Re: Apple ][ using PC floppy

[vladitx <vladitx@nucleusys.com>]

On Dec 31, 8:18 am, mdj <mdj....@gmail.com> wrote:

> The Spartan 3's have a Vin(max) of 4.4V, but the internal 3.3V pullup
> will provide sufficient voltage for a 5v logic high.

4.4V is absolute maximum, point beyond which you're damaging the gate
oxide.

Look at "Iik" parameter - Input clamp current per I/O pin. It's rated
at max 100 mA for input range of -0.5V .. Vcco + 0.5V.
WIth Vcco = 3.3V the clamp current is rated only up to 3.8V. Consider
also few pins' clamping diodes conducting simultaneously.

I think you either need external buffer or count on the 3.3V be enough
to feed TTL input (all trace length/load considered).

> As far as the other direction goes, a 10k resistor to limit the
> current is all you really need, but if you're paranoid and/or have an
> input device that'll generate a large Vh, a simple clamping diode does
> the trick without having to resort to another SMD.

The internal ESD clamp or external one?
There are also big spikes generated by RF noise (cellphones, etc).