Ah, thanks, it was an OSCCAL RETLW problem. I'd set and checked the calibration value using
pk2cmd -PPIC12F675 -MC -MP -F obj/blink.hex -YP -E -T -R -GP3FF-3FF -U80
which gave the value at location 3FF as 000080 - i.e. the calibration value was set but the RETLW instruction was missing. Solution:
pk2cmd -PPIC12F675 -MC -MP -F obj/blink.hex -YP -E -T -R -GP3FF-3FF -U3480
to write both the value and instruction - and lo, the scope shows that my LED is happily blinking
I still have the same code, CONFIG, and VDD-first settings, so I will watch out for silent corruption of that data, but a few reprogrammings without the -U option have not clobbered it yet.
Out of interest, I had a play with the extremes of the calibration value, and found that the speed ranged from 5.3MHz at FF, to 3.3MHz at 00.
Many thanks for your help.
post edited by tangent - 2013/12/15 19:44:26