diff --git a/avr/modules/0BUILD b/avr/modules/0BUILD
index 077e1e4..4ee28d5 100644
--- a/avr/modules/0BUILD
+++ b/avr/modules/0BUILD
@@ -3,6 +3,7 @@
 <gwbuild>
 
   <subdirs>
+    basetimer
     bmp280
     cny70
     com2
diff --git a/avr/modules/basetimer/0BUILD b/avr/modules/basetimer/0BUILD
new file mode 100644
index 0000000..8038787
--- /dev/null
+++ b/avr/modules/basetimer/0BUILD
@@ -0,0 +1,12 @@
+<?xml?>
+
+<gwbuild>
+
+  <extradist>
+    main.asm
+  </extradist>
+
+</gwbuild>
+
+
+
diff --git a/avr/modules/basetimer/main.asm b/avr/modules/basetimer/main.asm
new file mode 100644
index 0000000..5607ff2
--- /dev/null
+++ b/avr/modules/basetimer/main.asm
@@ -0,0 +1,188 @@
+; ***************************************************************************
+; copyright   : (C) 2024 by Martin Preuss
+; email       : martin@libchipcard.de
+;
+; ***************************************************************************
+; * This file is part of the project "AqHome".                              *
+; * Please see toplevel file COPYING of that project for license details.   *
+; ***************************************************************************
+
+;
+; The base timer makes sure that "onSystemTimerTick" is called about every
+; 100ms.
+; The setup depends on hardware and clock.
+;
+
+
+
+; ***************************************************************************
+; data
+
+.dseg
+
+baseTimerModuleData:
+baseTimerModuleReloadValue: .byte 1
+baseTimerModuleTickCounter: .byte 1
+baseTimerTicksSinceLastRun: .byte 2
+baseTimerModuleData_end:
+
+
+
+
+; ***************************************************************************
+; code
+
+.cseg
+
+BASETIMER_BEGIN:
+
+
+
+; ---------------------------------------------------------------------------
+; @rotuine BaseTimer_Init @global
+;
+; @clobbers r16, r17, x
+
+BaseTimer_Init:            ; setup timer for IRQ every 100ms
+  ; reset data in SDRAM
+  ldi xh, HIGH(baseTimerModuleData)
+  ldi xl, LOW(baseTimerModuleData)
+  ldi r16, 0
+  ldi r17, (baseTimerModuleData_end-baseTimerModuleData)
+  rcall Utils_FillSram
+
+  ldi r16, (1<<CS02) | (1<<CS00)  ; Prescaler 1024
+  out TCCR0B, r16
+  
+  ldi r16, (1<<WGM01)             ; CTC mode
+  out TCCR0A, r16
+
+  ldi r16, (1<<OCF0A)             ; clear pending interrupts
+  out TIFR0, r16
+  
+  ldi r16, (1<<OCIE0A)
+  out TIMSK0, r16
+
+;
+; Settings for clock 1Mhz (default)
+; use timer0 with OCR0A=98-1 (irq every 97.65625 millisecs), baseTimerModuleReloadValue 1
+;
+.if clock == 1000000
+  ; CMP-A interrupt about every 100ms
+  ldi r16, 98-1      ; (1,000,000/1024)/10 = 97.65625
+  out OCR0A, r16
+
+  ldi r16, 1
+  sts baseTimerModuleReloadValue, r16
+.endif
+
+;
+; Settings for clock 8Mhz
+; use timer0 with OCR0A=78 (irq every 9.984 millisecs), baseTimerModuleReloadValue 10
+;
