Hs Pll

[Ron Marcus]

This is one for the "Gurus" out there. I would like to use an external clock source at say 6 MHz on the XTAL input pin with the PLL enabled to get a 24 MHz system clock. My question is, would the internal clock stop if the input clock stopped? I am doing a video overlay for a client, and the horizontal syncs by stopping the clock. The internal clock must stop when the external clock stops. It works fine with the HS setting, but I would like to raise the clock speed without pushing the cmos gated oscillator too high in frequency.
With other PLL systems, when the sync pulse is removed, the internal oscillator free runs. This would be a bad thing! Any ideas?

Thanks,
Ron

[skimask]

Which internal clock are you referring to? Which PIC are you using?
If you're talking about the machine clock stopping when you remove the 6mhz external clock, yes, it will stop because the internal 24mhz clock is being driving by the external 6mhz clock.
If you're talking about the internal 8mhz clock which is seperate, then, no, it won't stop and you can make use of the clock fail interrupt and switchover bits.
Don't forget that any clock switching takes xx milliseconds to restart itself internally.
Or maybe I'm missing what you are doing here.
The PIC runs, does some stuff, stops and waits for an H-sync, then starts up again? Could you sit in a tight loop waiting for an H-sync on an interrupt pin, then do what has to be done?
Or again, maybe I'm missing the point entirely...
JDG

[Ron Marcus]

The loop and wait idea is just not exact enough to keep the screen from jittering. As I said, it works with the HS setting, but is not quite fast enough.If the PLL really stops and starts accurately enough, it should give me the speed to update the pixel registers without losing a horizontal line. Right now, the characters have a one line space between each pixel. It's still very legible, but I'd like the letters in solid blocks. The character generator runs in machine language,currently on a '819. The update is performed by a seperate PIC using PBP with a shiftout subroutine.