diff --git a/include/display.h b/include/display.h
index 62cdf363..70187893 100644
--- a/include/display.h
+++ b/include/display.h
@@ -5,6 +5,7 @@
 #include "cyclic.h"
 #include "qrcode.h"
 #include "power.h"
+#include "timekeeper.h"
 
 #if (HAS_DISPLAY) == 1
 #include <OneBitDisplay.h>
diff --git a/include/irqhandler.h b/include/irqhandler.h
index 8b14b293..eacbd35f 100644
--- a/include/irqhandler.h
+++ b/include/irqhandler.h
@@ -24,7 +24,7 @@
 void irqHandler(void *pvParameters);
 void mask_user_IRQ();
 void unmask_user_IRQ();
-void doIRQ(int irq);
+void IRAM_ATTR doIRQ(int irq);
 
 extern TaskHandle_t irqHandlerTask;
 
diff --git a/include/timekeeper.h b/include/timekeeper.h
index 956595df..88084916 100644
--- a/include/timekeeper.h
+++ b/include/timekeeper.h
@@ -20,8 +20,9 @@ extern const char timeSetSymbols[];
 extern Ticker timesyncer;
 extern timesource_t timeSource;
 extern TaskHandle_t ClockTask;
-extern bool volatile TimePulseTick; // 1sec pps flag set by GPS or RTC
+extern DRAM_ATTR bool TimePulseTick; // 1sec pps flag set by GPS or RTC
 extern hw_timer_t *ppsIRQ;
+extern portMUX_TYPE mux;
 
 void IRAM_ATTR CLOCKIRQ(void);
 void clock_init(void);
diff --git a/src/display.cpp b/src/display.cpp
index 10722fa3..e41dea81 100644
--- a/src/display.cpp
+++ b/src/display.cpp
@@ -317,7 +317,9 @@ void dp_drawPage(bool nextpage) {
 
 #if (TIME_SYNC_INTERVAL)
     timeState = TimePulseTick ? ' ' : timeSetSymbols[timeSource];
-    TimePulseTick = false;
+    portENTER_CRITICAL(&mux);
+    TimePulseTick = false; // flip global variable pulse ticker
+    portEXIT_CRITICAL(&mux);
     time(&now);
     localtime_r(&now, &timeinfo);
     strftime(strftime_buf, sizeof(strftime_buf), "%c", &timeinfo);
diff --git a/src/main.cpp b/src/main.cpp
index 4bc007ab..9cd922fc 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -48,37 +48,31 @@ Low priority numbers denote low priority tasks.
 -------------------------------------------------------------------------------
 0	displayIRQ -> display refresh -> 40ms (DISPLAYREFRESH_MS)
 1 ppsIRQ -> pps clock irq -> 1sec
+2 (unused)
 3	MatrixDisplayIRQ -> matrix mux cycle -> 0,5ms (MATRIX_DISPLAY_SCAN_US)
 
 
-// Interrupt routines
--------------------------------------------------------------------------------
-
-irqHandlerTask (Core 1), see irqhandler.cpp
-
-fired by hardware
-DisplayIRQ      -> esp32 timer 0
-CLOCKIRQ        -> esp32 timer 1 or external GPIO (RTC_INT or GPS_INT)
-MatrixDisplayIRQ-> esp32 timer 3
-ButtonIRQ       -> external GPIO
-PMUIRQ          -> PMU chip GPIO
-
-fired by software
-TIMESYNC_IRQ    -> setTimeSyncIRQ() -> Ticker.h
-CYCLIC_IRQ      -> setCyclicIRQ() -> Ticker.h
-SENDCYCLE_IRQ   -> setSendIRQ() -> xTimer
-BME_IRQ         -> setBMEIRQ() -> Ticker.h
-
-ClockTask (Core 1), see timekeeper.cpp
-
-fired by hardware
-CLOCKIRQ        -> esp32 timer 1
-
-
 // External RTC timer (if present)
 -------------------------------------------------------------------------------
 triggers pps 1 sec impulse
 
+
+// Interrupt routines
+-------------------------------------------------------------------------------
+
+ISRs fired by CPU or GPIO:
+DisplayIRQ      <- esp32 timer 0
+CLOCKIRQ        <- esp32 timer 1 or GPIO (RTC_INT or GPS_INT)
+MatrixDisplayIRQ<- esp32 timer 3
+ButtonIRQ       <- GPIO <- Button
+PMUIRQ          <- GPIO <- PMU chip
+
+Application IRQs fired by software:
+TIMESYNC_IRQ    <- setTimeSyncIRQ() <- Ticker.h
+CYCLIC_IRQ      <- setCyclicIRQ() <- Ticker.h
+SENDCYCLE_IRQ   <- setSendIRQ() <- xTimer
+BME_IRQ         <- setBMEIRQ() <- Ticker.h
+
 */
 
 // Basic Config
diff --git a/src/timekeeper.cpp b/src/timekeeper.cpp
index fec827a4..84445e77 100644
--- a/src/timekeeper.cpp
+++ b/src/timekeeper.cpp
@@ -19,7 +19,9 @@ static const char TAG[] = __FILE__;
 // G = GPS / R = RTC / L = LORA / * = no sync / ? = never synced
 const char timeSetSymbols[] = {'G', 'R', 'L', '*', '?'};
 
-DRAM_ATTR bool volatile TimePulseTick = false;
+portMUX_TYPE mux = portMUX_INITIALIZER_UNLOCKED;
+
+DRAM_ATTR bool TimePulseTick = false;
 timesource_t timeSource = _unsynced;
 TaskHandle_t ClockTask = NULL;
 hw_timer_t *ppsIRQ = NULL;
@@ -211,7 +213,9 @@ void IRAM_ATTR CLOCKIRQ(void) {
 // flip time pulse ticker, if needed
 #ifdef HAS_DISPLAY
 #if (defined GPS_INT || defined RTC_INT)
-  TimePulseTick = !TimePulseTick; // flip pulse ticker
+  portENTER_CRITICAL(&mux);
+  TimePulseTick = !TimePulseTick; // flip global variable pulse ticker
+  portEXIT_CRITICAL(&mux);
 #endif
 #endif