+.if clock == 8000000
+  ; CMP-A interrupt about every 10ms
+  ldi r16, 78-1
+  out OCR0A, r16
+
+  ldi r16, 10
+  sts baseTimerModuleReloadValue, r16
+.endif
+
+  sec
+  ret
+
+
+
+; ---------------------------------------------------------------------------
+; @routine BaseTimer_Fini @global
+;
+
+BaseTimer_Fini:
+  ret
+; @end
+
+
+
+; ---------------------------------------------------------------------------
+; @routine BaseTimer_Run @global
+;
+; @return CFLAG set if something done, cleared otherwise
+; @clobbers all
+
+BaseTimer_Run:
+  push r15
+    in r15, SREG
+    cli
+      lds r24, baseTimerTicksSinceLastRun
+      lds r25, baseTimerTicksSinceLastRun+1
+      clr r16                               ; replace with 0 for next IRQ
+      sts baseTimerTicksSinceLastRun, r16
+      sts baseTimerTicksSinceLastRun+1, r16
+    out SREG, r15                           ; restore global IRQ flag
+  pop r15
+  sbiw r25:r24, 0
+  clc                                     ; flag "nothing done"
+  breq BaseTimer_Run_End
+
+BaseTimer_Run_loop:                       ; for every timer tick
+    push r24
+      push r25
+        rcall onSystemTimerTick
+      pop r25
+    pop r24
+    sbiw r25:r24, 1
+  brne BaseTimer_Run_loop
+  sec
+BaseTimer_Run_End:
+  ret
+; @end
+
+
+
+; ---------------------------------------------------------------------------
+; @routine baseTimerIrqOC0A @isr
+;
+; OC0A interrupt handler
+;
+; Increments baseTimerModuleTickCounter, if it reaches 0 then baseTimerTicksSinceLastRun
+; is also incremented and the baseTimerModuleTickCounter reloaded.
+; The rest is done outside ISR baseTimerTicksSinceLastRun in BaseTimer_Run.
+
+baseTimerIrqOC0A:
+  push r15
+    in r15, SREG
+
+    push r24
+      push r25
+        lds r24, baseTimerModuleTickCounter
+        dec r24
+        breq baseTimerIrqOC0A_timerElapsed
+        sts baseTimerModuleTickCounter, r24
+        rjmp baseTimerIrqOC0A_end
+        
+baseTimerIrqOC0A_timerElapsed:
+        lds r24, baseTimerModuleReloadValue  ; reload counter
+        sts baseTimerModuleTickCounter, r24
+        
+        lds r24, baseTimerTicksSinceLastRun
+        lds r25, baseTimerTicksSinceLastRun+1
+        adiw r25:r24, 1
+        sts baseTimerTicksSinceLastRun, r24
+        sts baseTimerTicksSinceLastRun+1, r25
+        
+baseTimerIrqOC0A_end:
+      pop r25
+    pop r24
+    out SREG, r15
+  pop r15
+  reti
+; @end
+
+
+
+BASETIMER_END:
+.equ MODULE_SIZE_BASETIMER = BASETIMER_END-BASETIMER_BEGIN
+
+
+
diff --git a/avr/modules/comproto/msg_sysstats.asm b/avr/modules/comproto/msg_sysstats.asm
index 203de7a..f000df4 100644
--- a/avr/modules/comproto/msg_sysstats.asm
+++ b/avr/modules/comproto/msg_sysstats.asm
@@ -38,9 +38,8 @@ CPRO_WriteSysStats:
       lds r17, com2Interrupts+1
       st X+, r17                             
 
