Some notes on low power modes

code/playground/nrf_sdk/rtc/config/sdk_config.h

@@ -69,7 +69,6 @@
 #ifndef NRF_CLOCK_ENABLED
 #define NRF_CLOCK_ENABLED 1
 #endif
-
 #ifndef CLOCK_CONFIG_LF_SRC
 #define CLOCK_CONFIG_LF_SRC 0
 #endif

code/playground/nrf_sdk/rtc/rtc_gcc_nrf52-32.ld

@@ -0,0 +1,64 @@
+/* Linker script to configure memory regions. */
+
+SEARCH_DIR(.)
+GROUP(-lgcc -lc -lnosys)
+
+MEMORY
+{
+  FLASH (rx) : ORIGIN = 0x0, LENGTH = 0x80000
+  RAM (rwx) :  ORIGIN = 0x20000000, LENGTH = 0x10000
+}
+
+SECTIONS
+{
+}
+
+SECTIONS
+{
+  . = ALIGN(4);
+  .mem_section_dummy_ram :
+  {
+  }
+  .log_dynamic_data :
+  {
+    PROVIDE(__start_log_dynamic_data = .);
+    KEEP(*(SORT(.log_dynamic_data*)))
+    PROVIDE(__stop_log_dynamic_data = .);
+  } > RAM
+  .log_filter_data :
+  {
+    PROVIDE(__start_log_filter_data = .);
+    KEEP(*(SORT(.log_filter_data*)))
+    PROVIDE(__stop_log_filter_data = .);
+  } > RAM
+
+} INSERT AFTER .data;
+
+SECTIONS
+{
+  .mem_section_dummy_rom :
+  {
+  }
+  .log_const_data :
+  {
+    PROVIDE(__start_log_const_data = .);
+    KEEP(*(SORT(.log_const_data*)))
+    PROVIDE(__stop_log_const_data = .);
+  } > FLASH
+  .log_backends :
+  {
+    PROVIDE(__start_log_backends = .);
+    KEEP(*(SORT(.log_backends*)))
+    PROVIDE(__stop_log_backends = .);
+  } > FLASH
+    .nrf_balloc :
+  {
+    PROVIDE(__start_nrf_balloc = .);
+    KEEP(*(.nrf_balloc))
+    PROVIDE(__stop_nrf_balloc = .);
+  } > FLASH
+
+} INSERT AFTER .text
+
+
+INCLUDE "nrf_common.ld"

code/playground/nrf_sdk/rtc/src/main.c

@@ -13,7 +13,7 @@
 #define LED_PIN 3
 
 // Seconds between RTC COMPARE0 events.
-#define COMPARE_COUNTERTIME (1UL)
+#define COMPARE_COUNTERTIME (3UL)
 
 const nrf_drv_rtc_t rtc = NRF_DRV_RTC_INSTANCE(0);
 
@@ -49,7 +49,8 @@ static void rtc_config(void) {
   nrf_drv_rtc_overflow_disable(&rtc);
   nrf_drv_rtc_counter_clear(&rtc);
 
-  // Set compare channel to trigger interrupt after COMPARE_COUNTERTIME seconds.
+  // Set compare channel to trigger interrupt after COMPARE_COUNTERTIME
+  // seconds.
   err_code = nrf_drv_rtc_cc_set(&rtc, 0, COMPARE_COUNTERTIME * 8, true);
   APP_ERROR_CHECK(err_code);
 
@@ -64,8 +65,8 @@ int main(void) {
 
   rtc_config();
 
-  // NRF_POWER->SYSTEMOFF = 1;
   while (true) {
+    // NRF_POWER->SYSTEMOFF = 1;
     __SEV();
     __WFE();
     __WFE();

resources.md

@@ -358,4 +358,28 @@ Changing the following on framework-arduinoadafruitnrf52/variants/feather_nrf528
 - [devzone thread](https://devzone.nordicsemi.com/f/nordic-q-a/12731/clocks-timers-rtc-why):
   - Timers use the HFCLK; better resolution, more power usage
   - RTC uses the low frequency source LFCLK; less accurate, less power usage
-I want RTC, since I don't need to be super precise and I want to save as much battery as possible.
+I want RTC, since I don't need to be super precise and I want to save as much battery as possible.
+
+## Power down
+- [powerdown examples](https://github.com/NordicPlayground/nrf51-powerdown-examples) for nrf51 and nrf52
+- How to measure current with the dev kit using an oscilloscope. [Docs by nordic](https://infocenter.nordicsemi.com/index.jsp?topic=%2Fug_nrf52832_dk%2FUG%2Fnrf52_DK%2Fhw_meas_current.html)
+- Good thread on devzone about current measurement [link](https://devzone.nordicsemi.com/nordic/short-range-guides/b/hardware-and-layout/posts/current-measurement-guide-measuring-current-with-n)
+
+There is something very weird going on. The current usage of my module varies wildly with the input voltage (Vcc). Something is off:
+- Vcc = 2.5 => 15uA
+- Vcc = 3.3V => 1ma
+- Vcc = 3.6 => > 2ma
+
+Very weird. What I did:
+- Tried the same sketch with a barebones nrf52832 module (E73-2G4M04S1B). It seems to behave as expected: around 2uA current in the input range 2V-3.6V;
+- Soldered _another_ nrf52840 to another breakout board. Now I'm seeing ~4uA in the input range 2V-3.6V;
+
+This suggestes there was something wrong with my previous nrf52840 module. Maybe I abused it too much or shorted something while soldering (but I wasn't able to find it with a contuniuity test). Also the system on might be because the nrf52840 has more RAM to keep powered on. We can selectively disable some, I think.
+
+Anyway, I'm considering this solved.
+
+A side note: the E73-2G4M04S1B module uses the "old" nrf52832 module, but it shows lower current _and_ it has a 32kHz crystal built in, which the E73-..C does not. I think the old module might be a better choice for us. The only downside is that it's slightly larger, but shouldn't matter at all.
+
+So the latest data is:
+- System ON sleep: ~4uA
+- System OFF sleep: 0.3uA (pretty cool!)