STOP / P-STOP / WAIT

[manishsangram]

Hi,

We are using S12ZVMC128 in our project.

We are trying to minimize heat as there is very poor or zero air flow around the MCU and considerable ambient heat. We are not interested in saving power except to the extent it reduces heat.

We are trying to determine the best option to select between Stop / Psedo Stop and Wait from perspective of heat reduction only.

Is it is okay to STOP and wakeup every 1ms for long periods of time. We are not using any external XTAL. The manual says frequent STOP can stress the resonator, is this statement for internal or external components? We are not using any external XTAL.

For us the wakeup time from STOP of 25uS is acceptable.

We would be happy for example if we could save 70-80% on heat just by entering WAIT state and having all peripherals on.

So what we want to determine is where will the most savings in heat come from. Should we put it in WAIT mode? or should we P STOP? Are there any details on exact power consumption or something that gives some sort of indicative idea of the savings or rule of thumb ?

[RadekS]

Hi Manish,

The generated heat is proportional to consumed power (consumed current).

From this point of view is definitely better to use STOP mode (~100uA) instead WAIT mode (~40mA).

The Wait mode just stops core and wait for an interrupt. The Stop mode stops (almost) all clocks.

Note: The current consumed by MCU itself depends mainly on bus clock frequency. So, the third way how to decrease consumed current (generated heat) is a change of your clock settings.

However, since you use the internal voltage regulator, the STOP mode will not have any influence on heat produced by currents to external loads (pin voltage levels are unchanged during STOP mode).

Since you don’t use the external oscillator, the Pseudo STOP mode has no advantage for you.

Just for sure: I suppose for now that you don’t use CAN communication in your application. In opposite case, I definitely recommend using an external crystal as CAN module clock source. The internal oscillator isn’t enough accurate for CAN communication.

About wakeup time)

Here it depends on how the wake-up time is defined. I suppose that MCU is wake by one of the interrupts. In that case, the core will execute code in interrupt service routine and after that it will continue in main code (if there no other pending interrupts). Any entering and exiting to/from ISR need approximately 10 bus cycles for stacking,… Additionally, the clock module needs some time to stabilize it = until PLL lock.

How to enter into STOP mode)

As first, we have to clear S bit in CCR register to enable stop mode. For example:

  asm ANDCC #0x7F;  //clear S bit - enable the stop mode. If the S bit is not

//cleared then STOP instruction has no effect and is

//executed as a dummy instruction NOP.

The S bit is set again by next MCU reset

If we want to wake-up MCU by any I bit maskable interrupts, we have to enable interrupts = clear I bit. For example:

EnableInterrupts;

Finally, the entering into stop mode is caused by STOP instruction. For example:

asm STOP; //STOP mode

Please look at Table 1-16. Interrupt Vector Locations in RM for the check which interrupts may be used for wake-up from Wait/Stop mode.

Note: The floating inputs (digital input pins without any pull-up or pull-down) may cause increasing power consumption when they work in the linear area near to VDDX/2. So, I would like to recommend to keep all unused pins in some of the defined states = digital output/digital input with internal/external pull up/down.

I hope it helps you.

Have a great day,
Radek

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