diff --git a/ion/src/device/n0100/drivers/config/timing.h b/ion/src/device/n0100/drivers/config/timing.h
index 71d6c3f3d..27757c153 100644
--- a/ion/src/device/n0100/drivers/config/timing.h
+++ b/ion/src/device/n0100/drivers/config/timing.h
@@ -8,6 +8,8 @@ namespace Device {
 namespace Timing {
 namespace Config {
 
+#warning N100-calibration
+//TODO: calibrate LoopsPerMillisecond now that we decrease AHB clock while msleeping
 constexpr static int LoopsPerMillisecond = 8852;
 constexpr static int LoopsPerMicrosecond = 9;
 // CPU clock is 96 MHz, and systick clock source is divided by 8
diff --git a/ion/src/device/n0101/drivers/config/timing.h b/ion/src/device/n0101/drivers/config/timing.h
index 0c0e48a4b..747611b5c 100644
--- a/ion/src/device/n0101/drivers/config/timing.h
+++ b/ion/src/device/n0101/drivers/config/timing.h
@@ -8,6 +8,7 @@ namespace Device {
 namespace Timing {
 namespace Config {
 
+// TODO: calibrate msleep
 constexpr static int LoopsPerMillisecond = 8852;
 constexpr static int LoopsPerMicrosecond = 9;
 // CPU clock is 96 MHz, and systick clock source is divided by 8
diff --git a/ion/src/device/shared/drivers/board.cpp b/ion/src/device/shared/drivers/board.cpp
index a93a5145c..f37af00e5 100644
--- a/ion/src/device/shared/drivers/board.cpp
+++ b/ion/src/device/shared/drivers/board.cpp
@@ -57,6 +57,21 @@ void shutdownPeripherals(bool keepLEDAwake) {
   Display::shutdown();
 }
 
+void setClockFrequency(Frequency f) {
+  switch (f) {
+    case Frequency::High:
+      RCC.CFGR()->setHPRE(RCC::CFGR::AHBPrescaler::SysClk);
+      return;
+    default:
+      assert(f == Frequency::Low);
+      /* We could divide the system clock by 512. However, the HCLK clock
+       * frequency must be >= 14.2MHz and <=216 MHz which forces the
+       * AHBPrescaler to be below 192MHz/14.2MHz~13.5MHz. */
+      RCC.CFGR()->setHPRE(RCC::CFGR::AHBPrescaler::SysClkDividedBy8);
+      return;
+  }
+}
+
 }
 }
 }
diff --git a/ion/src/device/shared/drivers/board.h b/ion/src/device/shared/drivers/board.h
index 0e314c10f..372843077 100644
--- a/ion/src/device/shared/drivers/board.h
+++ b/ion/src/device/shared/drivers/board.h
@@ -15,6 +15,13 @@ void shutdownClocks(bool keepLEDAwake = false);
 void initPeripherals();
 void shutdownPeripherals(bool keepLEDAwake = false);
 
+enum class Frequency {
+  Low = 0,
+  High = 1
+};
+
+void setClockFrequency(Frequency f);
+
 }
 }
 }
diff --git a/ion/src/device/shared/drivers/timing.cpp b/ion/src/device/shared/drivers/timing.cpp
index 24be59ca9..e2a30d100 100644
--- a/ion/src/device/shared/drivers/timing.cpp
+++ b/ion/src/device/shared/drivers/timing.cpp
@@ -1,4 +1,5 @@
 #include "timing.h"
+#include "board.h"
 #include <ion/timing.h>
 #include <drivers/config/timing.h>
 
@@ -12,9 +13,12 @@ using namespace Device::Timing;
  * precision, we could use the controller cycle counter (Systick). */
 
 void msleep(uint32_t ms) {
+  // We decrease the AHB clock frequency to save power while sleeping.
+  Device::Board::setClockFrequency(Device::Board::Frequency::Low);
   for (volatile uint32_t i=0; i<Config::LoopsPerMillisecond*ms; i++) {
       __asm volatile("nop");
   }
+  Device::Board::setClockFrequency(Device::Board::Frequency::High);
 }
 void usleep(uint32_t us) {
   for (volatile uint32_t i=0; i<Config::LoopsPerMicrosecond*us; i++) {