-      lds r17, timerInterrupts
-      st X+, r17                                ; timer interrupts
-      lds r17, timerInterrupts+1
+      clr r17
+      st X+, r17                                ; timer interrupts (not used anymore)
       st X+, r17
     pop xl
   pop xh
diff --git a/avr/modules/timer/main.asm b/avr/modules/timer/main.asm
index 5fda4d3..c0d878d 100644
--- a/avr/modules/timer/main.asm
+++ b/avr/modules/timer/main.asm
@@ -26,9 +26,7 @@
 
 timerModuleData:
 timerModuleTickCounter:      .byte 1
-timerTicksSinceLastRun:      .byte 2
 timerModuleCounterSecs:      .byte 4
-timerInterrupts:             .byte 2
 timerModuleData_end:
 
 
@@ -44,15 +42,11 @@ TIMER_BEGIN:
 
 
 ; ---------------------------------------------------------------------------
-; Timer_Init
+; @routine Timer_Init
 ;
-; IN:
-; - nothing
-; OUT:
-; - nothing
-; USED: r16, r17, x
+; @clobbers r16, r17, x
 
-Timer_Init:            ; setup timer for IRQ every 100ms
+Timer_Init:
   ; reset data in SDRAM
   ldi xh, HIGH(timerModuleData)
   ldi xl, LOW(timerModuleData)
@@ -60,107 +54,63 @@ Timer_Init:            ; setup timer for IRQ every 100ms
   ldi r17, (timerModuleData_end-timerModuleData)
   rcall Utils_FillSram
 
-; CTC mode (WGM2:0=2, OCR0A=value, OCF0A Flag =1,  -> IRQ_OC0A
-
-  ; CMP-A interrupt about every 100ms
-  ldi r16, (1<<CS02) | (1<<CS00)  ; Prescaler 1024
-  out TCCR0B, r16
-  
-  ldi r16, (1<<WGM01) ; CTC mode
-  out TCCR0A, r16
-  
-  ldi r16, 98-1      ; (1,000,000/1024)/10 = 97.65625
-  out OCR0A, r16
-
-  ldi r16, (1<<OCF0A) ; clear pending interrupts
-  out TIFR0, r16
-  
-  ldi r16, (1<<OCIE0A)
-  out TIMSK0, r16
+  ldi r16, 10                         ; every sec
+  sts timerModuleTickCounter, r16
   
   rcall timerInitTimers
-  
   ret
+; @end
 
 
 
 ; ---------------------------------------------------------------------------
-; Timer_Fini
+; @routine Timer_Fini
 ;
-; IN:
-; - nothing
-; OUT:
-; - nothing
-; USED:
 
 Timer_Fini:
   ret
+; @end
 
 
 
 ; ---------------------------------------------------------------------------
-; Timer_Run
+; @routine onSystemTimerTick
 ;
-; IN:
-; - nothing
-; OUT:
-; - CFLAG: set if something done, cleared otherwise
-; USED: r15, r16, r24, r25 (more depending on called routines)
+; @return CFLAG set if something done, cleared otherwise
+; @clobbers all
 
-Timer_Run:
-  in r15, SREG
-  cli
-    lds r24, timerTicksSinceLastRun
-    lds r25, timerTicksSinceLastRun+1
-    clr r16                               ; replace with 0 for next IRQ
-    sts timerTicksSinceLastRun, r16
-    sts timerTicksSinceLastRun+1, r16
-  out SREG, r15                           ; restore global IRQ flag
-  sbiw r25:r24, 0
-  clc                                     ; flag "nothing done"
-  breq Timer_Run_End
-Timer_Run_loop:                           ; for every occurred 100ms irq
-    push r24
-      push r25
-        rcall timerRunTimers
-        rcall onEvery100ms
-      pop r25
-    pop r24
-    lds r16, timerModuleTickCounter
-    inc r16
-    cpi r16, 10
-    brcc Timer_Run_SecondElapsed
-    sts timerModuleTickCounter, r16
-    rjmp Timer_Run_loop_end
-Timer_Run_SecondElapsed:
-    clr r16
-    sts timerModuleTickCounter, r16
-;    push r24
-;      push r25
-;        rcall timerRunTimers
-;      pop r25
-;    pop r24
-Timer_Run_loop_end:
-    sbiw r25:r24, 1
-  brne Timer_Run_loop
-  sec
-Timer_Run_End:
+onSystemTimerTick:
+  lds r16, timerModuleTickCounter
+  dec r16
+  breq onSystemTimerTick_SecondElapsed
+  sts timerModuleTickCounter, r16
+  rjmp onSystemTimerTick_call
+
+onSystemTimerTick_SecondElapsed:
+  ldi r16, 10                         ; reload counter (10=every sec)
+  sts timerModuleTickCounter, r16
+  ldi xl, LOW(timerModuleCounterSecs) 
+  ldi xh, HIGH(timerModuleCounterSecs)
+  rcall Utils_IncrementCounter32      ; inc uptime counter
+
+onSystemTimerTick_call:
+  rcall timerRunTimers
+  rcall onEvery100ms
   ret
+; @end
 
 
 
