11 years, 4 months ago.

SPI / I2C clock defect? (frequency irregular above 20kHz)

I'm trying to get the lpc1768 SPI port (p5,p6,p7) running. I started using the example code in the Handbook section ( https://mbed.org/handbook/SPI ) but i could not establish communication with the connected device (LCD display EA DOGM162). (Additionally, i used pin p15 as DigitalOut for chipSelect and pin p16 as DigitalOut for registerSelect).

Trying to check the SPI clock frequency, i measured the ouput signal on pin p7 (sck) with a Tektronix Oscilloscope (TDS460, up to 350MHz). Using the following code, i expected to measure a constant frequency of 380 kHz on p7:

#include "mbed.h"

SPI spi(p5, NC, p7); // mosi, sclk 
  
int main() {
    spi.format(8,3);
    spi.frequency(380000);
 
    while(1) 
    {
       spi.write(0x00);         
    }
}

I varied the value of spi.frequency and measured the following results on pin p7 (sorry, i don't know how to make a table...):

spi.frequency():       measured frequency p7:        measured amplitude p7:

     10 kHz                 10 kHz                   4.4 V (range: 0 - 4.4 V)
     20 kHz	            20 kHz		     3.5 V (range: 0.2 - 3.7 V)
    100 kHz		    constant (DC) 	     noise (level: 2 V)
    380 kHz		    20 kHz		     0.2 V (range: 1.9 - 2.1 V) 
     1  MHz		    irregular (10-20kHz)     0.1-0.2 V (range: 2.0 - 2.2 V)

Formerly, I also couldn't get the I2C ports running because there also the clock didn't seem to work.

Question: is my mbed defect (which seems very probable to me) or did i miss to activate / declare / connect something very elementary?