Comparator External Connections

RE: Comparator External Connections

SteveHx — Tue, 13 Jan 2026 10:53:18 GMT

Nope, I specifically need a comparator and I need it to trigger on two external valtage levels (ratio'ed to one changing external voltage) that are changing. I'm just thoroughly disappointed that the part doesn't do what the datasheet says, and Nordic still publishes the datasheet saying it works. I wasted several weeks.

RE: Comparator External Connections

snoopy20 — Tue, 13 Jan 2026 09:05:39 GMT

BTW I don't know what you're detecting but it's starting to sound like a high and low signal. Use GPIOTE for that.

RE: Comparator External Connections

snoopy20 — Tue, 13 Jan 2026 09:02:25 GMT

I've not used external reference so haven't hit that.

All you need is two resistors to limit the input voltage to a value lower than an internal reference then you can use that. Standard practice.

RE: Comparator External Connections

SteveHx — Mon, 12 Jan 2026 22:54:05 GMT

I just discovered this erratum. So, Nordic is just going to throw up their hands and say I bought a pig in a poke?!? I'm glad I didn't actually lay out a board based on the chip working as advertised!

3.33 [113] COMP: Single-ended mode with external
reference is not functional
This anomaly applies to Revision 2, build codes CIAA-Ex0, QFAA-Ex0, QFAB-Ex0.
It was inherited from the previous IC revision Revision 1.
Symptoms
COMP output is not correct.
Conditions
COMP is used in single-ended mode with external reference.
Consequences
COMP cannot be used in this mode.
Workaround
None.

RE: Comparator External Connections

SteveHx — Fri, 09 Jan 2026 14:20:59 GMT

Slight misunderstanding on the 5V. I have no intention of running there. This was just an accidental, momentary, and current-limited blip to 5V that caused an unexpected event. After it happened it, I reported it as a potential troubleshooting clue.

Likewise RefSel 3V was a coincidental discovery as I tried all values of REFSEL. Again no intention of using it.

COMP works just as documented for all values of REFSEL except the one I really need to use, which is 7 - ExtRef. I'm also thinking there's something I'm not understanding correctly, in that the hysteresis using the other reference choices works only if I keep the limits near min & max; i.e. UP at 0x38 or above, and DN at 0x04 or below. Also why it doesn't work when I set ExtRef (on Ain7) to the same 3.3V as Vdd. REFSEL = 4 works, REFSEL = 7 does not, regardless of TH settings.

RE: Comparator External Connections

snoopy20 — Fri, 09 Jan 2026 11:09:04 GMT

Careful on the clamp diodes they'll take very little. You should never see 5V on a scope as the pin will be Vcc+0.3v max otherwise it's passing current to Vcc through the ESD diode. 5V will be a big current and will blow them. Use a resistor ;)

"I need to set up the comparator to use an external reference"

I think you've already got what I was about to say which is do it the other way around, internal reference and external input, single ended, and adjust the lower&upper limits (these are basically acting as configurable hysteresis).

I can't really understand what you're trying to do. If the input is 3v+ then refsel 0-3 won't work. Vdd will work if the chip is getting more than 3v. Same with Adc pin but remember Vdd must be greater than Adc pin voltage.

Of course Vdd might be noisy, that's the whole point to the Adc ref pin. Maybe you should just do the normal thing of using refsel 0-2 and dropping the 3v+ down with a voltage divider (aka two resistors). Any reason why you haven't? For sure you need to if it gives to 5v!

* I have guessed you're using an NRF52 series.

RE: Comparator External Connections

SteveHx — Tue, 06 Jan 2026 21:02:31 GMT

Further (strange) info. I accidentally blipped 5V into Ain7, and things started working unexpectedly. Without doing that, only 4 (Vref = Vdd) works, and only if TH up & down are kept near the rails. But if I select other values for REFSEL, start it up, then blip Ain7 to 5V momentarily and back to 3V, I get the following results for the stated values of REFSEL:

0 - 1.2V ref
1 - 1.8V ref
2 - 2.4V ref
3 - 3.0V ref <<<<<undocumented
4 - Vdd ref
5 - no action
6 - no action
7 - no action <<<<<should use the AinX pin selected by EXTREFSEL

I know about the parasitic clamp diodes, but why should smacking Vdd with a gentle nudge start COMP working? My voltage calibrator signal "overload" at 5V, so clearly the clamp diodes are doing there thing.

RE: Comparator External Connections

SteveHx — Tue, 06 Jan 2026 20:33:29 GMT

The above works only if I select 4 (Vdd) for Vref. Per datasheet Figure 112: Comparator in single-ended mode, I should be able to feed Vref via Ain7 by selecting REFSEL 7 and EXTREFSEL 7. Even if I feed the same voltage as Vcc (external voltage calibrator), I can't get it to run correctly. Has anyone actually used these documented features of the COMP? Do they actually work? Nothing in the errata to suggest a problem.

