On the first release (a and b) datasheet, Microchip don't specific either
the voltage range accepted for vref.
I think I have right. I haven't time to test right now, but the answer
came by measuring the current flow on both pairs with and without the AD
switched on.
I don't recommend to supply the chip until it sink 250mA or 500mA !
( nor to load the io pins at vcc=5V to get Isupply = 250mA )

Vasile

On Tue, 23 Oct 2001, Kevin Blain wrote:

> Which would explain why microchip don't specify which is which.
>
> It strikes me anyway that on the devices with more pins (40) that extra
> power and ground are a good idea anyway, as there would be a greater
> requirement for current to drive the I/O pins.
>
> The datasheet (18cxx2 for example) says
>
> "Maximum current into VDD pin : 250mA"
>
> Do you think that having two pins means you can shift 500mA on that device,
> within the constraints of the specified 1.0 Watt power dissipation???
>
> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Byron A Jeff
> Sent: 23 October 2001 12:28
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: 16F877 - Why two sets of power pins?
>
>
> On Tue, Oct 23, 2001 at 10:00:37AM +0100, Kevin Blain wrote:
> > which is which?
>
> It's not as cut and dried as analog/digital. The bottom line is that a chip
> is laid out in 3 dimensional space. As real estate fills up it becomes more
> difficult to route power and ground to some areas of the chip. Hence the
> multiple power pins.
>
> This is a non-problem that one should not overthink. Always connect all
> power
> pins unless the data sheet gives specific guidelines as to why all power
> pins
> should not be connected.
>
> BAJ
> >
> > -----Original Message-----
> > From: pic microcontroller discussion list
> > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Vasile Surducan
> > Sent: 23 October 2001 07:31
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: Re: [PIC]: 16F877 - Why two sets of power pins?
> >
> >
> > analogic and digital section.
> > Vasile
> >
> > On Tue, 23 Oct 2001, Bala Chandar wrote:
> >
> > > While using the 16F877, I have been connecting only one set of power
> > supply
> > > pins (+5V and Gnd to pins 32 & 31) and haven't faced any problems so
> far.
> > >
> > > Sometime back, someone in this list had asked why there are two Vdd and
> > two
> > > ground pins in 16F877. I was under the impression that it is there
> merely
> > to
> > > ensure pinout compatibility with 28 pin devices, so that the socket in
> the
> > > programmer is suitable for both.
> > >
> > > But when I checked with the multimeter, I was surprised to find a
> > resistance
> > > of around 2.5 ohms between the two Vdd pins (11 & 32) and the two ground
> > > pins (12 & 31). If the two pins are internally connected, should not the
> > > resistance be zero ohm? Or is it necessary to tie both the pins together
> > for
> > > some reason?
> > >
> > > Any comments?
> > >
> > > Regards,
> > > Bala
> > >
> > > --
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