Sleep Mode PIC microcontrollers - Explanation !!

Can any one explain how exactly does the sleep mode save power?
Just by making the processor idle with NOP until an interrupt or WDT ??
What will happen to the existing values of I/O ports ?
Will it continously draw current in the sleep mode to maintain the previous tasks ?

Thanks !

PIC and sleep command.
this'll help

Thanks for the reply!
I read that post actually. It basically explains the various options to wake the processor up. What I was looking for is on how the processor saves power. Like what happens during sleep mode ? - shuts down the CPU ?
No instructions execution ?
How much difference in current consumption does it make ??

The PIC really does go to sleep, even the clock stops to save power so no instructions at all are executed. You need some way of waking it up but this is done electrically rather than by reading the pin states. An interrupt while in sleep mode will restart the clock and let the program commence again and the rest pin will of course start it up and jump to the reset vector. Note that 10Fxxx PICs do not resume running, they reset from start when woken up.

Ports are essentially 'frozen' during sleep so if you want to minimize power consumption, make sure they are not supplying current to something else.

Brian.

Thanks a lot Brian!
By frozen, did u mean that the port pins retain their values ?
And some of the features in the controller like A/D converter and comparator functions in the sleep mode. What will be the status of clock in these cases ?
And i got your point on the triggering by external interrupt to wake the processor up, but how does the watch dog timer do that ? Does it have a separate clock running ?
And when it come to practical applications, how much power can the controller actually with being in the sleep mode with all the peripherals and pins turned off ?
Sorry for all these basic stupid questions, but i m actually trying to get a deeper understanding of the sleep mode and have to use an ultra power sleep mode.
Any reference or any kind of materials would be of great help!!
Thanks again!

Most power is consumed by clocking things. If you shut down the high speed clock you save a lot of power. Some processors have a 32kHz clock which can be used and therefore takes less power. Others just rely on the watchdog timer to wake the processor up which is slower and takes even less power. Or you can stop all clocks and rely on an even - interrupt or port pin change - to wake it up. Holding port pins or memory locations in a particular state takes no power provided you aren't trying to get power out of the pins.

Keith

Thanks Keith!
I ll make sure that not to extract power from the pins while in sleep mode!
I m trying to use the ultra sleep mode with the PIC 18F47J53 controller.
Let me know concepts,any kind of info or reference to the various sleep modes and power save modes.
Thanks again for helping me out!!

You need to make sure you disable just about everything to get the lowest sleep current (assuming something external is going to wake it up). That will include ADC and comparators I would think - check the data sheet.

Keith.

Adding to what Keith has said, the WDT is an independent oscillator, it runs at slow speed and is not frequency stabilized to keep its current as low as possible. If you disable the WDT it stops it oscillator to save even more consumption but then you can't use it to wake the device up again. It's main use is in 'crash control' not timing though.

By 'frozen' I meant the pins stay in the state they had at the time when the PIC went to sleep. That's the reason for making sure nothing is drawing power from them unless that is what you intend.

Brian.

Thanks Keith!
Thanks Brian!