F super-cap placed for fast VIN rise time, but same result.
4) SOP0 High, SOP2 low, SOP3 low for external serial boot-loader mode (does not need super-cap anyhow in this condition, as it does not get to "APP CODE LOAD" - hence I can try try it also without the 1F super-cap). I am unable to load the Application code into flash cause of now 32KHz.
5) I have the nReset pin also tied to the open drain POK (power OK) output of the DC-DC converter (AMS: AS1337B-BTDT see DigiKey: AS1337B-BTDTTR-ND). The POK is released when the VIN pins (2.5V) is within 10% is 2.5V (i.e. When > than 2.25V). Besides item B (below) shows that after the super Cap is charged (2.5V), that togging the nReset line does not start the 32KHz.
Observations:
A) I observe with a scope that the normal sequence is nReset rises then the 32kHz slowly starts up.
B) I have observed, on a working unit (and when these intermittent ones work), that the 32KHz internal inverter works OK (I observed this, by forcing pin 51 high and see that pin 52 goes low, and visa-versa). Once it is working OK (intermittent 32kHz oscillator at first power up), I can reset the CC3200 chip (pull nReset) low, and the 32KHz Crystal always starts up reliably. But often when power is first applied, the 32KHz does not oscillate after nReset rises. It seems to be in external 32KHz clock source mode (the 32Kz internal inverter is no longer functional).
C) Then it does not work, no matter how I try to pull Pin 52 high or low (hold high or low, 100kR or not) while toggling only the nReset pin, it remains in this "external" 32KHz clock source mode. Then after many power on-off cycles (with Super cap discharged, and even without super cap placed), I can get 32KHz to run, and I can observe the normal "inverter" pin 52, 51 function on the scope.
D) Swapping the 32KHz crystals around (see item 1 above) shows the same result, interestingly placing the through hole common, round-can, 32KHz crystal (Abacorn: AB38T-32.768KHZ, 32KR ESR, 12.5pF, Digikey: 535-9034-ND) on the Launchpad reference design now also is intermittent on Launchpad. - Has the 50KR RSR spec been exceed by heat from soldering????
This is a worry, cause when I move to my volume 10k production run, I may have many intermittent CC3200 32KHz crystals not running.
Questions:
a) I am about to purchase a whole lot of other crystal samples, what specs would be best (lower ESR? lower load capacitance)???
b) I am also about to see if it is in external 32KHz clock source mode, by connecting an external clock source. What ways can I force it into internal 32Hz mode (I know about the 100KR pull-up for setting it into crystal mode - see item C above)?
c) How could the super-cap slow VIN rise time with POK nReset signal cause this problem??? It actually isn't cause it has the same problem without the super cap placed. See item 3 above.
d) Can a wrong (or bad) 32KHz cause the the CC3200 to start up in external 32KHz clock source mode? How can it? See items B and C above.
e) Has anyone got nay suggestions???... I will keep you posted as I diagnose further today.....
