Keeping Track of Time With Mbed - time

Question

I'm using the mbed platform to program a motion controller on an ARM MCU.
I need to determine the time at each iteration of a while loop, but am struggling to think of the best way to do this.
I have two potential methods:
1) Define how many iterations can be done per second and use "wait" so each iteration occurs after a regular interval. I can then increment a counter to determine time.
2) Capture system time before going into the loop and then continuously loop, subtracting current system time from original system time to determine time.
Am I thinking along the right tracks or have I completely missed it?

Answer 1

Your first option isn't ideal since the wait and counter portions will throw off the numbers and you will end up with less accurate information about your iterations.
The second option is viable depending on how you implement it. mbed has a library called "Timer.h" that would be an easy solution to your problem. The timer function is interrupt based (using Timer3 if you use a LPC1768) you can see the handbook here: mbed .org/ handbook /Timer. ARM supports 32-bit addresses as part of the Cortex-M3 processors, which means the timers are 32-bit int microsecond counters. What that means for your usability is that this library can keep time up to a maximum of 30 minutes so they are ideal for times between microseconds and seconds (if need more time than that then you will need a real-time clock). It's up to you if you want to know the count in milliseconds or microseconds. If you want micro, you will need to call the function read_us() and if you want milli you will use read_ms(). The utilization of the Timer interrupts will affect your time by 1-2 microseconds, so if you wish to keep track down to that level instead of milliseconds you will have to bear that in mind.
Here is a sample code for what you are trying to accomplish (based on an LPC1768 and written using the online compiler):
#include "mbed.h"
#include "Timer.h"
Timer timer;
Serial device (p9,p10);
int main() {
device.baud(19200); //setting baud rate
int my_num=10; //number of loops in while
int i=0;
float sum=0;
float dev=0;
float num[my_num];
float my_time[my_num]; //initial values of array set to zero
for(int i=0; i<my_num; i++)
{
my_time[i]=0; //initialize array to 0
}
timer.start(); //start timer
while (i < my_num) //collect information on timing
{
printf("Hello World\n");
i++;
my_time[i-1]=timer.read_ms(); //needs to be the last command before loop restarts to be more accurate
}
timer.stop(); //stop timer
sum=my_time[0]; //set initial value of sum to first time input
for(i=1; i < my_num; i++)
{
my_time[i]=my_time[i]-my_time[i-1]; //making the array hold each loop time
sum=sum+my_time[i]; //sum of times for mean and standard deviation
}
sum = sum/my_num; //sum of times is now the mean so as to not waste memory
device.printf("Here are the times for each loop: \n");
for(i=0; i<my_num; i++)
{
device.printf("Loop %d: %.3f\n", i+1, my_time[i]);
}
device.printf("Your average loop time is %.3f ms\n", sum);
for(int i=0; i<my_num; i++)
{
num[i]= my_time[i]-sum;
dev = dev +(num[i])*(num[i]);
}
dev = sqrt(dev/(my_num-1)); //dev is now the value of the standard deviation
device.printf("The standard deviation of your loops is %.3f ms\n", dev);
return 0;
}
Another option you can use are the SysTick timer functions which can be implemented similar to the functions seen above and it would make your code more portable to any ARM device with a Cortex-Mx since it's based on the system timer of the microprocessor (read more here: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0497a/Babieigh.html). It really depends on how precise and portable you want your project to be!
Original source: http://community.arm.com/groups/embedded/blog/2014/09/05/intern-inquiry-95