Resistors between LCD and 8051

[MEnsing]

Hello,

I hope that this is the correct place for my question.

I have a schematic of a pre-amplifier in which an 8051 is controlling an LCD display. I am wondering why there is an resistor network (RN1) between the 8051 and the LCD display. The LCD display is connect by an connector (P1).

[error401]

Originally posted by MEnsing
Hello,

I hope that this is the correct place for my question.

I have a schematic of a pre-amplifier in which an 8051 is controlling an LCD display. I am wondering why there is an resistor network (RN1) between the 8051 and the LCD display. The LCD display is connect by an connector (P1).

The image isn't really clear enough to read easily, but it looks to me like these are just series protection resistors. Since no appreciable current actually appears on these control lines, a resistor (of any value, really) can be placed in series. This serves to protect the microcontroller from potential shorts or other fault conditions in the LCD panel or cabling. If such a short happened and the microcontroller tried to drive the two pins with alternate polarity, it would quickly fry itself due to the lack of current limiting. With the series resistors in place, any short to power, ground or each other shouldn't cause any damage to the microcontroller. It's just good design practice to include them, even though they're not required.

I do see that there's a VCC connection nearby that seems like it might connect to the resistor array as well, so it's possible that these are pullups. I doubt it though, that doesn't really make much sense in the application.

[HawkStudio]

It's really hard to see in the drawing but unless my eyes are tricking me it looks like a resistor pack with one side common going to 5V, yep...pull ups. It doesn't make sense to me unless the port pins are open collector outputs. Its been many years since I've played with the 8051 so I don't remember.

[error401]

Originally posted by HawkStudio
It's really hard to see in the drawing but unless my eyes are tricking me it looks like a resistor pack with one side common going to 5V, yep...pull ups. It doesn't make sense to me unless the port pins are open collector outputs. Its been many years since I've played with the 8051 so I don't remember.

Well, I checked out an Atmel 8051, and it looks like Port 1 is open-drain, so maybe it does make sense after all.

If you have a better scan of the schematic...

[HawkStudio]

Open drain....yep...thats it.

[MEnsing]

Thanks for the answers. That helped me a lot

Regards,

Marco