clarification about GRTC.SYSCOUNTER when in low-power sleep

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

Emil Lenngren — Fri, 17 Apr 2026 14:39:42 GMT

It is necessary to set the CLKCFG.CLKSEL register of GRTC to LFXO instead of SystemLFCLK to get the lowest power consumption, and that applies even if the LFCLK.SRC register is set to LFXO. Otherwise, the clock signal will take a longer trip through PCLK32KI which apparently costs extra energy, instead of using the direct path called "GRTC.LFXO". Not sure if this is documented anywhere but that's my observation at least.

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

Håkon Alseth — Fri, 17 Apr 2026 07:12:31 GMT

Hi,

I am glad to hear that it runs as expected at this time. Let us know if you run into any issues or have questions in the future.

I wish you a great weekend!

Kind regards,

Håkon

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

bios-bob — Thu, 16 Apr 2026 13:37:39 GMT

everything works as expected on the v1.0.0 DK.... my measurements are consistent with yours....

earlier in this issue, i identified an "extra" step needed to setup the CLKCFG register to use LFXTAL.... (i haven't verified whether this is "not necessary" on the more recent silicon -- but for the cost of one instruction, why bother....)

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

Håkon Alseth — Tue, 14 Apr 2026 07:00:44 GMT

That sounds good. Let me know how it goes!

Kind regards,

Håkon

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

bios-bob — Mon, 13 Apr 2026 13:22:28 GMT

i'm at v0.9.2 -- but do have another board coming tomorrow.... we'll pick this up then....

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

Håkon Alseth — Mon, 13 Apr 2026 12:17:03 GMT

Hi,

Thank you for attaching the test fw. I am measuring 1.2 uA "between the events" on this firmware:

I locked the scale here, to show where the ULP (ultra low power) regulator starts "taking over". Measurements before this period is relying on stored energy on the external caps from the previous active power mode, so will be lower than 1.2 uA until it switches fully over to ULP mode.

Setting the same retention configuration in NRF_MEMCONF as my former example gives the same, ie. 1.7 uA, as I previously measured.

[quote user="bios-bob"]

(i must have "old" silicon on my DK???)

[/quote]

It does indeed sound like there is something leaking on the DK that you have. It should be half of what you measure.

I have tested on nRF54L15-DK "PCA10156 v1.0.0" on my side. What does yours state?

Have you turned off features in nRF connect for desktop -> "Board configurator" application? These features, uart/external flash etc, can potentially leak smaller currents when enabled, which _could_ be the reason for the added around 1 uA floor that you're seeing.

Kind regards,

Håkon

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

bios-bob — Fri, 10 Apr 2026 22:33:59 GMT

i can confirm that there is a 0.2uA difference when i tested your project on my end.... my measurements were 2.5uA (without GRTC) and 2.7uA (with GRTC) at 3V.... (i must have "old" silicon on my DK???)

i'm attaching the program Pal Haland submitted.... when idle, it only saves one 16K block of SRAM.... otherwise, i believe all peripherals are disabled when sleeping -- except for the GRTC....

devzone.nordicsemi.com/.../1373.beacon_5F00_advertiser.zip

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

Håkon Alseth — Thu, 09 Apr 2026 13:55:34 GMT

Hi,

I setup a test project in zephyr (first 4 blocks on, system on idle):

devzone.nordicsemi.com/.../system_5F00_on_5F00_idle_5F00_test_5F00_nrf54l15.zip

Where I tried to replicate the setup that you have. Sorry for the magic numbers on the register level.

Testing on nRF54L15-DK, with "almost everything" disabled in the board controller application, and 3.0V:

I measure a difference of approx. 0.2 uA if I start the LFXO + setup the GRTC as you initially mention.

No LFXO: 1.6 uA

LXFO + RTC: 1.77 uA

As there is a write to the registers mentioned in the docs, the LFXO/GRTC will be kept on, accounting for the added ~0.2 uA:

If you are unsure of the difference between the SoftDevice setup vs. the bare-metal approach, and you think it is related to the clock configuration, you could use Segger Ozone to enter debug mode and look at the register content in both projects, to see if there is anything directly different between the two setups.

Kind regards,

Håkon

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

bios-bob — Wed, 08 Apr 2026 15:12:05 GMT

using the PPK, i just verified that the nordic code referenced above consumes about 1.74uA when sleeping; when awake it transmits a pre-specified BLE advertising packet....

my own code (completely bare-metal, no zephyr, no softdevice, touching registers only) just measured 2.14uA on the same board.... there is clearly some register i "forgot" to setup correctly....

another major difference.... the nordic code (based on a sample project in zephyr) is about 92K in size; my code (which performs the identical BLE advertising task) is about 2.5K in size.... and it clearly outperforms the nordic stack when active -- less energy consumed to perform the same task....

another major difference.... my code is written in a novel (open-source) programming language described at openem.org/emscript.... i could certainly use a review from nordic people who clearly know the subtleties of the nRF54l better than i....

if you're willing to help, just message me and we'll figure out how to proceed.... just trying to get the most out of your silicon....

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

Håkon Alseth — Wed, 08 Apr 2026 14:34:41 GMT

Hi,

If you only retain 16k RAM, the floor current should be below 2 uA.

Could you share the test-case that you currently have, so I can try to recreate the scenario on my side?

Kind regards,

Håkon

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

bios-bob — Wed, 08 Apr 2026 13:31:36 GMT

i'm not touching any of the GRTC registers which would make SYSCOUNTER active.... i've also verified that the ACTIVE bit is indeed 0....

what i recently discovered was that i was *not* setting up the CLKCFG register -- which by default is set to LFCLKSRC.... now i have setup the LFCLK to use the LFXTAL -- but apparently i should also explicitly set CLKCFG to LFXTAL as well....

while everything "works", i found that explicitly setting CLKCFG saved about 0.2uA when sleeping.... my test case is *only* retaining a single 16K SRAM block, which puts me closer to the 2.2uA range....

i'm trying to match the sleep current achived in nordic's submission to the BlueJoule benchmark (2uA), found at https://github.com/em-foundation/BlueJoule/blob/main/captures/nrf-54-dk/zephyr2-3V0-J/ABOUT.md

i (wrongly) presumed that the default setting for CLKCFG would be sufficient -- especially since i already bound LFXTAL as my LFCLK.... for whatever reason, it introduced about 0.2uA of additional sleep current....

i'm still not quite in parity with nordic's code; another 0.2uA to go....

RE: clarification about GRTC.SYSCOUNTER when in low-power sleep

Håkon Alseth — Wed, 08 Apr 2026 09:28:37 GMT

Hi,

If you write to any of the registers mentioned here, it will be active in sleep mode:

https://docs.nordicsemi.com/bundle/ps_nrf54L15/page/grtc.html#d763e406

[quote user=""]

all other things being equal, i find that this (benign) setup adds 0.4uA of *extra* current consumption when sleeping.... (2.1uA without, 2.5uA with)....

[/quote]

Which mode(s) are you testing?

The expected SystemOnIdle current, with full RAM retention, with- and without GRTC, is specified to be just below 3 uA (2.7 vs 2.9):

https://docs.nordicsemi.com/bundle/ps_nrf54L15/page/chapters/current_consumption/doc/current_consumption.html#ariaid-title4

Kind regards,

Håkon