65C02 and SCR latch-up

[banedon]

Hi guys

I was wondering if any of you ahve ever experienced SCR latchup, in particular with the WDC W65C02S? I know that most CMOS devices are subject to this possibly happening and I think it's caused by the input or output pins signals being 0.6V or more above VDD.
The reason I ask is 1) simple interest, 2) should I bother with something like clamping diodes in y 6502 designs?

[GARTHWILSON]

I've been using a WDC 65802 for 20 years and never had such a problem with it. I have had the problem with Rockwell VIAs, where I/O pins went to a printer across the room that was powered by a different mains circuit and one circuit could have a spike without the other one, but it has been rare, and I haven't seen it happen in many years. The static-protection diodes on the I/O pins are Schottky already (with forward voltages of around a quarter of a volt), so putting more diodes on probably won't help a bit.

[BigDumbDinosaur]

banedon wrote:

I was wondering if any of you ahve ever experienced SCR latchup, in particular with the WDC W65C02S? I know that most CMOS devices are subject to this possibly happening and I think it's caused by the input or output pins signals being 0.6V or more above VDD. The reason I ask is 1) simple interest, 2) should I bother with something like clamping diodes in y 6502 designs?

Like Garth, I've never experienced a latchup problem. Unless you see severe ringing, clamping diodes are, in my opinion, an unnecessary measure.

[banedon]

Cool. A number of parts that I don't have to worry about . Thanks, guys.

[bogax]

GARTHWILSON wrote:

The static-protection diodes on the I/O pins are Schottky already (with forward voltages of around a quarter of a volt), so putting more diodes on probably won't help a bit.

Are you sure about that? (cite?)

Thats not my understanding (I'm far from an expert though)

The protection diodes are part of the problem and the idea
of external clamps would be to keep current out of the (internal)
protection diodes

[GARTHWILSON]

The external diodes would have to turn on at a lower voltage than the internal ones in order to keep the internal ones from conducting. If the internal ones start turning on at around 0.15V, I don't know how to get anything lower than that externally.

[BigDumbDinosaur]

GARTHWILSON wrote:

The external diodes would have to turn on at a lower voltage than the internal ones in order to keep the internal ones from conducting. If the internal ones start turning on at around 0.15V, I don't know how to get anything lower than that externally.

Supposedly the Schottky arrays, e.g., the 74S1053, turn on at lower voltages than the internal ESD protection diodes. The 74S1053 and its cousins were intended to suppress ringing on longish bus lines, rather than deal with ESD. Nevertheless, the 74S1053 has a pretty high forward current rating per diode and turn-on of approximately 0.4 volts. I do have to question the notion that the internal ESD diodes in a chip turn on at a lower voltage, since there are hard limits to the Schottky technology.

It should be noted that some ringing on the buses isn't necessarily an evil condition. Unless ringing is sufficient to cause circuit malfunction there's no reason to use a diode array to terminate a bus, other than perhaps on an expansion bus into which cards would be plugged.

[bogax]

GARTHWILSON wrote:

The external diodes would have to turn on at a lower voltage than the internal ones in order to keep the internal ones from conducting. If the internal ones start turning on at around 0.15V, I don't know how to get anything lower than that externally.

Sorry, I wasn't clear.

What I'm questioning is whether the internal
protection diodes on CMOS are Schottky

[GARTHWILSON]

I suspect they are Schottky, because the "Absolute maximum ratings" in the data sheet say no pin should go outside the power rails by more than 0.3V. I would consider a standard silicon diode to take around 0.5V to start conducting (it depends on how tiny you want to put the threshold current), and won't usually be damaged by high current until the voltage across it gets well over 0.7V, sometimes even over 1V. I have not looked at the curve for Schottky, so I don't know how its shape compares to that of standard silicon diodes. Wikipedia says Schottky diodes start conducting at 0.15V. I haven't experimented. I suppose I should now, now after having used them in switching power supplies so many times.