Understanding the First 10K Resistor

I created these notes to help better understand the role of the first 10K resistor we encounter when soldering. They also contribute towards an understanding of the schematic. Please feel free to suggest changes.

The First 10K Resistor

The role of this first 10K Ohm resistor you solder is to keep the Chip Enable (CE) line of the LCD high. If you find a datasheet for the LCD 5110 online you will see that the CE pin is active-low on this device which means that high voltage means off and low voltage means on. So it needs to be kept high while not in use. When the microcontroller wants to communicate with the display, it will pull it low (connect it to ground) to sink all the current coming through the resistor and thereby make the pin go to low voltage. This happens on pin 24 of the microcontroller, labelled LCD_CE on the schematic.

Looking at the schematic and then the board you might get confused where the V_BAT connection happens. The positive side of the capacitor connects to a pin on “Switch 1” which is on the V_BAT line which isn’t completely drawn in the schematic but sometimes replaced by the ‘V_BAT’ label. There’s a connection between all ‘V_BAT’ labels but it isn’t indicated exactly how it’s made - a compromise we accept when neatening-up schematics with labels.