LPC4357 clock configuration

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LPC4357 clock configuration

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quyenngo
Contributor II

Hi all,

I have a question about clock configuration.

LPC43xx user manual document say that:

  The following procedure shows how to change the default setting of the core clock
(BASE_M4_CLK = 96 MHz; IRC = PLL1 clock source) to an operating frequency above
110 MHz while also changing the clock source from IRC to crystal oscillator:
1. Select the IRC as BASE_M4_CLK source.
2. Enable the crystal oscillator (see Table 127
).
3. Wait 250 s.
4. Reconfigure PLL1 as follows (see Table 138
):
– Select the M and N divider values to produce the final desired PLL1 output
frequency f
.
– Select the crystal oscillator as clock source for PLL1.
outPLL
5. Wait for the PLL1 to lock.
6. Set the PLL1 P-divider to divide by 2 (DIRECT = 0, PSEL=0).
7. Select PLL1 as BASE_M4_CLK source. The BASE_M4_CLK now operates in the
mid-frequency range.
8. Wait 50 s.
9. Set the PLL1 P-divider to direct output mode (DIRECT = 1).
The BASE_M4_CLK now operates in the high-frequency range.

In step 5. Wait for the PLL1 to lock, what is the maximum time system can wait, and what will system do if the PPL1 is unstable for a long time?

Thank you and best regards.

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1 Solution
733 Views
jeremyzhou
NXP Employee
NXP Employee

Hi Quyen Ngo,

The PLL setting process can be completed in 100us, and definitely the MCU can't work properly if the system clock become unstable.
Have a great day,
Ping

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2 Replies
733 Views
quyenngo
Contributor II

Thank you so much jeremyzhou,

Another question, in the lpc4357_oem_board library, when configure clock, it not check the PLL1 lock status follow the above procedure (step 5: Wait for PLL1 to lock). But the clock is still working well, how they do that?

Very best regards.

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734 Views
jeremyzhou
NXP Employee
NXP Employee

Hi Quyen Ngo,

The PLL setting process can be completed in 100us, and definitely the MCU can't work properly if the system clock become unstable.
Have a great day,
Ping

-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------