The NHD-C12832A1Z-FSW-FBW-3V3 is a FSTN LCD display and has a resolution of 128 x 32 pixels. In the design the display is connected via a SPI based interface. Detailed information about the display can be obtained from the display datasheet (NHD-C12832A1Z-FSW-FBW-3V3 from New Haven Displays) and from the display controller datasheet (ST7565R from Sitronix).
The external circuitry of the display is configured to boost the 3.3V supply voltage by a factor of 3 to ~10V. However the typical supply voltage of the display (contrast control) is 6V and therefore the boosted supply must be adjusted, and this must be done by software when the display is configured. Following formula is used when the voltage is adjusted by software:
V0: Display voltage (contrast control)
Voltage regulator internal resistor ratio
VREG: Internal fixed voltage supply (typically 2.1V)
α: Electronic volume level, 1 to 64 are possible values
The recommended configuration for the display is to use
| Electronic volume register value (a) | Display supply voltage V0 [V] | Contrast |
|---|---|---|
| 20 | 6.44 | Too strong |
| 21 | 6.40 | |
| 22 | 6.35 | |
| 23 | 6.31 | |
| 24 | 6.26 | |
| 25 | 6.22 | |
| 26 | 6.17 | |
| 27 | 6.13 | |
| 28 | 6.08 | |
| 29 | 6.03 | |
| 30 | 5.99 | Optimal |
| 31 | 5.94 | |
| 32 | 5.90 | |
| 33 | 5.85 | |
| 34 | 5.81 | |
| 35 | 5.76 | |
| 36 | 5.72 | |
| 37 | 5.67 | |
| 38 | 5.63 | |
| 39 | 5.58 | |
| 40 | 5.54 | Very weak |
The display backlight is controlled by a FET which is by default in an off state but it is possible to turn the backlight on with the XMEGA by driving the gate of the FET high. On the XMEGA pin PE4 is connected to the gate of the FET. The pin PE4 is also an output of an on-chip timer module and because of that it easy to do dimming of the backlight by using PWM.