RE: Comparator External Connections

SteveHx — Tue, 06 Jan 2026 00:32:22 GMT

A closer read of the datasheet shows that, counterintuitively, I must run the comparator in single-ended mode to accomplish what I want. It appears to work correctly with REFSEL set to 4 (Vdd) and adjusting the values in the TH registers works as expected - except that they must be kept close to their limits. I can live with that. But according to the DS I should be able to set REFSEL to 7 (ext ref) and then select an ADC pin to use as the reference. I have an external voltage calibrator connected to my selected AinX pin, but even with the calibrator voltage set the same as Vdd (3.3V), I don't get any action from the comparator. There is no value given in the datasheet for AREFMIN - the only reference to that value is under REFSEL.

It would be most helpful to have an actual schematic of what's going on inside the chip, because not all of the registers have the stated effects.

RE: Comparator External Connections

SteveHx — Mon, 05 Jan 2026 22:27:37 GMT

I can, if I must, use the 50 mV hysteresis with an external reference voltage. But the best I can do so far, whether diff or SE, etc., is to have switching at 150 mV & 350 mV. None of the register settings seem to have any influence.

RE: Comparator External Connections

L.B. — Mon, 05 Jan 2026 09:23:39 GMT

Unfortunately, I have no installation of the Nordic SDK at the moment. And I only used the nrfx library, so I have no experience on a register level. By looking at the datasheet, I can theorize that you can turn on/off a small hysteresis of 50 mV with the HYST register. I guess this is for the differential input of the operational amplifier (like setting the input offset voltage to 50 mV). My guess is that you would have to use external resistors and implement a classical Schmitt trigger circuit.

RE: Comparator External Connections

SteveHx — Sun, 04 Jan 2026 23:21:15 GMT

I started this thread several years ago, indeed I've forgotten what project was involved. It came up in a search just now, completely different project.

I need to set up the comparator to use an external reference, with hysteresis specified by the TH register. That is, I need the thresholds to be proportional to an external voltage, but with the up and down thresholds significantly separated. The documentation is not very clear as to exactly how to set that up. I have tried a lot of combinations of HYST, TH, REFSEL, EXTREFSEL, etc, and can't seem to get what I need. Hints?

RE: Comparator External Connections

snoopy20 — Sun, 03 Dec 2023 23:30:18 GMT

For anyone searching this, the comp if wiring both EVENT_DOWN and EVENT_UP to the GPIOTE_SET AND GPIOTE_CLEAR will eventually miss an event and go out of sync.

The trick instead is to use the single EVENT_CROSS and wire it to GPIOTE_OUT with the polarity set to invert.

To do this you must have a known condition with the comparator and enable GPIOTE default output to match. Then leave this be and don't change it. If you need to bypass the pin output use the GPIO drive high/low disconnect and pullup/pulldown if you need to.

The comparator still needs to be faster than your input signal, I just leave it in high speed and it is fine with a 1MHz signal.

I can confirm this works well!

Andrew

RE: Comparator External Connections

L.B. — Fri, 28 Jan 2022 08:20:13 GMT

Hi, I'm doing the same thing - I'm building an RC generator that involves the COMP module.

So far I haven't seen any glitches, so I decided to share my init code:

#define CONFIG_COMP_OUT_PIN 28

static nrf_ppi_channel_t m_ppi_channel;

nrfx_comp_config_t const comp_config = {
	    .reference = NRF_COMP_REF_VDD,
	    .ext_ref = NRF_COMP_EXT_REF_7, //P0.31
	    .main_mode = NRF_COMP_MAIN_MODE_SE,
	    .threshold = {
	    		.th_up = 34,
				.th_down = 30
	    },
	    .speed_mode = NRF_COMP_SP_MODE_Low,
	    .hyst = NRF_COMP_HYST_50mV,
	    .isource = NRF_COMP_ISOURCE_Off, 
	    .input = NRF_COMP_INPUT_1, //P0.03
	    .interrupt_priority = 0
};

void nrfx_comp_irq_handler(void){

}

void init(void){
	uint32_t gpiote_task_addr;
	uint32_t comp_event_addr;

	nrfx_comp_init(&comp_config, nrfx_comp_irq_handler);
	nrfx_comp_start(NRFX_COMP_EVT_EN_CROSS_MASK, 0);

	nrf_drv_ppi_init();

	nrf_drv_gpiote_out_config_t config = GPIOTE_CONFIG_OUT_TASK_TOGGLE(false);
    nrf_drv_gpiote_out_init(CONFIG_COMP_OUT_PIN, &config);

	nrf_drv_ppi_channel_alloc(&m_ppi_channel);

	comp_event_addr = nrf_drv_comp_event_address_get(NRF_COMP_EVENT_CROSS);
	gpiote_task_addr = nrf_drv_gpiote_out_task_addr_get(CONFIG_COMP_OUT_PIN);

	nrf_drv_ppi_channel_assign(m_ppi_channel, comp_event_addr, gpiote_task_addr);
	nrf_drv_ppi_channel_enable(m_ppi_channel);

	nrf_drv_gpiote_out_task_enable(CONFIG_COMP_OUT_PIN);
	nrf_drv_gpiote_set_task_trigger(CONFIG_COMP_OUT_PIN); //Start oscillations
}

I have inputs on P0.03 (-) and P0.31 (+), and an output on P0.28 (OUT) with this code.

