MC9RS08KA4/KA8 and watch crystal for RTC, how low will it go

[PeterK]

Hello,
 
 I am trying to understand if the new KA4/KA8 devices can be used as low power
 real-time clock and how low it will go power-wise.

  The datasheet seems to hint that it will go down to 3.6uA using a 1MHz external clock,
  I think being divided down by some factor (or divisor really), bypassing the FLL, which
  is turned off anyway, and clocking  the 16 bit timer, that eventually flows over and generates
  an interrupt, waking up the MCU from sleep/stop/wait. I'm not sure of the terminology here.
  (neither is the datasheet, which does not seem to always make a clear distinction between
  crystal, oscillator, and (external) clock)

The 3.6uA which the datasheet seems to indicate, would  surprise me (too low at that high a frequency),

  We''ll really I am interested in the case where you would use a 32.768 watch crystal.
  This should be even (much) better, but I do not feel confident that I interpret
  the datasheet right (I'm used to TI's msp430).

Being new to Freescale MCUs I'd appreciate any help or pointers.


thanks for reading this far,
  Peter

[RickN]

Hi PeterK,

Section 3.6 Stop Mode, from the RS08KA8RM states:  

Exit from stop is done by asserting reset, any enabled asynchronous interrupt, or the real-time interrupt. The asynchronous interrupts are the KBI pins, LVD interrupt, ADC interrupt or the ACMP interrupt.

Notice that the real-time interrupt is what wakes you up from Stop mode, not the timer/pwm or modulo timer.  You need to look at Section 5.7 Real Time Interrupt, to figure out what interrupt period you could get (Table 5-7)  using the 32.768kHz watch crystal as external oscillator.  You will be limited to 8 choices for the real time interrupt period, which will wake you from Stop mode.

I can't tell what the RTI adder from stop would be with 32.768 external crystal from the data sheet.

I can suggest that you take a close look at the 9S08QE8, if low power is your main concern.  It will cost a bit more than the RS08KA8, but will have much better numbers for power consumption.

   Thanks,

        Rick

[PeterK]

RickN,

you wrote:
-----------------------
"Notice that the real-time interrupt is what wakes you up from Stop mode,
 not the timer/pwm or modulo timer."
-----------------------

thanks for pointing that out  ... uh, .. font sticks

yes, could save $ if this part will do the RTC at a low markup in uA.
TI has had a monopoly on the inexpensive deep low power MCU
RTC for a couple of years now. Prices reflected that.
 It would be nice if they got some more competition. They've got some
from the QE8 recently and now perhaps also the KA4/8. They dropped
prices somewhat about a month ago so they've taken notice. It's a nice
architecture though but at the low end flash is not as generous as Freescale.
The KA4/8 seems ... uh, ... cribbled is a strong word, I would not use that, and
you sure get some bang for your bytes, with 8 bit instruction lengths. Perhaps
that's why all the distributors are out of stock? Yes, you're right, I'm rambling

It's getting late here in Ontario thus my boring stream of unconsciousness
but I'd like to know, or ask you, if you would agree that for the 1MHz crystal
the datasheet seems to imply a (mere) 1.6 uA adder at 3v?

nite & thanks
PeterK
 
 

[RickN]

Hi PeterK,

   Sorry about the font.

   Yes, the RS08KA8 Data Sheet specifies a 1.6uA addition in Stop mode for a 1MHz External Oscillator.  I'll try to see what it is for the 32.768KHz crystal.

   If you send me a message with your address I can send you a 9S08QE8 demo board and an RS08KA8 demo board.  You can experiment and measure the Stop currents.

   I would recommend using the buy direct feature of the Freescale website for these parts, if your disty is out of stock.     

    Thanks,

     Rick

[PeterK]

Hi again Rick

 >>  Yes, the RS08KA8 Data Sheet specifies a 1.6uA addition in Stop mode for a
 >> 1MHz External Oscillator.  I'll try to see what it is for the 32.768KHz crystal.
thanks

>>   If you send me a message with your address I can send you a 9S08QE8 demo board
>>  and an RS08KA8 demo board.  You can experiment and measure the Stop currents.
  I will send you a message
>>

appreciate it a lot
  PeterK

[JimDon]

Yes, you can use 32.786K xtal.

Clock Source Options
– Oscillator (XOSC) — Loop-Control Pierce oscillator;
crystal or ceramic resonator range of 31.25 kHz to
39.0625 kHz or 1 MHz to 5 MHz

[PeterK]

JimDon,
thanks for replying,

when using a watch crystal
any experience with the "adder from stop" (hope I'm using right terminology)
the adder from stop you can expect, e.g. how many added uA?

cheers
  PeterK