I am currently doing (very) preliminary investigations on a similar project (PIC->ADC0804). The ADC0804 handles up to about 1MHz but really wants 640kHz. Currently i'm looking at driving it off an RC network (as shown in specs). This would only need a couple of components and no chips so should be fairly space efficient (not sure if an RC is up to 1MHz tho). As the ADC interrupts on data ready the timing doesn't matter I just wait for the interrupt (it takes a variable no. of cycles as it's succesive approx. so knowing timing is no help). The specs for the ADC also give a way of wiring it up in a free running mode. I believe you tie the interrupt to the "Start Conversion" line so it continuously samples. If you only need to monitor 1 line this could work. You could just leave the ADC running and che check it's output whenever you want. In that way you only need 8 pins (for ADC0804) and timing is not an issue. You could still recieve the interrupt (9 pins) and thus only sample data when it was new but this isn't really neccesary IMHO. Just keep monitoring when you have a handful of spare cycles and if it's changed resend it. Another option would be to do what the PIC does (in external hardware) and get a 1/4 speed clock from the osc. and drive the ADC of this. This should give you a ~1.25MHz clock. Sorry but I'm pretty new to PICs so can't remember the circuit. It takes something like 3 Flip Flops to do it so one quad FF chip should do it. Hope that helps, Tom. ----- Original Message ----- From: Graham, Peter Subject: Generating clock signal > I am trying to modify an existing data logger that uses a 16c63 and an > AD7890. > > The ADC needs an external clock signal to do the conversion which at the > moment is generated in software on a normal output pin. My problem is > that because the signal is generated in software I can't do anything > during that time and I would also like to increase the frequency of the > signal to reduce the conversion time. > > The AD7890 has a max frequency of 2.5Mhz and the oscillator driving the > PIC is at 4.9152Mhz, so I can't use the osc. directly. > > I would like to have a freq of at least 1Mhz. Am I dreaming or is this > possible preferably without external components as there is no room on > the existing board design.