40x2 Power Pins

StigOfTheDump

Senior Member
Referring to the following question
  • If it is a PICAXE with more than one power pin per rail, are they all connected?
on thread http://www.picaxeforum.co.uk/showthread.php?25280-READADC10-Bias I looked into what was necessary. I've got some 40x2s but haven't used any yet on a project. I couldn't find any info on the forum, the picaxe website or scanning through Microchip's 560 page datasheet. It must be there somewhere, just that I can't find it. Page 35 of manual 3 shows the minimum circuit for 28 pin, with both the 0v connections made, but there isn't a similar diagram for the 40 pin devices.

Googling some other electronics forums seemed to suggest that all should be connected. Most posts suggested that each should be decoupled individually, one even suggesting that separate power supplies could be used on sensitive applications, as different power pins supply different parts of the chip circuitry.

Could somebody please explain what is the correct procedure.

In particular:
If using 2 power supplies, how closely would they need to be matched?
Linking the 2 power pins underneath the chip, inside the socket on stripboard, when operating from a single supply, is very convenient. As this would short together the separate decoupling caps, should it be avoided?

No particular project planned. Just thought I would get it fixed in my head whilst it is at the front of my mind.

Thanks

Stig
 

hippy

Senior Member
The official Microchip info on power considerations is here ...

http://ww1.microchip.com/downloads/en/DeviceDoc/power.pdf

Many PICmicro MCUs have more than one VSS or VDD pin. This is common on devices with higher pin counts. These pins are generally connected internally but through a finite impedance. To insure proper operation, all VSS and VDD pins need to be properly connected externally.
There's a smidgen of ambiguity in that but I read it as "properly connected together externally". Either way, simply connecting the the two V+ and two 0V together as bfgstew says would meet the stated requirements, even if something else may also be allowed.

The really big "don't" is to put 0V into one pin and take it out the other 0V pin, either deliberately or through forgetting to link the two on the board.

It is possibly recommended to have decoupling caps each side but would doubt it matters for most circuits and one would likely be enough.

As for two separate supplies that doesn't seem very wise given it's impossible for mere mortals to know exactly which parts of the chip are internally connected to each other. Microchip's description of "connected internally but through a finite impedance" would suggest any imbalance could cause more problems than it intends to solve.
 

StigOfTheDump

Senior Member
Use them all.

Thanks for the link Hippy.
It explains it very clearly in layman's language.

I had assumed (dangerous thing assumption) that they were internally connected and were just supplied both sides for convenience, power sharing and to make the package up into a standard size.

When I did measure between the 5v pins of a 40x2, on a normal resistance range, I got s/c one way and a small resistance the other.

It seems there are some Microchips where 2 power supplies are recommended but these are clearly labelled as VDD, VSS for the normal supply and AVDD, AVSS (just for the analogue circuitry). Maybe we will see some of them in the future.
 

inglewoodpete

Senior Member
I had assumed (dangerous thing assumption) that they were internally connected and were just supplied both sides for convenience, power sharing and to make the package up into a standard size.

When I did measure between the 5v pins of a 40x2, on a normal resistance range, I got s/c one way and a small resistance the other.
The power and 0v pins are connected together internally but, due to the fine wires between the silicon and the external pins ("legs") they should not carry much current. I usually run the power tracks under the chip when designing the PCB.

Since the pins are side-by-side, it is easy to get two surface-mount capacitors soldered between the PCB pattern, between the pins. Neat, efficient solution. I'll see if I can find a photo of one, on my other PC.

BypassCap1a.jpg
 
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srnet

Senior Member
The power and 0v pins are connected together internally but, due to the fine wires between the silicon and the external pins ("legs") they should not carry much current
Up to the maximum rated current of the particular PIC I would expect, 200ma in this case.
 

hippy

Senior Member
The power legs will support the max rated current of the chip but only when all power legs are used. One path could handle a quarter of the current and the other three quarters.

If only one leg is connected you can end up with all current through a path which cannot handle that. The current which should be going through one leg instead passes through the internal link to the other leg. That can cause the internal link to heat up or be damaged if the current is too great.

The internal link resistance can also act as the low side of a potential divider so the unconnected 0V rises above 0V. Any outputs switching to that 0V won't reach as low as 0V measured at the point where 0V is connected.
 
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