Is there a min/max ramp time for the K22F power?

[davepfaltzgraff]

I have a design that is basically working, but we are finding that some units do not want to power-up cleanly. I am stil investigating possible causes, but I was wondering if there is a min or max ramp rate for the power to the K22F family?

Along the same ideas, is there a min or max ramp rate for the RST pin? I know that the RST signal is open collector, is there a maximum RC value?

Thanks...

[davepfaltzgraff]

So, OK. It looks as though I'm good on that account. But, I still need to explore what's causing my problem...

Is the part subject to latch-up? What I mean by this is: If there is a voltage at a pin that may be higher than Vcc, might it cause latch-up of the part by sourcing a current through the protection diodes?

What I'm exploring here is that an external circuit may be providing a voltage source prior to the part being fully powered. It's sort of a shot in the dark, but I need to go there.

Thanks

[xiangjun_rong]

Hi, David,

If an external circuit may be providing a voltage source prior to the part being fully powered, I suggest that you limit the current to several mA via serial resistors.

What is your issue now?

BR

Xiangjun Rong

[davepfaltzgraff]

As stated in the original posting, the circuit "basically works" but some units would not "come up" under certain circumstances. The design is a two board system where one board has the K22F acting as the primary controller (user interface, data repository, etc.) and the other board has a PIC18 processor acting as the real-time, low power data interface.

By eliminating various possibilities, like RST rise times, etc., we narrowed the problem down to a few possibilities, one of them being latch-up. Since the external voltage was controlled by the PIC18, we delayed the application of that voltage until it was known that the K22F was fully operational. Now, although testing is not yet complete, the system appears to power up correctly all the time.

Your suggestion of a current limiting resistor may have taken care of this problem, but the circuit is in production and making the firmware change was easier.

Thanks for your help,

Dave

[xiangjun_rong]

Hi, David,

I do not think we have defined the minimum/maximum value for both power-up voltage and Reset signal, anyway, when the VDD reaches up to 1.75V, the core will start to work.

For the Reset pin, the Reset pin is open drain and bi-directional signal, internal pull-up resistor is connected,and multiple internal modules can drive the Reset pin, during power-up process, the Reset pin is driven to be LOW by internal modules, when all internal module does not drive the Reset pin, external RC circuit can drive the pin in turn. The Reset pin minimum width is 100nS, with the spec in mind, you can compute the RC value.

Hope it can help you

BR

Xiangjun rong