Hot insertion setup??

[hello188]

Hi. I am trying to design a board that is to be hot inserted to already powered back plane.

Some of the IO pins needs to be connected outside for data_exchange, configuration, etc.

How should I protect the exposed IO pins and power supply pin for MCU(3.3V and 5V) from transient voltage surge ?

What I am thinking is just connecting shunt TVS diode to ground.

However, which ground should it be connected to ? Should it be connected to the same ground as the CPU? or the true ground(Frame ground or earth ground)?

My CPU is isolated. So, wouldn't connecting one end of the TVS diode(even for protection) to somewhere other than isolation defeat the purpose of the isolation?

Also, I know that microchip has a library for start-up diagnosis for safe start-up. Does CCS have one? Or can I just use what's from Microchip and integrate it into CCS compiler ?

Any advice for hot-swapping, would be greatly appreciated.


Thank you always.

[RF_Developer]

Hot swapping, aka Hot plugging, generally needs careful system design. Its not just a firmware issue, or a hardware issue: its all systems aspects.

Hot swappable devices normally use connectors specially designed for hot swapping. USB connectors are probably the commonest example, the more complex Positronic PCIM34W connectors, used for backplane and power supply applications, are another example. These, and USB, use pins at different depths to ensure critical signals and power are applied to the hot swappable device in a controlled sequence.

I don't think CCS has a "library" for the functions that the Microchip code provides. Instead it has the simple "restart_cause()" function, which returns a code corresponding the the reason for any PIC start/restart.

A good starting point would be to keep things as simple as you can. USB has just power and minimal data connections. If you're talking a full systems bus with address, data and all its associated control signals then you're going to have lots of trouble. At the very least, you are going to have to ensure that your plug-in device doesn't interfere with any backplane signals until its fully up and running. The timings and other requirements for hot swapping should have been fully defined when the backplane was designed, and you will have to meet them.

In this context I have to ask if your system was designed to support hot swappable? If not, then its unlikely that you'll be able to add on hot swapping support.

I haven't even mentioned any of the issues you've asked about. Power should probably be the least of your concerns right now, though it too is important.

RF Developer

[gpsmikey]

As RF Developer indicates, there are a whole host of issues when "hot plugging" something and it is up to you to keep that under control. Typically, the GND gets connected first, followed by power then the signal lines, but the signal lines need to be buffered and controlled so they don't affect the bus until the processor is up and stable then they can be enabled depending on what else is going on. You also need to address the issue of "hot unplugging" - it needs to gracefully release from what it is controlling on the bus and do a clean shutdown. It is not as complicated if the device you are working with is the only one on the bus, (instead of multiple devices on a backplane), but those issues all need to be addressed and it is a combination of the hardware and the software to work the problem (including a special connector typically that controls the connection sequence as indicated). If you don't design for all conditions, they will happen to you (like someone slowly plugging/unplugging the device instead of quickly doing it).

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

[temtronic]

and....

You have to figure out what _will_ go wrong to whatever the CPU card is controlling when yanked out or rammed in!
Whatever the device is controlling the 'backplane' needs real good 'hard' defaults to not turn on valves, motors, whatever else that could cause 'bad things to happen'.

Real world testing is critical and do not trust the system without several people doing the hot swapping.

hth
jay

[westom]