KL25Z Comparator library

Dependents:   ComparatorIn_demo TEMT6200_demo 05_comparator_demo 05_comparator_demo ... more

KL25Z Comparator library

Comparator features

  • Operational over the entire supply range
  • Inputs may range from rail to rail
  • Programmable hysteresis control
  • Selectable interrupt on rising-edge, falling-edge, or both rising or falling edges of the comparator output
  • Selectable inversion on comparator output
  • Capability to produce a wide range of outputs such as:
    • Sampled
    • Windowed, which is ideal for certain PWM zero-crossing-detection applications
    • Digitally filtered:
      • Filter can be bypassed
      • Can be clocked via external SAMPLE signal or scaled bus clock
  • External hysteresis can be used at the same time that the output filter is used for internal functions
  • Two software selectable performance levels:
    • Shorter propagation delay at the expense of higher power
    • Low power, with longer propagation delay
  • DMA transfer support not yet implemented in this library
    • A comparison event can be selected to trigger a DMA transfer
  • Functional in all modes of operation
  • The window and filter functions are not available in the following modes:
    • Stop
    • VLPS
    • LLS
    • VLLSx

Block diagram



This library allows us to create comparator objects between different inputs.
The comparator + and - inputs can be routed to one out of eight reference inputs (see table).
Input selection

Pin name#MUX inputCMP0Comment
PTE30100IN4CMP0_IN412-bit DAC0
110IN61V internal Bandgap*
NC111IN7internal 6-bit DAC0
(table 1)

* Not yet implemented

Using the library

Selecting pins on initialisation
Upon initialisation, the comparator registers are set to following values:

    CMP0->CR0   = 0x00;  // Filter and digital hysteresis disabled
    CMP0->CR1   = 0x17;  // Continuous mode, high-speed compare, unfiltered output, output pin disabled
    CMP0->FPR   = 0x00;  // Filter disabled
    CMP0->SCR   = 0x06;  // Disable all interrupts and clear flags (flags are cleared by this write)
    CMP0->DACCR = 0xE0;  // DAC enabled, Vdd is 6bit reference, threshold set to 1/2 of full-scale (1.65V)

The library accepts two input parameters:

ComparatorIn compi(PTC8, NC);
  • First parameter : + input pin.
  • Second parameter : - input pin.
    Every pin from the 'Pin name' column in (table 1) can be selected.


  • IN6 (internal 1V bandgap reference) has no pin name and is not selectable on init.
    However, we can use the SwitchPlus and SwitchMin functions to change the corresponding input to IN6.
  • There are two special cases for the input parameters:
    • NC : Connect the internal 6-bit DAC0 to IN7.
    • PTE30 : configures PTE30 as 12-bit DAC0 output and connect to IN4.
      IMPORTANT: Make sure no external output is connected to PTE30 when you declare this pin, otherwise you WILL destroy the external device and/or the CPU.

Following code demonstrates interrupt callback and polling mode.
Note: Polling the comparator this way won't always trigger the printf because of the wait() instructions (if we go above and below the threshold during the wait() time, then the polling cannot not see this change).

#include "ComparatorIn.h"
#include "mbed.h"

DigitalOut blinker(LED_BLUE);
DigitalOut cmpled(LED_GREEN);
DigitalOut cmp_en (PTD5);
AnalogIn cmp_lvl (PTB0);
ComparatorIn compi(PTC8, NC); // in+ = PTC8, in- = internal 6-bit DAC 

// Comparator callback functions
void cmp_rise_ISR(void)
    cmpled = 0;

void cmp_fall_ISR(void)
    cmpled = 1;

int main()
    cmp_en = 1;
    cmpled = 1;

    compi.rising(&cmp_rise_ISR);                // Set pointer to rising interrupt function
    compi.falling(&cmp_fall_ISR);               // Set pointer to falling interrupt function
    compi.treshold(0.5);                        // Set comparator threshold to 1.65V = 32 * 3.3V / 64

        printf("Light sensor : %7.5f Volt\n",cmp_lvl*3.3);
        blinker = 1;
        blinker = 0;
        if (compi.status() == 0x01)
            printf("*** Treshold reached : %7.5f\n",cmp_lvl*3.3);


  • When we enable the comparator interrupt, we need to declare user callback function(s) (rising and/or falling interrupt) AND initialise the function pointer(s).

  • Currently, following functions are not yet implemented:
    • On the fly MUX switching for + input (SwitchPlus function).
    • On the fly MUX switching for - input (SwitchMin function).
    • DMA transfer.