Michael,
I put your RTC in place this morning. My system xtal is 9.8304mHz and the interrupt is now slightly shorter than my earlier version by about 1us. The green pulse is the period of the ISR and the yellow is my 1/10sec heartbeart which i use for timeout for do loops etc. Vert scale is 2v/major div. Hor scale is 5uS/major div
Mark thanks for your ideas. I hope you are right and it is hardware,/noise. I take your point about sw being repeatable but I have 3 asynchronous RS232 devices plus a 1wire connected to the PCB as well as the 1/10 sec ISR which I thought may be interacting in an irregular fashion. I went through the code today and disabled timer1 in all my subs where 1/10sec time was not needed. i realise that this will cause my RTC to run slow by a few ms but it is updated every 15 min from the modem which will correct it. Hopefully this will point to hw or sw in a few weeks.
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