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Altera_Forum's avatar
Altera_Forum
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15 years ago

EPM240T100 CPLD Ouput Overvoltage?

I have an dedicated output tied to a shared SPI bus. One of the other bus drivers, drives to 5V. My CPLD is powered by 3.3V. When my CPLD output is not is use, so high impedance, is there a spec on how high this can be driven. Also, there is a total of 200 ohms of series resistance between any bus driver and the other so current would be limited, although I don't think there is a diode clamp to take advantage of that anyway.... All works well, and has for a long time, but I fear this will need to be addressed/fixed...

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  • Altera_Forum's avatar
    Altera_Forum
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    15 years ago

    By MAX II specification, it's not o.k. to drive 5V to a pin. You need to add a clamp diode. According to the device manual, internal PCI clamping diodes can be enabled for part of the MAX II pins (if I remember right, only with larger packages). But they are not active before user mode starts, so you may want to have a safe hardware clamp instead.