ENET Clocking Problem[MCP5748G]

[medhedibasly]

Hello,

I'm working with MPC5748G, i first start my project on Eval Board from NXP which have a 40MHz external oscillator, after that when the code arrived to a mature state, we changed the project to it's real target board, which is custom board with external oscillator at 16MHz,

when the code is downloaded to the target board with 16MHz oscillator. no PLL configuration is done but some communication modules kept working fine which are CAN, I2C and Ethernet.

I analysed the situation and i figured out that for the CAN and the I2C there is really no problem about the clock wrong frequencies but for the ETHERNET i think that we should have a problem. but we don't !!

So her is my question, with 16MHz external oscillator and the same PLL config from the eval board that give 160 from 40. PLLDV[MFD] = 16, PLLFD[MFN] = 0, PLLDV[PREDIV] = 1 }==> 40 *(16/4) = 160MHz.

but with 16MHz } 16 *(16/4) = 64MHz 

==> so my system is runnig at 64MHz

in reference manual Paragraph 9.7.2 according to clock restriction my configuration does not respect the ENET AHB restrictions nor the ENET timer clock period which connected to F40 = 16MHz due to wrong config ==> period 62.5ns

The problem is that the ENET module works fine in both cases and i can't find why.

Any explanation would be appreciated. just ask me me if you need more information.

MC_CGM_AC2_SC is not reconfigured in my code so ENET is clocked from F40.

for exemple when system clock is 160MHz the F40 is 1/4 which is 40Mhz, when we have 64Mhz does the ration 1/4 for F40 keep having sens to give F40 = 16Mhz ? 

[petervlna]

Hi,

what could lead you to do such thing.

If you keep the 40MHz clock settings and use only 16Mhz XTAL you will most probably violate VCO range.

Your mode transitions will fail if your PLL is not locked as you are asking micro to use it as new mode target clock.

If you are using 16MHz external clock source I recommend you to do the PLL module configuration for such clock source.

Otherwise you will run into issues.

Peter

[medhedibasly]

Hello,

Thank you Peter for your reply, I'm totally with you concerning the PLL configuration.

But my question is concerning the ENET clock, with the wrong PLL config i should have my CPU running at 64MHz which will give wrong clock configuration for the ENET module but my ENET is working correctly without correcting the PLL configuration. 

Actually i changed the PLL configuration and my system is running at 160MHz again with no problems, but i want to know how every thing is working to be able to predict the impact of the modification on my system behavior.

Hedi,

[petervlna]

Hi,

I think it is just pointless to discuss about the configuration which this micro does not support.

Even if it may run in your setup, did you tried it in temperatures (-40 +150) ? Did you evaluate such configuration in all scenarios? Imagine what has design and test to do in order to ensure that datasheet defined values are valid for all use-case scenarios.

Peter

[medhedibasly]

Hi,

Your reply make sense, this project is a bit critical so the more we take caution the better it is.

Thank you for your reply,

Hedi