MC9S12D64 Oscillator Configuration

[sfb]

Hi,

We use MC9s12D64 our project. I need 20MHz bus speed. And I know that there is two alternatives for this:

1- Using 40MHz xtal osc. And this divided by two. So I will have 20 MHz bus speed.

2- Using for example 10 MHz xtal and by using PLL I will have 20 MHz bus speed.

Can anybody explain me what is the difference between this two choices. For example EMC chacarteristics of product which one is better. Or when communicating more than one uC by SCI, IIC or CAN bus is there any problem using PLL (for example baud rate differences...).

[Lundin]

The strongest argument is that using the PLL is generally more unpleasant for the designer. Personally I avoid the PLL because of all the fuss it creates.Debuggers, flash programmers and other such programs tend to get lost when they face S12 with the PLL activated. The Codewarrior debugger (at least the version I'm using) will for example get lost in space if you use the PLL.

From an EMC point of view, I don't think there is any difference between the alternatives, but that depends on how noisy the external oscillator is compared to the "Pierce" oscillator in HCS12.

When communicating with other devices, the PLL might be to prefer if you have very high demands on baudrate accuracy. A 20MHz bus will for example be good for CAN baudrates, but not ideal for SCI, and so on. You could in theory change the bus frequency depending on which peripheral you are using, but I think that is to ask for a punch in the face - troubleshooting will become unnecessary tough.