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3v to 5v level translation

I'm connecting a 5-volt HD44780 LCD controller to a 3-volt microcontroller. I'd planned to use a level translator IC, but I just switched to an MCU with

5-volt tolerant inputs. Actually, I don't need inputs - just 6 outputs. My question is this - assuming the slew rate is acceptable, is there any reason I can't just connect each I/O line to a pullup to 5 volts, set the port to all 0's, and then use the data direction register to switch between high-impedance input mode and output to control the 5-volt signals?

Seems to me like it should be fine, but I can't recall having seen it done that way before.

Thanks,

Scott

Reply to
Scott Miller
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What you're saying makes sense - I've done weird things with these protection diodes on other MCUs - but if that's the case, how is it supposed to work with 5-volt inputs? The specs for the chip (AT91SAM7S64) say Vih can be up to 5.5 volts. It does say not to drive the inputs over VddIO while the internal pullup is enabled, which makes sense. But wouldn't there have to be some provision in the design for dealing with 5-volt inputs without raising Vdd?

I also see now that it's got an open-drain output mode, which is pretty cool, and should simplify things a bit.

Thanks,

Scott

Reply to
Scott Miller

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There are 'reversed' diodes on each I/O pin to the internal Vdd and Vss of the uP. When the I/O is high impedance current will flow through the pull-up resistor and through one of these diodes to Vdd (which would otherwise be at

3 volts.) Hence the uP has now has a forced Vdd of 5 less the v drop across the resistor and diode - which will be very small as the uP current is small. Whether or not this matters is dependant upon the which uP you are using. Ensuring a load, or even better a clamp, on the supply to the uP that can sink this current without causing the volts to rise too much will solve the problem (with a decent cap) However the pull-up will now only rise to 3 volts when the I/O is high impedance. If this does not matter why not just connect the uP as it is. If this does matter - that's why it ain't done!
Reply to
R.Lewis

Some HD44780 Coppies Can run at 3V but the V0 LCD cross the LCD glass needs to be 5V.

So the copy IC can direcly interface to your 3V MCU.

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Reply to
Joe.G

Look at the Voh of your mpu. It should be something like 2.4V, or 2.6V, or

2.8V output with a Vdd of 3V. Now look at the Vih on the HD44780 mpu. It should be something like 2.0V If it is, you should not have a problem driving a "1" into the LCD display.

Do the same exercise with Vol of the mpu, and Vil of the LCD display. If Vol is less than Vil, then you should have have a problem driving a "0" into the the dispaly.

If you detect a problem, go to the TI web site, and find a logic family which will do what your want, maybe 74HCT.

Jeff Stout

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Reply to
Jeff Stout

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