115200 on MC9S08QE128 with a high bus and system clock

[Limestone]

Hi!

I am developing a system that will need to communicate at 115200 in the temperature span of +20 to +70 celcius. As internal oscillator are known to be temperature dependent, I guess I have to use an external crystal (Correct?). I also need a fairly fast bus clock and system clock.

Right now, during development, I am using the internal oscillator at 36 kHz, trimmed it during programming to 36 kHz. I have a bus clock of ~18 MHz and and a FLL output clock of ~36 MHz.

I can use the same values with an external crystal that matches the internal oscillator. A 36 kHz crystal, but as they are about ten times as expensive as for instance a 3,6864 cyrstal, I wonder if there are any other settings (using a more common crystal) that will allow me to have a relative high system speed and 11520 baud rate.

Best regards

Martin

[tonyp]

You can get usable 115K baud even with the internal oscillator, if you trim the device.  By the datasheet, the oscillator deviation percentage (2%) is valid throughout the operating temp range.  So, you should be OK.  Still, you may want to use a crystal for lower noise.

I like the 4MHz [or 8MHz] crystal which after dividing down (RDIV) by 128 [or 256] (for 31.25KHz base frequency) and using the 1536 FLL multiplier, you get 48MHz (24MHz bus) which is near the top end of the specs, and also provides exact real-time clock.