The two jumpers
Two pairs of pads on the back, both open as the board ships. Solder across BL-3V3 and the backlight is on whenever there is power, with no BL wire. Solder across CS-GND and the display is always selected, with no CS wire, but it must then be alone on its SPI bus. Each saves a GPIO; each needs its pin left unconnected afterwards.
Two pairs of pads
On the back, beside the transistor, a pair of pads is marked BL and 3V3. Down by the header, another pair is marked CS and GND. Each pair is a solder jumper: two pads close together, open until a blob of solder joins them. The board's files show both open as it ships.
BL-3V3: the backlight always on
Bridged, it ties Q2's gate to 3V3, so the backlight is on whenever the board has power. BL needs no wire, and a board short of pins gets one back.
What it costs: no dimming, and no switching the backlight off to save power. And BL on the header is now 3V3. Leave it unconnected. A pin driving it LOW would connect 3V3 to GND through your board's output.
CS-GND: always selected
Bridged, it holds CS LOW for good, so the display always listens and CS needs no wire. Give the library -1 for CS, or a spare pin that nothing is on.
What it costs: the display takes every byte on SCL and MOSI as its own. So it has to be the only thing on that SPI bus: no SD card, no second display. And CS on the header is now GND. Leave it unconnected: a pin driving it HIGH would short to GND.
Solder, not a setting
Both are set with a soldering iron, with the power off, and read by nothing in code. A sketch cannot tell whether they are bridged, and the sketches in this book assume they are open: eight wires.
When it does not work
Look at the pads with a light behind you: open, there is a dark gap between the two gold pads; bridged, a bump of solder covers the gap. A multimeter on continuity across the pads settles it. As shipped, both are open.
Something is driving BL LOW, which now shorts 3V3 to GND through that pin. Unplug it, take the BL wire off the header, and check the sketch does not set a BL pin. With the jumper bridged, BL must stay unconnected.
The display now takes every byte on SCL and MOSI as its own, including the ones meant for the card. With CS tied to GND it has to be the only device on that bus. Remove the solder, or give the card a separate SPI bus.
Heat the blob with a clean iron and pull it away with solder wick, or with the iron's tip wiped clean. Check the gap with a meter afterwards: the pads are small and close together.
White, black, dim, or the wrong colours: what each one means.
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