Hope this helps!

Regards,

L. B.

p.s. My target is NRF52832 / nRF5_SDK_17.0.2_d674dde / gcc version 10.3.1 20210621 (release) (GNU Arm Embedded Toolchain 10.3-2021.07)

RE: Comparator External Connections

bjorn-spockeli — Fri, 04 Jan 2019 14:57:10 GMT

The SHORTS can be seen as an internal PPI system for each peripheral. You still need the COMP to generate the events for the SHORT to work, i.e. there must be a valid crossing on the pin that is detected by the COMP.

RE: Comparator External Connections

bjorn-spockeli — Fri, 04 Jan 2019 14:55:03 GMT

The PPI peripheral will only work with a Event register address as the EEP. Setting it to another address, e.g. the COMP's RESULT register will not work.

Bjørn

RE: Comparator External Connections

hmolesworth — Fri, 04 Jan 2019 04:07:42 GMT

I'm now wondering about the PPI 16MHz sample clock, which is effectively a synchronous sample of an asynchronous event (the comparator output transition) and is therefore subject to metastable issues. From the documentation:

On each PPI channel, the signals are synchronized to the 16 MHz clock, to avoid any internal violation of setup and hold timings. As a consequence, events that are synchronous to the 16 MHz clock will be delayed by one clock period, while other asynchronous events will be delayed by up to one 16 MHz clock period. Note that shortcuts (as defined in the SHORTS register in each peripheral) are not affected by this 16 MHz synchronization, and are therefore not delayed.

The shortcut register might be a useful alternative, but it's not immediately clear how that can be used on the comparator. It provides a fast (non-PPI) access to Start, Stop and Sample Tasks on the comparator. This implies a better way of updating the Result register without using the PPI:

The immediate value of the comparator can be sampled to RESULT register by triggering the SAMPLE task.

Worth trying the SHORTS, methinks; I see no link from Output so we can only try the Up, Down and Cross events to trigger Sample and hence bypass the PPI and potential metastable condition.

RE: Comparator External Connections

hmolesworth — Thu, 03 Jan 2019 17:17:05 GMT

Very interesting post; we would like the same functionality for a similar purpose. Given that the crossing event (being edge-triggered) may be missed, is it possible to connect the Compare Result register (0x400) to the PPI, or is that denied to it not being within the Events register groupings? My suggestion is based on the fact that the PPI link could in theory be set to any register, but I haven't tried this. The datasheet states Pointer to event register. Accepts only addresses to registers from the Event group but that might be misleading and any register address could perhaps work .. The Compare Result register register looks like t is the direct comparator output without any edge detection involved.

RE: Comparator External Connections

bjorn-spockeli — Thu, 03 Jan 2019 13:46:16 GMT

Yes, but my question was if there is a valid transition on the Comparator pin or not? IF there isnt, then is this transition is not detected by the comparator? If that is the case then outputing the comp output to a GPIO will not solve the problem. Or to you mean that the Comparator correctly detects the transition, but does not generate the transition event?

As far as I know there is no other way than using PPI and GPIOTE to output the Comparator output on a GPIO.

RE: Comparator External Connections

SteveHx — Thu, 03 Jan 2019 12:47:11 GMT

In essence I'm building a relaxation oscillator using the comparator. In my initial reading of the datasheet I missed the fact that it's apparently not possible to directly place the comparator output on an I/O pin, so I fell back to using the event / task / PPI mechanism. The transition event trips a task to flip the state of a GPIO pin. I a transition is ever missed, the output doesn't change state, and the oscillator therefore stops forever.

Fundamentally all I need is to be able to place the comparator output directly on an I/O pin. I know I could have firmware read the RESULT register and set a pin, but that's not feasible in my application. I have a stopgap solution whereby the firmware checks to see if the oscillator has stopped and gives it a kick start, but that's pretty messy and really not a good solution.

RE: Comparator External Connections

bjorn-spockeli — Thu, 03 Jan 2019 12:32:01 GMT

HI Steve,

[quote userid="68828" url="~/f/nordic-q-a/42019/comparator-external-connections"]Bottom line is that I have everything working nicely as it should, except that the comparator sometimes appears to miss a transition and thereafter is hung. I fundamentally just need to be able to place the comparator output on a digital output pin[/quote]

Could you elaborate on what you mean by the comparator missing a transition? In my mind If the comparator misses a transition, then the output from the comparator would be unchanged so it would not help to output the comp output to a GPIO.

Best regards

Bjørn