Hi!

I have salvaged an LCD & panel buttons from an old HP laserjet. It has a three-wire interface using 3.3V. MISO can go directly from the panel to µC/parallel port (later used for testing).
CLK, CS, MOSI have to be adjusted 5V>3.3V - I wanted to use a '244 for the datasheet says output high is typically 3.4V which should be within specs. But practically the '244s I have output with 3.7V - which in the long run probably would be too much. (So far 3.7V did no harm...)

I know that there are other ways for level shifting (passive w/ resitors or diodes, active ones), but would it be possible to use the '244. I thought of reducing it's power supply to 4.75V hoping that the output would go down 0.25V too - then being within spec. The datasheet doesn't tell me if it would be okay for it's input being driven above Vcc.

Ralf

I'm not sure about the 244 but the 245 is pretty popular if you only need one way level conversion. The outputs use whatever you feed to the Vcc pin, regardless of the voltage on the inputs.

The VOH rating of 3.4V typical is at VCC = Min and IOH = Max. No wonder, that you are seeing a higher output voltage. The clamping diodes on the receiving side should kick in at 3.3V + .5V, so 3.7V is on the edge but still below the typical absolute maximum ratings for input circuits with clamping diodes.

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6502 sources on GitHub: https://github.com/Klaus2m5

One thing i forgot to tell: the '244 is not only used for inputs, I abuse it to get the 3.3V voltage - so driving the panel permanently with 3.7V - is that too much?

There are some logic chips made specifically for interfacing between 5V and 3.3V systems. Look at the 74LVX logic family, the 74LVX4245MTCX is a level converting version of the 74245 logic chip.

_________________
Regards, KM
https://github.com/floobydust

YIK

But I try to work with what I have at hand not what I could order...

You probably get closer to VIH = MAX and the 3.7V should fade... If neither side starts to emit smoke it is OK. However, it would work better, if you would sacrifice a chicken before applying power...

_________________
6502 sources on GitHub: https://github.com/Klaus2m5

Do you have any small MOSFETs like BSS138, AO3400, 2N7000 and some 10k resistors? Then you could build one of those simple bidirectional level signal converters (see "level shifter mosfet" in google), that should do the job for your SPI.

So far no magic smoke - but probably it will shorten lifespan...



Although I have raised and eaten some chickens (and all necessary stepsin between), I better not venture into voodoo



No MOSFETs here (or-- maybe some really BIG ones that fell out of a scavenged switching power supply) - but I don't need bidircetiona signals. CS, CLK, MOSI are all from master to slave, i.e. 5V --> 3.3V, and MISO is 3.3V --> 5V.


I probably change the power supply to Zener diode+resistor, and the signal shift to dividing voltage by using 1k+2k Ohm resistors.

I checked at reichelt - it seems 74LVX all have Vcc=3.3V. Is there a solution running at 5V?

The LVX parts are 24-pin vs their 20-pin non translating counterparts. The additional pins are for dual power supplies, 3.3V and 5.0V. Attached a datasheet.

_________________
Regards, KM
https://github.com/floobydust

Just to note, I used 4050 and 74125 chips for level shifting successfully.