Enable ICS for RTC module in Stop3 mode (Problem)

Showing results for 
Search instead for 
Did you mean: 

Enable ICS for RTC module in Stop3 mode (Problem)

Contributor III

Hi ,

I am trying to wake up from Stop3 mode by internal interrupt from RTC module in a MC9S08PA MCU.

Setting up the  RTC module Clock Source Select work with LPO(~1KHz) working properly in Stop3 mode so RTC waking up the MCU after desired time, BUT while I am setting the RTC module with the Bus clock as Selected Clock Source, the RTC module dose not wake up the MCU.

As I read in Stop3 state :

" To enter stop3, the user must execute a STOP instruction with stop mode enabled

(SOPT1[STOPE] = 1). Upon entering the stop3 mode, all of the clocks in the MCU are

halted by default, but OSC clock and internal reference clock can be turned on by setting

the ICS control registers."


So that's the reason RTC dose not generate wake up situation.


After that I have to Enable ICS in Stop3 mode in order to make the RTC module work with Bus Clock timing.


As I read in Chapter 8: Clock Managment:

" Stop (STOP)

In stop mode, the FLL is disabled and the internal clock source can be enabled or

disabled. The BDC clock is not available and the ICS does not provide MCU clock


Stop mode is entered whenever the MCU enters a stop state. In this mode, all ICS clock

signals are static except in the following cases:

• ICSIRCLK will be active in stop mode when all of the following conditions occur:

• ICS_C1[IRCLKEN] bit is written to 1

• ICS_C1[IREFSTEN] bit is written to 1

• OSCOUT will be active in stop mode when all of the following conditions occur:

• ICS_OSCSC[OSCEN] bit is written to 1

• ICS_OSCSC[OSCSTEN] bit is written to 1


The DCO frequency changes from the pre-stop value to its reset

value and the FLL need to re-acquire the lock before the

frequency is stable. Timing sensitive operations must wait for

the FLL acquisition time, tAquire, before executing."

The Problem is that until now I cannot start the ICS  in Stop3 mode in any way , while I have already done what the above informs me.

Please tell me if I miss something in order to start the ICS module in a Stop3 situation.

It is very critical for me to timing the RTC module with Bus Clock while we need more precise time on wake up situation, than LPO can give.


P.s. : I have already test that ICS work without FLL but direct with ICR(Internal Reference Clock) (ICS_C1_CLKS=01), and work properly, so the only problem is to make ICS module work at Stop3.


Thanks in advance

     Pelekis Marios

     Intelco Electronics

Labels (1)
0 Kudos
2 Replies

Senior Contributor II

I could be wrong but I don't think you can use the bus clock to exit STOP3 mode.

Have a look at this (also noting: "The system integration module (SIM) holds the CPU in a non-clocked state"):


0 Kudos

Contributor III

Hello Tony

Thanks for the reply.

CPU is like module for MCUs and is clocked from the ICS. Bus Clock is not generated from CPU but from ICS module as image above informs us.


So the Bus Clock that is controlled from ICS and it is equal to ICSCLK, must be on at Wati mode

and Optionally On at Stop3 mode.

Thanks anyway!

0 Kudos