Very poor performance on lpc1768

Mbed Studio Feedback
1

Hi,
I’ve been trying to do some simple loops with the lpc1768. Due to form factor, I’m using the mbed board.
The issue is extremely poor performance; trying to drive the dac I’m getting about 1 cycle every two seconds, generating a sine wave. I took everything out of my code save the loop in main (just to test) and
cannot explain this terrible performance. On a different form factor lpc1768 using keil, the performance is great, and I can do 8k sine waves.
Does anyone have any feedback about this? I’d like to dump the mbed.h and compile just my keil code, but I get a lot of duplicate defines, (__ARM, for example) that don’t exist in my code.
Has anyone else had these performance issues?

Thank you

2

Hi Jack,
Can’t say I’ve had any problems with my LPC1768.
Are you using the On-Line compiler and which library, MbedOS or Mbed 2?
Try it on both and if you are using MbedOS, try it using the baremetal build, see here:

https://os.mbed.com/docs/mbed-os/v5.15/reference/bare-metal-example-application.html

Otherwise can you post your code that you are experiencing issues with and I’ll try it.
BR
Paul

3

Hi,

Thank you for your quick reply. This isn’t my code, but an example I found:
#include “mbed.h”

DigitalOut myled(LED1);
AnalogOut Aout(p18);

//Note if w_hat is pi/4 this means 8 samples/period. If we measure frequency of this output signal.
//then reconstruction frequency is 8 times this.
//Could put it through a low pass filter to smooth out.
//Also check the offset.

//Sine wave reconstruction in this case is much faster(tahn test4_sine) since I calculate the sample values before hand.
//Sine wave period is 2.2*5us = 12us
//Sine wave freq = 83KHz
//Reconstruction freq is 666KHz

unsigned short n;
#define pi 3.1415192
#define N 64 //Numebr of samples in one period
#define w_hat 2*pi/N //Normalised radian frequency
float sine1 [N]; //contains sine wave sample values

void init_sine_array(void);

int main() {
init_sine_array();
n=0;
while(1) {

        Aout = sine1[n];
        n=(n+1)%N;
}

}

void init_sine_array(void)
{
unsigned short n = 0;
for (n=0;n<N;n++)
{
sine1[n] = 0.5 + 0.5sin(nw_hat); //note the 0.5 V of offset since DAC outputs voltage between 0 and 3.3V
}

}

My code is similar, but uses a file of data instead of the sine wave.

jack