New 65C816 datasheets say that Vih (for the data bus in particular) is 0.8Vdd, i.e. not TTL-compaible.

Old data sheets (for example, the one published in Apple IIgs Hardware Reference) say that at 5V +/- 5%, Vih is 2.0V.

Are old 65C816s TTL compatible? Was there some kind of change?

There has been a lot of talk about this but I can't find anything conclusive.

  • Many manufacturers often have an annoyingly large gap between the the combinations of input voltage, supply voltage, and temperature where their parts will work, and those where their parts will be guaranteed to work. In some cases, manufacturers will publish graphs showing typical performance characteristics, but unfortunately offer little guidance as to what engineers should do with the information contained thereon if the device will only be used under a narrow range of conditions?
    – supercat
    Commented Apr 29, 2019 at 15:23

1 Answer 1


Are old 65C816s TTL compatible?

If their Vih is 2.0v, they are certainly TTL-compatible.

Was there some kind of change?

All WDC chips seem to be verilog-reimplemented, so when they synthesized them into netlist, they've intentionally dropped TTL-compatibility of IO-pins.

The reason for that might be the need to have chips working in 1.8V..5.0V range (as per datasheet). The input buffers that support TTL levels at 5.0V and simultaneously have reasonable ranges at 1.8V might be either too complex or simply unavailable for the technology WDC uses.

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