CLKOUT maximum frequency

[marcoelia]

Hi gents,

S32K-DS rev.13 - par. 6.5.7 says that maximum supported clock out frequency is 20 MHz. Is this referred to all CLKOUT pins? Is it really true? Which is the reason of this limitation? 

Specifically, for a S32K148 device in QFP100 powered at 3.3V, why should not be possible to output a 25 MHz signal from a FAST GPIO pin like PTD10 (pin 36)? IMHO the scenario would be the same of other FAST GPIO pins like PTC2/PTD7/PTD12 in RMII mode or PTD11 in MII mode.

Please also note that "S32K1xx_Clock_Calculator_Rev1" does not throw an error when CLKOUT is configured for frequencies greater than 20 MHz.

Thanks to all

[Robin_Shen · Accepted solution]

Hi Marco,

I use CLKOUTSEL to select a frequency higher than 20MHz through the CLKOUT pin, and I can observe the waveform on the oscilloscope.

Regarding your proposed plan, use CLKOUT to feed the 25 MHz clock input of their on-board Ethernet PHY (KSZ8081RNA, pin 8).

After discussed with hardware team, we think it's possible to use in such case. But please remind:
1. select a fast type pad GPIO pin for CLKOUT;
2. configure corresponding PCR[DSE] = 1 to ensure its output strength;
3. recommend to add a clock buffer if BOM cost allowed.

Best Regards,
Robin
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