topic Re: MCF52259 Flashing with Battery Backup Connected in ColdFire/68K Microcontrollers and Processors

MCF52259 Flashing with Battery Backup Connected

seanmac — Tue, 09 Aug 2011 16:10:17 GMT

I am using a Colfire MCF52259 processor with a 3.3V battery connected to the Vstby pin.

With the battery connected (and circuit powered), I am unable to 'flash' program the proccesor using a P&E Micro BDM.

If I remove the battery, the device programs without any issue.

I have also noticed the processor will not start-up if the standby battery voltage is slightly high (3.6V).

Is this 'normal' operation for these devices?

Re: MCF52259 Flashing with Battery Backup Connected

FridgeFreezer — Wed, 10 Aug 2011 17:57:56 GMT

No, it's not normal, you need to check your circuit I think.

Re: MCF52259 Flashing with Battery Backup Connected

ebee — Fri, 06 Jan 2012 18:04:22 GMT

I have been experiencing the same start up problem as you but was coming at it from another angle.

With the battery connected the processor would not start up from a power on reset even if I performed an external reset (by pulling RSTI) after the power on. The only way that I could get the board to start was to disconnect the battery momentarily and then it would run. As soon as the battery was disconnected the application would run. Once it was going everything works OK whether the battery is fitted or not.

After one of my searches on the forums brought up this post I checked the voltage on the VSTBY pin and found it to be 3.6V from my 3V battery. After making some modifications to the board (fitting a 2.5V zener diode and series resistor between the battery and VSTBY) the board now powers up fine.

To me it does seem as if this is the effect of the higher than specified voltage on the VSTBY pin rather than a design error somewhere else.

Re: MCF52259 Flashing with Battery Backup Connected

boogy — Fri, 06 Jan 2012 22:50:26 GMT

Here's what I have discovered. If VSTBY is greater than VDD it doesn't boot.

It doesn't matter what the voltages are.

I searched through all the specs but didn't find anything that could explain this phenomenon.

Re: MCF52259 Flashing with Battery Backup Connected

TomE — Sun, 08 Jan 2012 13:48:49 GMT

rParker is having a similar problem on this forum. I wonder what the "common factors" are between you problems and hardware designs?

Here's some more complications concerning VSTBY and the RTC:

Current Consumption - higher than you might like if you're using a "button cell":

https://community.freescale.com/message/51910#51910

If you have a battery on VSTBY and it goes flat, it seems the RTC won't run, so you probably do need to run VSTBY from a diode from the battery and a diode from 3V3 (which would fix your current 3.6V battery problem).

https://community.freescale.com/message/82160#82160

> If VSTBY is greater than VDD it doesn't boot.

Do you mean by a particular amount, or does it fail at say 3.301V? there usually has to be a difference enough to forward-bias a junction for it to have an effect.

The Data Sheet apparently gives a maximum voltage for VSTBY of 3.5V, so that looks like "0.2V above 3.3V". So if your 3.3V is within spec but LOW and your battery is within spec but HIGH... If this is the problem then the spec for VSTBY should probably be "VDD+0.2V".

There's a very interesting section in the Data Sheet:

    RAM standby supply current
    * Normal operation: VDD > VSTBY - 0.3 V   = 5uA
* Standby operation: VDD < VSS + 0.5 V    = 20uA

So there's logic that switches at those relative voltages - or they're the minimum and maximum comparison voltages. The MCF52110 is quite different - check its specs for the strange "transient" operation.

I suggest you download all Reference, Data Sheet and Errata for every part that has a VSTBY pin (52259, 52110, 5213, 52223, 5282, 5301x) and see if some of the manuals tell you something interesting the others don't.

What is the timing relationship between your 3.3V turning on and the external reset signal going away (assuming you have an external reset controller)? If the Reset is going away before the 3.3V is fully established, then the problem might be the difference between VSTBY and VDD at that point. It may be related to when the crystal starts oscillating relative to the 3l3V ramp and the Reset signal.

Tom