Stop Microcontroller Brownout?

[steaksandwich21]

Hi,

I am having a problem whten connecting two microcontrollers together. I have a main board with power, and a hotswappable daughter board that takes its power from the first.
When I connect the two boards together (with the main already powered), the main will reset - as if the daughter requires too much current and the main board browns out.

HOWEVER, I have put a scope on the power lines going into the main microcontroller and the positive rail does not seem to drop.
The microcontroller has 100nF caps as bypass caps - I was wondering if I could/should but some larger caps where I connect the daughter board to act as a reservoir of sorts.

Does anyone have an experience dealing with the same issue - and how did they fix it?

Many thanks,
steaky

 

[dknguyen]

You scoped right where the microcontroller power pins are? If you scope too far from the MCU and too close to the power supply, you might not see any effective voltage drop because that's not where the cumulated trace inductance is at it's worst.

It could also just be the regulator not responding to the transient in time. In either case, bulk capacitance might help- 100nF is more for the current transients of the MCU itself- not big enough to protect from external current sucking transients.

It might also help to change the connector so that ground pins are longer so they make contact before any other pins make contact.

 

[Diver300]

You should have a current limit on the supply to the daughter board. As you plug it in, it is at zero volts. Any capacitance on the daughter board will drag down the power supply on the main board.

How much it drags it down depends on all sorts of things, like the size and ESR of the capacitors that you use.

A current limit circuit will make sure that the voltage on the mother board isn't dragged down.

 

[BrownOut]

I thought you were asking me how to stop a microcontroller.

~Brownout

 

[ben7]

Hi,

I am having a problem whten connecting two microcontrollers together. I have a main board with power, and a hotswappable daughter board that takes its power from the first.
When I connect the two boards together (with the main already powered), the main will reset - as if the daughter requires too much current and the main board browns out.

HOWEVER, I have put a scope on the power lines going into the main microcontroller and the positive rail does not seem to drop.
The microcontroller has 100nF caps as bypass caps - I was wondering if I could/should but some larger caps where I connect the daughter board to act as a reservoir of sorts.

Does anyone have an experience dealing with the same issue - and how did they fix it?

Many thanks,
steaky

You should always use a 100nF and at least a 100uf together.

Sometimes 100nF caps alone aren't enough

 

[ronv]

Assuming it's a power problem I think Diver has the right idea. Add a few ohms in series with the +5 going to the daughter board and add a 100 ufd. or so cap on the main board side of the resistor. This should slow down the transient as the caps on the daughter board get charged up.