PLL Problems?

jkroylance

I was messing with my protoboard the other day and replaced the 5MHz xtal with a 6MHz one and it ran fine with a 16x pll, but when I try to use the 6MHz xtal with another one of my projects that uses the 40-pin DIP version, it won't run @ 16x pll. If I tell it the xtal is 12MHz (double) and 8x pll (half), it works fine. As far as I know the chip has lived a sheltered life and not been over-voltaged or static zapped.

The program I was using to test just set a pin as an output to turn on an LED. Worked fine on the protoboard, but on the 40pin it acted as described above.

Any ideas?

Mike Green

The Propeller is not guaranteed to work with a system clock above 80MHz. Many Prop chips will work at 96MHz (6 x 16), but it's not guaranteed. Maybe you just found one that won't. The crystal speed you specify is only used for calculation of timings, like it's used for computing serial port delays and video timing delays, but it's not otherwise required. The PLL multiplier is used to configure the clock PLL. By setting it to PLL8X, you've set the system clock to 6 x 8 = 48MHz which is well within the spec.

Philldapill

So basically, with this setup you have, all your timing delays will be 2X slow. i.e., if you say 'waitcnt(clkfreq + cnt)', the system will wait for TWO seconds, instead of one. This applies to every other timing issue like serial data transfer, TV, etc.

jkroylance

I forgot to mention the 40pin also does this with a 5MHz xtal - 16x pll won't work, but 8x will...

Mike Green

You really haven't given enough information to figure this out. What you describe with the 40 pin Propeller can't really happen. Either the whole clock PLL goes or not. There must be something else going on.