topic ECLK or two crystals in the same PCB in S12 / MagniV Microcontrollers

ECLK or two crystals in the same PCB

arcano — Wed, 04 Jun 2008 08:15:19 GMT

Hello everyone.

I have a project, using two MCU: MC9S12UF32 and MC9S08AW60. I need to make them work at full bus frequency: 30MHz and 20MHZ respectively.

I could use MC9S08 internal self-clocked source but I need high clock accuracy.

Can I use two oscillator crystals of 12MHz and 4 MHz in the same PCB without inducting noise?
Or I can use the ECLK signal, in expanded mode, to drive the 8-bit MCU? If I do so, could I use 9S12 ECLK at 30MHz or 20MHz, driving 9S08 MCU in EXTAL pin at 15MHz or 20MHz bus frequency respectively?
I want also to use external memory, at full speed (30MHz bus frequency) if I can.

What do you recomend?
Thank you.
Best regards,
Hernán Acosta

Re: ECLK or two crystals in the same PCB

bigmac — Wed, 04 Jun 2008 11:51:16 GMT

Hello Herman,

There shouldn't be a problem using two separate crystals, one for each MCU. However, if you wish that the HCS12 provide a clock source for the HCS08 device, keep in mind the following -

If you intend using FEE mode for the HCS08, the external frequency must not exceed 10MHz.
If you use FBE mode (bypassing the FLL), the applied frequency has a maximum limit of 40MHz, but the maximum bus frequency would only be one half this frequency.

It would also be possible for the AW60 device to output a clock signal, via the MCLK pin, potentially to drive the HCS12 device. The maximum output frequency would be fbus/2. However, there might be an issue (for the HCS12) with the amount of jitter associated with the FLL signal.

Regards,

Mac

Re: ECLK or two crystals in the same PCB

arcano — Fri, 06 Jun 2008 22:23:28 GMT

Thank you for your response Mac! I'ts very helpful.

I think I'm going to use two separate crystals, temporaly in prototype. Then I will choose FEE mode.

Regards,

Hernán