variable clock source to drive the pll?
rwgast_logicdesign
Posts: 1,464
i was just lying in bed with a 74hc4060 on my mind specificly nxps 95mhz version http://www.google.com/url?sa=t&source=web&cd=4&ved=0CGQQFjAD&url=http%3A%2F%2Fwww.nxp.com%2Fdocuments%2Fdata_sheet%2F74HC_HCT4060.pdf&ei=hT_UT_WGFqrO2AXq9IiEDw&usg=AFQjCNEM021m4qsYenQlBu3GhmQDQcw2OA i remmbered the data sheet said somwthing about using the chip as an oscilator in which the freqency is determined by an rc circuit.
it made me wonder could one potentially write a simple driver to control a digital pot conected to a hi quality cap to set the oscillation of this chip and then connect the chip to xo and xi. this would mean you could adjust your clockspeed with software and not have to buy a ton of extra crystals to experiment with over clocking. i searched the data sheet for pom to see if it couuld make acreasonable clock src for the prop and found nothing. would this work if not is there a similar solution?
.f
it made me wonder could one potentially write a simple driver to control a digital pot conected to a hi quality cap to set the oscillation of this chip and then connect the chip to xo and xi. this would mean you could adjust your clockspeed with software and not have to buy a ton of extra crystals to experiment with over clocking. i searched the data sheet for pom to see if it couuld make acreasonable clock src for the prop and found nothing. would this work if not is there a similar solution?
.f
Comments
Within limits, yes. The wider the range of control, the lower the stability and precision.
RC circuits give very wide control ranges, but they struggle to go above one part in 10,000 in jitter/stability.
Fine for quick tests.
The next step, is a LC circuit, where a varicap can give a narrower control range, and higher stability.
A Cap range of over 4:1 is all you need to adjust 2:1, and beyond that binary scaling can be used.