gEDA-dev: GAF wish list addition - complex boards having multiple logic families

[Evan Lavelle]

DJ Delorie wrote:

> I suppose we could tag pins with Voh, VihMin, VihMax, etc.

That would be a *really* nice feature, and I think would be the only way 
to do this. I just had to manually review a (Protel) board with 4 supply 
voltages (CMOS FPGAs/ASICs and 74LVCH) and this would have caught maybe 
half a dozen show-stoppers.

A DRC check for over-voltages would be a good start but, to be really 
useful, it would need to be more complex. Some thoughts:

- you'd need to be able to calculate static currents in/out of input 
pins, including the case where the trace goes through a resistor (a 
current-limiting resistor, or a pull-up or pull-down). This is because 
some input structures will tolerate over-voltages if the current through 
the input protection diodes is limited (this is commonly done on 
mixed-voltage CMOS boards). So you'd need attributes for 
presence/absence of input protection diodes, and their current rating.

- for some families, "over" voltage is relative to the real current 
supply voltage to the chip; for others, its an absolute voltage level

- for older TTL families, you might also want to calculate static 
currents in/out of output pins, to make sure that you get a valid logic 
level. To do this, you'd also need an attribute like "this output can 
only sink 16mA", or whatever. This would let you do pullup and fanout 
checks as well as voltage level checks

- you could also check for 'reverse' currents: eg. a 3.3V pull-up, to a 
2.5V input, through a forward-biased protection diode, back to the 2.5V 
supply; you need to warn that the 2.5V supply might have a problem with this

- you need to warn about 'under-driven' inputs. It's common, for 
example, to drive 3.3V inputs with 2.5V. This is technically Ok (in the 
families I know about) but you get less noise immunity, and the designer 
should be reminded about this

- an obvious one - just checking that the supply voltage is in range; 
can be fiddly for all those low-voltage 7400 families, and when more 
complex chips have 3 or more supplies (core, I/Os, etc)

Evan