MC9S08SH8 with 20MHz bus clock???

[RogerSchaefer]

Hello,

The flyleaf page of the MC9S08SH8 data book says that with the internal clock source the MPU should "support bus frequencies from 2 MHz to 20 MHz".  When I tried setting the ICSTRM to get 20 or 16 MHz the chip failed.  No problem setting to 8 MHz.  When I tried to load a simple test program into the chip at a 12 MHz it seemed to work.  At 8 MHz the ICSTRM value for this particular chip was $BC and to get 12 MHz the value need was $3E.

The little test program  sends out a message at 9600 baud and blinks a LED works at 12 MHz but now I can no longer communicate with the chip with my OSBDM.  Does anyone have any idea why that may be?

Has anyone got the faster bus frequencies with the SH8?

Roger

[bigmac]

Hello Roger,

From the datasheet, it would appear that the trimmed frequency range for the internal reference must lie within the limits 31.25kHz to 39.0625kHz.  I would presume that trimming for operation outside these limits limits would not be guaranteed.  Therefore, the corresponding frequency range of the DCO would be 32 to 40MHz.

The bus frequency will depend on the BDIV setting.  For the reset default division by 2, the bus frequency range would be 8-10MHz.  If the BDIV setting is changed to division by 1, the bus frequency range would be 16-20MHz.

I would make two observations -

1. A bus frequency of 12MHz is not valid using the internal reference.
2. If the BDIV setting is changed from 2 to 1, this should be done after frequency trimming has taken place within the above frequency limits.  Otherwise the maximum bus frequency limit may be exceeded.

Regards,

Mac

[RogerSchaefer]

Thanks Mac,

I had forgotten about BDIV. 

I had a S08SH8 running at 8 MHz and I changed BDIV to divide by 1.  The bus speed jumped to 16 MHz.  I knew this because I had to change my terminal program from 9600 to 19200.

So to get 20 MHz the thing to do is set my trim program to find the proper trim at 10 MHz then change BDIV in the application program.

Roger

PS  It wouldn't hurt Freescale to put in a few example programs in their data books