# AT28C256 Page Mode Write Waveforms in datasheet

I read the datasheet for the AT28C256 EEPROM and in the page mode write section there is the following diagram.

Where $$\ \overline{OE} \$$ line is always in the HIGH state during operations whereas in the note (2) it says that it should be HIGH only when $$\ \overline{WE} \$$ and $$\ \overline{CE} \$$ are both LOW.

So I'm a bit confused, what am I missing?

• Just to include it, here's one example datasheet. Please note that the data bus is bidirectional. It can be either an input or an output, but cannot be both at the same time. It would make no sense at all for the output enable to be engaged while you are using the data bus to write values to the chip (and therefore must be an input to the chip.) Doesn't matter what the datasheet might seem to say.
– jonk
Apr 16 at 21:11
• @jonk, I think the OP has full understanding of the standard /RD /WR /CE bus controls. It's 'Note 2' being at odds with that standard behaviour that's confusing them. Hence my answer. Apr 16 at 21:36

The wording may not be ideal, but it's rather simple.

If CE is high, the chip ignores OE and WE so it can't be read or written, and when CE is low the chip reacts to the OE and WE lines, so it can be sent read cycles or write cycles. But you can only do either a read cycle or a write cycle, as both simultaneously is invalid operation.

If the chip is being sent a byte of data with a write cycle, conrolled by WR and CS being low at the same time, the data bus must be an input so obviously the OE pin must be held high, as the data bus would be an output when OE pin is low with CE being low.

What it means that OE can be low at some points during the cycles, but it cannot be low during the write cycle when both CE and WE are low. No matter if it is the CE or WE that initiates the write, OE being low is enough to inhibit a write operation.

You're missing nothing - it's bad phrasing in the datasheet.

Note 2 should say:

/OE must be high when /WE and /CE are both low


In other words, don't try to perform a read during a write. Which is standard and what you expected it to be saying, I imagine.