UART and Open Drain

[crocco]

Hi,

In my project I want to communicate my Arduino with TI BQ2018 Battery Monitor. Communication is done with proprietary HDQ and I cant find any Library supporting it. However in one of the manuals http://www.ti.com/lit/an/slva101/slva101.pdf they state that I could use UART with 57600, non-parity and 2 stop bits which should mean that I can send and receive 8-bit word for each bit of communication. I suppose I can use ordinary serial in my Mega?

But, to do so it says:

"Also note that the TX and RX of the UART must be tied together. In case the TX output is not
an open-drain output, it will need to be inverted and then drive the gate of an NFET with the
open drain tied to the RX input. See Figure 1.

http://www.tiikoni.com/tis/view/?id=d575932


My question is what inverter and NFET should I use? Any special considerations/specifications?

[Grumpy_Mike]

Any special considerations/specifications?

No, any inverter will work like a transistor, FET or logic gate. Like for example the 74HC04 has 6 inverters in it.
Any logic level N-type FET will work.

[crocco]

Thanks for fast response!/Crocco

[MarkT]

Any nFET with on-resistance in the 0 to 200 ohms should be quite happy to interface to logic - assuming using a
2k2 pull-up or so.  Clearly something cheaper than a large power MOSFET would be good!  Without knowing more
I don't see a reason why a bipolar NPN transistor can't serve the same role (remember base resistor).  Two NPNs
could be used, the first as an inverter...

A 74HC244 could also be used with the UART's TX going to one of the output enables, and LOW to a buffer's input -
then the buffer's output will work like open-drain.  For instance pin1 form TX, pin2 LOW, pin 18 to RX and the pull-up
resistor.  This works because the output enables are active low, so no inverter needed.