I am reading a book that teaches about programming the LPC1768.

I have the following exercise here: Using any digital output pin, write a program which outputs a square wave, by switching the output repeatedly between Logic 1 and 0. Use wait( ) functions to give a frequency of 100 Hz (i.e., a period of 10 ms). View the output on an oscilloscope. Measure the voltage values for Logic 0 and 1. How do they relate to Fig. 3.1? Does the square wave frequency agree with your programmed value?

My problem is that I don't have an oscilloscope. Without it, this exercise is sort of impossible for me but I am guessing that indeed my frequency will be higher than expected.

I am thinking that it will be higher due to the output timing being dependant on the internal clock.

Am I right in thinking this? If not, what sort of frequency would I get and why?

  • \$\begingroup\$ where is link to figure? \$\endgroup\$ Jan 23, 2019 at 16:41
  • \$\begingroup\$ At 100 Hz you can use a computer soundcard, there may well be distortion but you could see frequence, duty cycle and jitter (for example if any interrupts periodically fire at a slow rate) \$\endgroup\$ Jan 23, 2019 at 17:09

1 Answer 1


The comment of "Chris Stratton" is good for a simple measuring, and for learning

For the theoretically approach there are some basic you need to know about software: A while loop takes time (even the following empty one).



this means that the following code will take more time then 10ms


Further you need to know that the relation between frequency and time is


Which makes that more time is a lower frequency and vice versa.

At last i want to add that the use of a wait statement in embedded software is a vary bad practice. It makes it impossible to do multiple things simultaneously in a microprocessor. For timing critical software one should use interrupts, for non critical software one should use time-flags and and state-machines. So to create a stable signal of 100hz in a LPC1768 one should use the timer-matching-compare unit of the processor


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