3.3V and 5V Isolation

We read the books, we read the specs (religiously) and they said the chip is NOT 5V tolerant (in this case the MPC855T). We have some essential 5V stuff on the board. So how do we isolate it - cheaply.

We chose to use the 74LCX range of devices - we had them already on the board for other reaons and we could steal a few gates. BUT for the first time ever we decided that we would try not blowing something up before we read the specs again and ran some experiments to find out what would happen. This PDF (with text in both English and Francais) was the result of the tests using a 74LCX125 (low voltage Quad buffer with 5V tolerant Inputs and Outputs). If you really need true interworking with a 5V level on one side and 3.3 on the other you need to use the 74LVX range. We didn't!

Test results in PDF format

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