S08jm60 with USB and 32KHZ clock

[donw]

I wish to use the s08jm60 with USB, so the bus speed must be 24mhz.
The data sheet shows an example of using a 4mhz xtal which is divided down to 32khz
to drive the PLL which generates to 48mhz main clock.
I have an external 32khz clock source (from a RTC ic) . Can I use this to feed to PLL, rather than a 4mhz xtal?

The data sheet has AC specs for 5volt operation only.
Can the micro be run at full speed (24mhz bus) at 3.3Volt ?

[fabio]

Hello Donw,

According to the DS, that is not possible. Page 293 (USB clock requirements) states that the MCG must operate in PEE (PLL engaged external) mode with a 2 or 4MHz crystal.

The DS also states that the PLL must have a frequency within the range of 1 MHz to 2MHz after divided by RDIV. Section 12.4.6 (External reference clock) states that when using PEE and PDE modes, the external reference clock must be within 1MHz and 16MHz.

Best regards,

[Moizez]

Hi,

I understand that a 4MHz crystal would work fine for USB usage. However, I wonder why the DEMOJM board uses a 12MHz crystal. Is there any advantage in that?

Regards,
Moises