Another question about pin /VP

[cbmeeks]

In my design, I am putting a two pin jumper that connects /VP to GND. So, for WDC 65C02, the user would leave the jumper un-connected. For others, they would connect it so it would be grounded.

However, I'm also putting a female header for the /VP in case the user had a specific need to use it. But I got to thinking that if the user connected the header (shorting /VP to GND) *AND* decided to send a 5V signal to /VP with the female header, then they would more than likely damage the CPU.

So, would it be a good idea to put a 3.3k pull-DOWN resistor on one side of the jumper so that for non-WDC users, they would be using a pull-down to GND and if they actually used the /VP pin, they would still be safe?

Thanks.

[jac_goudsmit]

No. VPB (!VP) is an output, not an input, so no resistor is needed.

The !VP signal is used to override any memory mapping hardware to make sure that the same page is always mapped into memory when the CPU pulls a vector (NMI/IRQ/RESET) from memory.

Without WDC processor, this feature is unavailable, so there's no point in designing for it if you're going to allow non-WDC processors anyway. But in case you want to do that anyway, you can use a three-way jumper that either connects pin 1 to GND, or connects pin 1 to the input of your circuitry. And yes, that circuitry will probably require a pull-up resistor to simulate an inactive !VPB when pin 1 isn't connected to it, but there's no need for any resistor connected (permanently) to pin 1.

Code: Select all

            VCC
             o
             |
             \
             /  pull up resistor
     jumper  \
        +-+  |
        |o---+------> To memory glue logic
pin 1 ---o|
        |o---+
        +-+  |
             |
            ---
            GND

[GARTHWILSON]

I see Jac posted while I was writing; but I'll post this anyway (supporting what he says).

Pin 1 is never an input; only output (totem-pole, not open-drain) or ground, so there's no point in doing pull-up or pull-down resistors. If it's a ground, you'll only benefit from the connection if it's a solid, super-low-impedance connection.

[cbmeeks]

AH! Duh..I should have known that.

@jac, I like that idea. I think I will alter my design slightly to have the three-way header. Best of both worlds.

Thanks!