# MSP430FR6047: Calculating current limiter values for SCL,SDA,LCD-COM,TXD & RXD lines to increase battery life.

#### pralayp

Joined Apr 8, 2020
3
Hello Experts,

My application is a commercial battery powered device.

The 2 most important thing for me is to achieve "accuracy in metering" and "Increase battery life" of my product.

1st thing I'm able to achieve using MSP430FR6047 chip for water metering.

2nd thing is what I'm working on right now.

Logically current limiters are very helpful in achieving this, given I know how much of it to limit.

I'm unable to figure out where to start(optimization in software such as LPM mode's etc. already done).

Please guide me in this resistor calculation part as this is my first time doing so.

Any literature regarding the subject will also be highly appreciated.

Regards,

Pralay

#### DickCappels

Joined Aug 21, 2008
8,705
This an unusual idea and show that you can think or original approaches.

I don't know what LCD-COM is, but my bet is that it is a common connection for all LCD segments. If that is what it is then you can use any value resistor you want but as the resistance gets higher the less contrast you are likely to have because of the voltage drop across the resistance caused by the capacitive loading of the display segments. For the best display, do not put resistance in series.

SCL and SDA are usually open collector outputs and they are for a reason (bidirectional signaling). They need external pull-up resistors if you are going to use the TW (I2C) interface and you can save power by using the highest value resistor that does not cause communication errors. The resistance if dependent upon the devices connected to the bus, the bus speed, and your layout or other routing of those wires.

TXD and RXD
The device driven by TXD usually has a fairly high input impedance and any series resistor that reduce current is likely to cause communication errors. RXD in an input, and it will be a very high impedance, and will now draw appreciable power.

In other words, these lined are best left without series resistors.