diff --git a/ion/src/device/device.cpp b/ion/src/device/device.cpp
index da42108ed..9aaddae1c 100644
--- a/ion/src/device/device.cpp
+++ b/ion/src/device/device.cpp
@@ -201,6 +201,9 @@ void initPeripherals() {
   Console::Device::init();
   SWD::Device::init();
 
+  // CPU clock is 96 MHz, and systick clock source is divided by 8
+  // To get 1 ms systick overflow we need to reset it to
+  // 96 000 000 (Hz) / 8 / 1 000 (ms/s) - 1 (because the counter resets *after* counting to 0)
   CM4.SYST_RVR()->setRELOAD(11999);
   CM4.SYST_CVR()->setCURRENT(0);
   CM4.SYST_CSR()->setCLKSOURCE(CM4::SYST_CSR::CLKSOURCE::AHB_DIV8);