Maximum Input Frequency of LPC81x?

[tiberiumcode]

I'm trying to make a high frequency counter out of an LPC81x. I can use an external crystal to make the LPC81x run at 24 MHz, but I'd like to count the frequencies of square waves in the 100MHz-200MHz range. Can I configure the SCT and its prescalers to count faster than the bus clock, or do I need to use a clock divider chip external to the LPC81x? I cannot find the dynamic characteristics of the microcontroller's inputs in either the UM10601 LPC81x User Manual or the LPC81x datasheet.

[jeremyzhou]

Hi Daniel Vibbert 

Thank you for your interest in NXP Semiconductor products and for the opportunity to serve you.
Actually, the duration of the high and low levels on the input pin must be greater than 1 clock cycle, and the frequency of the input must be less than one-quarter of the clock rate.
About your question, the LPC81x is incapable of detecting the input PWM whose frequency exceeds the 100 MHz.
Have a great day,
TIC

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