 ; ---------------------------------------------------------------------------
-; Set timer value.
+; @routine Timer_SetValue @global
 ;
+; Set timer value.
 ; Setting a timer to 0 effectively stops the timer.
 ;
-; IN:
-; - r16: new timer value (low)
-; - r17: new timer value (high)
-; - X  : pointer to timer value in SRAM
-; OUT:
-; - nothing
-; REGS: X
+; @param r16 new timer value (low)
+; @param r17 new timer value (high)
+; @param X   pointer to timer value in SRAM
+; @clobbers X
 
 Timer_SetValue:
   push r15
@@ -171,17 +121,17 @@ Timer_SetValue:
     out SREG, r15
   pop r15
   ret
+; @end
 
 
 
 ; ---------------------------------------------------------------------------
+; @routine Timer_SetValueTo1s @global
+;
 ; Set timer value to 1s.
 ;
-; IN:
-; - X  : pointer to timer value in SRAM
-; OUT:
-; - nothing
-; REGS: X
+; @param X pointer to timer value in SRAM
+; @clobbers X
 
 Timer_SetValueTo1s:
   push r16
@@ -192,10 +142,18 @@ Timer_SetValueTo1s:
     pop r17
   pop r16
   ret
+; @end
 
 
 
 
+; ---------------------------------------------------------------------------
+; @routine timerInitTimers
+;
+; Init timers in table.
+;
+; @clobbers all
+
 timerInitTimers:
   ldi zl, LOW(timerList*2)
   ldi zh, HIGH(timerList*2)
@@ -220,13 +178,18 @@ timerInitTimers_writeInitial:
 
 timerInitTimers_end:
   ret
+; @end
 
 
 
+; ---------------------------------------------------------------------------
+; @routine timerRunTimers
+;
+; Run timers in table.
+;
+; @clobbers all
+
 timerRunTimers:
-  ldi xl, LOW(timerModuleCounterSecs)
-  ldi xh, HIGH(timerModuleCounterSecs)
-  rcall Utils_IncrementCounter32
   ldi zl, LOW(timerList*2)
   ldi zh, HIGH(timerList*2)
 timerRunTimers_loop:
@@ -260,20 +223,23 @@ timerRunTimers_reachedZero:
   rjmp timerRunTimers_loop
 timerRunTimers_end:
   ret
+; @end
 
 
 
 ; ---------------------------------------------------------------------------
-; read time table entry.
+; @routine timerReadTableEntry
 ;
-; IN:
-; - Z      : pointer to time list entry (suitable for LPM)
+; read timer table entry.
+;
+; @param Z      pointer to time list entry (suitable for LPM)
 ; OUT:
-; - r19:r18: handler routine
-; - X      : SRAM address for counter
-; - r21:r20: initial value
-; - r22    : flags
-; - r23    : reserved
+; @return r19:r18 handler routine
+; @return X       SRAM address for counter
+; @return r21:r20 initial value
+; @return r22     flags
+; @return r23     reserved
+
 timerReadTableEntry:
   lpm xl, Z+    ; SRAM address of counter
   lpm xh, Z+
@@ -284,45 +250,19 @@ timerReadTableEntry:
   lpm r20, Z+   ; initial value
   lpm r21, Z+
   ret
+; @end
 
 
 
+; ---------------------------------------------------------------------------
+; @routine timerCallR19R18
+; Call route at address r19:r18
+
 timerCallR19R18:
   push r18
   push r19
   ret
-
-
-
-; ---------------------------------------------------------------------------
-; OC0A interrupt handler
-;
-; Called every 100 milliseconds, increments timerTicksSinceLastRun. The rest is done outside ISR
-; in Timer_Run.
-
-timerIrqOC0A:
-  push r15
-    in r15, SREG
-
-    push r24
-      push r25
-        lds r24, timerTicksSinceLastRun
-        lds r25, timerTicksSinceLastRun+1
-        adiw r25:r24, 1
-        sts timerTicksSinceLastRun, r24
-        sts timerTicksSinceLastRun+1, r25
-        
-        lds r24, timerInterrupts
-        lds r25, timerInterrupts+1
-        adiw r25:r24, 1
-        sts timerInterrupts, r24
-        sts timerInterrupts+1, r25
-
-      pop r25
-    pop r24
-    out SREG, r15
-  pop r15
-  reti
+; @end