> doesn't work below about 1200hz

Fran, here's why (as I see it)

PIC processing speed = 5MHz
Timer1 can record 65,535 before overflowing
16 waves to count

700Hz = 7,143 cycles @ 5MHz  => * 16 = 114,286. Too many for
your measurement routine. The result is Timer1 = 48750 and TMR1IF
set (a total of 65536 + 48750). Your s/w can add a 'TMR1IF set' test

Re your observation that ~ 1200Hz is the lower limit

65,535 / 16 = 4,095.9 cycles per wave = 1221Hz

Using a 2:1 pre-scaler that will extend the lower limit to 610Hz

Your 1/25th Hz precision is still well within spec with a pre-scaler. At 
4kHz 1/25th of a wave is 10 cycles. A 2:1 pre-scaler reduces this to 5
cycles for just one wave measurement. At 512 waves the reduction /
error is negligible

wbr 
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