MC9S08QG8: 32.768Khz External Xtal or RTC - Current consumption?

[PeterK]

Hi,

 I am new to Freescal MCUs, what I know is just from reading the app notes and DSs.

 Coming from the msp430 with extremely low current consumption with an active RTC

 during "sleep" (msp430 LPM3 ~ Freescale stopx) I have been trying to understand if

 the same low current consumption (msp430F20XX: 500nA)  is  possible with Freescale MCUs.

 (I am not trying to promote the msp430 here, in fact I am considering a defection), so this

 thread was of great interest to me.

 However, the reason I am jumping in is the mentioning in the previous postings

 of the need for an external RTC IC.

 There is probably something I have missed here. It seems to me this is possible with just

 an external 32,768 kHz crystal on the MC9S08QG8 (16 pin version) EXTAL/XTAL pins?

The DS "MC9S08QG8 and MC9S08QG4 Data Sheet, Rev. 1.01" (page 264).

Current consumption during stop3 would then be around 6uA (3v).  

10x the msp430F20XX, but still so low that a coin cell battery, say CR2032,  would last 4-5 years,

which is acceptable (not great) for my application.

I hope somebody in-the-know in this news group could clarify this, e.g. for deep low current consumption, do

you need an external RTC IC or will a locally connected watch crystal do (if you can accept 6uA during stop3)?

Thank You

 Peter K

(Alban split to new post: new question = new post to ease clarity and search)

Message Edited by Alban on 2007-01-27 05:09 PM

[Alban]

Hi Peter,

You don't need an external RTC.

The QG8 contains an AWU or PWU = Auto/Periodic Wake-Up module.

This is using an RC oscillator with low power consumption.

The software can correct any lack of precision this clock might have if this is critical.

A crystal will be sufficient (and much cheaper) and will not be powered when in very low consumption mode.

The consumption of the QG8 is lower than 6uA. The measure is a max @ 85C.

If you look, the typical value is 750nA.

To lower consumption, you can also play with clock speeds. Not to use full speed when not required will lower the overall consumption.

Cheers,

Alban.

[Nabla69]

Hello,

A small tip as you are new to Freescale MCUs.
Freescale specifications are re-known to be extremely worst case.
Your device will much more probably be closer to the typical than the maximum.

It will depend a lot on the temperature profile of the application.

Jojo.