MCF52259 Dueling Table Data

[GaryOlmstead]

Hi --

The MCF52259 Data Sheet says (Table 2.1, page 23) that the RAM standby supply voltage (Vstby) must be 1.8V to 3.5V.  While Table 2.6 (pg 28( says that Standby voltage Vstby must be 3.0V to 3.5V.  I assume this means that 1.8V is OK if you aren't running, but not if you are.

My interest is not in the RAM, but the RTC, which is also fed from Vstby.  Does it work at < 3.0V?  The Data Sheet doesn't mention the RTC.  The '59 Reference Manual has a whole chapter on the RTC, but doesn't say how it gets its power.

So, can the RTC keep time at less than 3.0V?

[mjbcswitzerland]

Hi Gary

You should find that this has been discussed in detail here:

http://forums.freescale.com/freescale/board/message?board.id=CFCOMM&message.id=6407&query.id=118275#...

Regards

Mark

[GaryOlmstead]

Hi Mark --

Yes, I read that thread, but it only talks about the RAM.   MacL says the RTC is backed up at 1.8V, but I need it to run.

I have a '59 demo board, and a 1.8V - 3.0V variable power supply, guess I'll have to write some code to start the RTC, and find out what happens.

By the way, how did you measure 3.9uA?

Regards

Gary

[mjbcswitzerland · Accepted solution]

Hi Gary

The tests were with running RTC, which worked down to about 2.0V. Somewhere under 2.0V the oscillator would stop - so the 1.8V limit may be correct but it was difficult to get to this point using a voltage generator (possibly due to small voltage jumps when adjusting the level, which won't happen with a battery being drained to this level).

I measured the Vstby current with a uA meter - no official calibration so tolerance not known. These are therefore ball-park figures and if you need high accuracy it will be necessary to use calibrated equipment in a suitable test environment.

Regards

Mark

[GaryOlmstead]

Hi Mark --

OK, I missed the part about the RTC running.  Thanks for the clarification.  That's really good to know.  Although two batteries in series would not have been the end of the world.

A uA meter... of course, how stupid of me.  I may have to invest in one of those.  Or rent one, considering how often I need one.

Thanks for the info.

Gary