![]() ![]() Case LH: IRQB output is Low: M1 is conducting, and nothing else is pulling the IRQB input line low. As for the HH case, both sides of the diode are at the same potential and no current will flow. Case LL: IRQB out at Low level: M1 is conducting, and another peripheral is pulling the IRQB line low. The diode is reverse polarized and no current will flow. Case HL: When the IRQB output of the 65C22 is at High level, and another peripheral is pulling the IRQB input line low, the cathode will be at Vcc (M2 on) while the anode will be at about 0V. Explanation (see attached picture): Case HH: When the IRQB output of the 65C22 is at High level, and nothing else is pulling the IRQB input line low, both sides of the the diode will be at Vcc: the cathode thanks to M2 conducting, the anode due to the pull-up resistor R. So it seems you have a W65C22S? The datasheet requires the diode to be "forward biased when IRQB is low": this means the cathode must be on the 65C22 side. ![]()
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