I have a new fav Microprocessor. The Microchip Pic 18F2455. This is a feature-full yet powerful little device has almost limitless possibilities. Alas, you must understand that I am coming from the Pic 16F77, which lacks USB support, but is far more simple to get "blinky lights".
What the 18F2455 achieves in features, it gives up in ease of use (for newbies like myself). The external oscillator settings where an epic nightmare to get right! I believe the reason for this is twofold:
1. Nanowatt (tm) specifies the use of 2 clocks/oscillators for the 18F2455. The first clock for the main clock source to run the cpu itself; the ALU, the USB port(s) and all that. The second clock runs timers. This setup allows the main clock (and CPU) to power down leaving the second clock running to handle checking for new data on the USB port and running timers.
2. The USB port needs a strict 48 MHz clock to run, but Microchip still wanted to allow users of these chips to use oscillator configurations that they were used to. For example, for me, I have always clocked my chips at 20 MHz and I have a surplus of those crystals, so using a 20 MHz clock source would be really nice. I can still do this, but I need to use internal chip configurations to make use of the chip's internal pre/post oscillator scalers.
Here is how it all breaks down for me:
using 18F2455 and a 20 MHz external crystal. Please open the data sheet for the 2455 and refer to figure 2-1.
diagraming it out:
20 MHZ --> pins 9,10 (on pic)
you can see from Figure 2-1 that whatever you hook up to pins 9,10, you are feeding a PLL prescaler internally and you MUST feed the subsequent PLL prescaler a 4 MHz clock source as its input. This means that for me, I have to prescale my 20 MHz oscillator to 4 MHz. This means that we need to divide 20 MHz by 5. You can see from figure 2-1 that this bit setting is 100 on the PLL DIV MUX.
Now you have 96 MHz at the exit of the second PLL that is feeding another divider and also feeding a PLL postscaler. So then you can set the CPU DIV for the appropriate div for the main CPU clock source. This is 1 in my case because I want the main CPU to be clocked at 20 MHz.
If you are thinking that this is pretty confusing, you are right! There are a lot of choices (12 different oscillator modes) in oscillator setup and configuration. This leads to confusion and dismay. Mayhem might ensue. But, I think if you take the time to read Fig. 2-1 and learn the register bits that control which mode the clocks can be in, then I think this is an extremely powerful tool.