Hello all, I am attempting to interface with an old Intel encryption chip, 8294a.
However the interrupt output pins have a strange configuration. They show them running to what looks like an inverter, with OC inside it.

From what I can tell "OC" is an open collector inverter. But does this mean the output itself is open collector and needs a pull up?
If so, is the inverter necessary or can I just read the output pin and add a pull up?

Right now if I try to read the pins directly without any pullup or inverter, they never change state from zero.

I have attached the datasheet. The relevant figures are on page 8 of the pdf, figures 13 & 14.

Thanks alot for any help!

 

The way I see it is that they are recommending external open collector buffers to allow multiple devices to share the interrupt lines of the CPU.
Figure 15 shows SRQ and OAV driving gate inputs directly without pullups.

I wonder if the reason you are always seeing a zero on these lines is that the interrupt is being handed too quickly for you to see.

 

The open-collector inverters are external to the 8294 chip. Open-collector, open-drain or tri-state bus driver are needed when interfacing to party-line interrupt bus signals.

The SRQ, OAV and CCMP interrupt signals from the chip are not open-collector outputs and hence do not need pull-up resistors.
Make sure that you send a SET MODE command to enable the interrupts.

 

Thanks guys. I was hoping that was the problem. But It looks like its not.
I am puzzled by it. I do send the "set mode" command but the interrupts do not ever get asserted.

 

Hello all, I am attempting to interface with an old Intel encryption chip, 8294a.
However the interrupt output pins have a strange configuration. They show them running to what looks like an inverter, with OC inside it.

From what I can tell "OC" is an open collector inverter. But does this mean the output itself is open collector and needs a pull up?
If so, is the inverter necessary or can I just read the output pin and add a pull up?

Right now if I try to read the pins directly without any pullup or inverter, they never change state from zero.

I have attached the datasheet. The relevant figures are on page 8 of the pdf, figures 13 & 14.

Thanks alot for any help!

Are you talking about the Interrupt lines? Yes, they should be open collector so more than one output can be connected on that line. Yes, they need a pull up resistor, and yes, it will be quick.

 

Hi, thanks for responding.
Yes I am referring to the interrupt lines. and I am only driving one input with the interrupt line.
Also, are you saying that the interrupt would only be high for a short time? I would think that once it went high it would stay that way until another input was sent to the chip. At which point the interrupt would go low until the chip is done processing the new command/data, and then go high again.
The 8294a is designed to be operated asynchronously.

Could you explain your reasoning a bit since your answer is different from the other two?

 

Ok, so I just got the interrupts to respond. Pull up Resistor was not required.
I mis-understood Command 5 and thought you could perform that at the same time as Set Mode.

Thanks again to all who responded.