MC9S08GT60 + SCI @ fast rate: should I use Internal or Exterrnal clock?

[ehestigoni]

Hi Guys,

I 've been reading some information in this forum about clock configuration, however I couldn't make a decision yet...

I will be sampling 4 analog signals via 4 ADC pins and sending them via SCI (through an FTDI chip) to an USB port. So that it is very interesting to reach the maximum speed I can.

In order to use a rate of 250000 baud (it is tha maximum USB 2.0 speed, right?) do I need (or is it better) to use an external cristal in the MCU? As far as I understand, the fastest  cristal I can use is 16MHz... Is it right?

If using the internal oscilator, can I reach the same speed I would reach by using the external cristal?  Or, is it fast enough to sample all these data?


Please let me know what is the best option.


Cheers,

Ed.

[peg]

Hi Ed,

16MHz is as fast as you can go with a crystal and no FLL. This is because 16 MHz is the fastest crystal you can connect. However you can use a slower xtal (or the internal clock) in combination with the FLL to get the GT going at or very near its maximum or 20MHz buss speed. This gives a maximum SCI baudrate of 1250000 although the CPU will struggle to keep up with this.

[ehestigoni]

All right Peg, Thanks for your advice.

Now I've got another one for you...

I was intending to do some tests with the GT60 on the breadboard and ordered the SDIP parts... Now I just realized that they are not PDIP and they don't fit on it...  Any idea of how I could still use the SDIP MCUs for prototyping?

And also, they don't have the device pin diagram of the SDIP package on the datasheet. I've been trying to figure it out but the other packages have 44, 48 and 64 pins instead of 42 as SDIP. Do you know where I could find it?


Cheers mate,


Ed.

[Alban]

Please, one subject = one thread, otherwise we end up with totally useless subject lines.
Thread is now locked as both subjects closed.




Message Edited by Alban on 2007-09-12 10:44 AM

[peg]

Hi Ed,

I guess you could use a pin adapter PCB to convert it to PDIP.

But I would suggest a M68DEMO908GB60 board from FSL or Axiom. Far easier, just use the big header to cable over to your other circuitry. Also serves as a standard base to sort out issues as others can duplicate.

The 42-pin SDIP package pinout is shown on page 42 of the GB60 manual.

[ehestigoni]

All right,

I had the GB60A (GT60A) manual... 
Just downloaded the GB60 (no A) datasheet and found it.

Thanks mate.


cheers,

ed.

 

[peg]

Woops!

Correction to previous post:

The pinout for the 42-pin SDIP GT60 is on page 26 of datasheet MC9S08GB60/D Rev. 2.3 12/2004