[GPS_Standard] Problems PN2222A Inverter missing occasional 1PPS

[Bob Stewart]

Michal and all,

I'm still getting the occasional rare (maybe 5 or 10 a day?) glitch in my 1PPS pulse.  The symptom is a 2D00 instead of a 9680 in the oscillator count value.  This was puzzling until I realized that it's simply 2 times 0x9680, which means that from time to time I am not getting a 1PPS pulse.  As mentioned, the circuit is a simple inverting amp with 1800 ohms in the base lead to the 1PPS signal from the Adafruit, and 1800 ohms in the collector lead.  So, should I use a different transistor, or different values for the two resistors?  I want to keep this as simple as possible, so I would prefer not to have to add a chip, even if that would be the best solution.  I am not a hardware person, and just blindly designed the amp for low current.

Bob - AE6RV





>________________________________
> From: Michal <michal at e2000.gdynia.pl>
>To: bob at evoria.net 
>Sent: Friday, November 15, 2013 2:12 AM
>Subject: Fwd: Re: [GPS_Standard] GPSstd_PLL testing: V0.13 with an improved State Machine 2
> 
>
>
>Hi Bob,
>
>My one cent about 2N2222 inverter used on PPS input between Adafruit receiver and 5V PIC input.
>>All precise timing are referred to the rising edge on
                  the PPS output signal. So if unchanged in GPS or PIC
                  software, the microcontroller will look for relatively
                  slow rising edge on its input (rise time = multiply of
                  input capacitance and pullup resistor). Events on the
                  falling edge of PPS signals are not defined such well
                  as raisng edge so uncontrolled jitter may occur
                  between succesive falling PPS pulses. I would suggest
                  to use two 74HCT series inverters in series with extra
                  470ohm 5V pullup resistor on the PIC input istead of
                  2N2222 inverting stage. HCT logic works well with 3.3V
                  CMOS outputs.
>>
>>
>>Best 73,
>>Michal
>>sp2iqw
>>
>