MCF52259 RTC details

[FFlOM]

Hello,

Can I use a 3V battery for the RTC backup on the MCF52259 ?

I am not sure to read well the datasheet.

thanks.

[FFlOM]

Hi,

In the datasheet of the MCF52259, there is Vstby maximum rating 1.8V to 3.5V and Vstby DC electrical specification 3V to 3.5V

So I don't know if I will have functional backup if I put a 3V lithium cell ?

And if I choose a capacitor, what caracteristics I take to calculate the backup time ?

Thanks

[mjbcswitzerland]

Hi

I suspect this means that during normal operation the Vstby should be between 3.0V and 3.5V (since the RAM must be used). In standby mode the data will however be retained down to 1.8V.

A Lithium battery is suitable for the backup periods. By using diodes to connect a lithium battery and also the main 3V3 power (when powered up) the specification at run time and also in standby should be achieved.

Regards

Mark

[Daughenbaugh]

Mark-  I hope that you are right regarding the voltage spec.  Limiting the standby voltage drop to 3.0V would be crippling.  

The related spec that I am chasing is the standby current.  All that I can find is "RAM standby supply current"  which specs 5uA max when in "normal operation" and max 20uA when in "standby operation," which seems kind of backwards in itself, but also doesn't indicate if it includes the RTC oscillator, etc.  I have requested more info from our FAE, but maybe someone here has more info?  I will post anything I find out.

Thanks!

Jason Daughenbaugh

Advanced Electronic Designs, Inc.

[mjbcswitzerland]

Hi Jason

I will see whether I can measure the current consumption on my EVB and verify that it holds data and the RTC values down to under 2V when the CPU is not powered.

I won't be able to do this until late tomorrow so do post any official info if you receive it beforehand.

Regards

Mark

[mjbcswitzerland]

Hi All

These are my results:

a) Power consumption from battery connected to VSTBY when the board is powered:

VSTBY 3.5V : 3.9uA

VSTBY 3.0V : 3.9uA

VSTBY 2.5V : 3.7uA

VSTBY 2.0V :  - 70uA (minus 70uA)

VSTBY lower than 2V - the negative current increases rapidly since the battery is now sinking current.

b) Power consumption from battery when board is powered down (RTC configured and running):

VSTBY 3.5V : 17.7uA

VSTBY 3.0V : 17.2uA 

VSTBY 2.5V : 31.7uA 

VSTBY 2.0V : 22.7uA

Lower than 2V the current consumption drops to about 0.2uA due to the fact that the RTC stops working.

c) RAM tests showed that the RAM contents were lost when the board power was removed, irrespective of the battery voltage. This suggests that VSTBY doesn't power the RAM when the device is powered down.

In the user's manual the block diagram doesn't show that the VSTBY is connected to the RAM but in the test it states that it supports both RTC and SRAM.

My opinion is now that the block diagram is correct (assuming no testing errors).

My conclusion as to the RTC is that a battery above 2V will allow backup. It seems a good idea to put a diode in series to stop it sinking current if it is faulty or its voltage can drop below this threshold. Furthermore I found that the RTC operates normally when the board power is supplied, even when VSTBY is not connected. The fact that the VSTBY delivers current when the board is powered, and the fact that a battery starts sinking current as its voltage falls to around 2V, suggests that there is an internal supply with switchover; this means there is never a need to supply VSTBY externally from 3V3 through a diode.

Regards

Mark

www.uTasker.com

[Daughenbaugh]

Mark - thanks for your testing!

Unfortunately they don't jive with my "official" response from Freescale.  I am told that it will draw 20uA max for both SRAM retention and RTC, and that it will work down to 1.8V.  And yes, the SRAM data will be retained...

So if we compare Freescale's spec to your performance data:

20uA max: nope, you measured over 30uA!

1.8V min: nope, you found that the RTC stopped at 2V.

Data retained: nope.

Jason 

[mjbcswitzerland]

Hi Jason

The problem with measurement results is that they have some inaccuracy and tolerance therefore my results are indicative. I don't want to disprove any official values, especially if the inaccuracies and tolerances are worst than expected (or even measurement errors were involved).

But, yes I certainly measured a current peak at around 2.5V battery backup and I am not at all sure whether SRAM is really backup up. My test results consistently showed that I could reset the board and reconnect the debugger without losing RAM content but the content was consistently random after a power down between disconnecting and reconnecting the debugger. Also the fact that backup current dropped to only 0.2uA after the RTS was stopped rather questions whether SRAM was really powered.

My observation was that the RTC stopped (somewhere) under 2.0V so may be it managed 1.8V (just).

So I don't want my results to be used as evidence but rather as a starting point for any discussion and also as a base for some confirmation that the RTC will operate successfully down to say at least 2.0V (preferably with a diode in series) and in the worst case may be 30uA in backup should be taken into account. If any advances can be made on these values it will be positive and - if any successful use of battery backed up SRAM can be confirmed I will be more than happy learn what went wrong with my test!!

Regards

Mark

[macl]

Vstby backs up the first 16K of SRAM and the RTC when the main power is shut down.  We are working to clarify the voltage spec but experimental results show that it works fine down to somewhere near 1.8V.  When the main power is turned on, the SRAM and RTC get powered from Vdd.  It's only when this source is removed that Vstby kicks in.  Its good to ensure that Vstby is always above 1.8V before the main power is turned on.

MacL

Message Edited by macl on 2009-03-26 12:23 PM

[mjbcswitzerland]

Hi

That is interesting news about the 16k. 

When I did my tests I was checking a block just above the 32k mark since this was above the stack pointer (project configured to only use 32k) and so was guarantied to to not get modified by any initialisation code.

This would explain why tests were negative.

This piece of information doesn't seem to be mentioned any where in any data sheets or manuals. Is it valid for all other V2 MCU types which have Vstby?

- M52100, M52110 - have both 16k SRAM

- M52210, M52211 - have both 16k SRAM

- M52212, M52213 - have both 8k SRAM

- M52221 - has 16k SRAM

- M52223 - has 32k SRAM

Regards

Mark

www.uTasker.com

- OS, TCP/IP stack, USB, device drivers and simulator for M5221X, M5222X, M5223X, M5225X. One package does them all - "Embedding it better..."

[w2vy]

The latest datasheet (MCF52259RM Rev. 2 8/2009) states:

Up to 64-Kbyte dual-ported SRAM on CPU internal bus, supporting core, DMA, and USB

access with standby power supply support for the first 16 Kbytes

and

Standby power supply (Vstby) keeps the RTC running when the system is shut down

I do not see the Hardware Specs... 1.8 v would be nice...

tom

[mjbcswitzerland]

Hi

The RTC in the M5225x is battery backed (see Vstby).

Also take a look at the M52259EVB circuit diagram, which has an optional battery.

See comparison of the different chips here:

http://www.utasker.com/forum/index.php?topic=256.0

Regards

Mark

www.uTasker.com

- OS, TCP/IP stack, USB, device drivers and simulator for M5221X, M5222X, M5223X, M5225X. One package does them all - "Embedding it better..."