Waveshare 3.7" Pi Pico and FireBeetle support

Add support for a Waveshare Pi Pico 3.7" display connected
to a DFRobot FireBeetle.

Notable things in this example include:
 - Use u8g2 functions to get string/font sizes rather than
   display.getTextBounds, as it seems more accurate when using
   u8g2 fonts, and makes drawString[MaxWidth] easier to place.
 - Use the calibrated ESP32 adc functions to measure the battery
   voltage, and use a simpler algorithm to predict percentage.
 - For the overnight long sleep period, do a single long sleep
   rather than wake every 30 minutes to check.
 - When the battery voltage is below a dangerous level, print
   a large obvious battery warning on the screen!

Signed-off-by: Graham Whaley <[email protected]>

README.md

@@ -18,6 +18,7 @@ Download the software to your Arduino's library directory.
    - Waveshare_2_13
    - Waveshare_2_7
    - Waveshare_2_9
+   - Waveshare_3_7
    - Waveshare_4_2
    - Waveshare_7_5
    - Waveshare_7_5_T7 (newer 800x480 version of the older 640x384)
@@ -38,9 +39,9 @@ Download the software to your Arduino's library directory.
 
 5. Save your files.
 
-NOTE: See schematic for the wiring diagram, all displays are wired the same, so wire a 7.5" the same as a 4.2", 2.9" or 1.54" display! Both 2.13" TTGO T5 and 2.7" T5S boards come pre-wired.
+NOTE: See schematic for the wiring diagram, all displays are wired the same, so wire a 7.5" the same as a 4.2", 2.9" or 1.54" display! Both 2.13" TTGO T5 and 2.7" T5S boards come pre-wired. The 3.7" FireBeetle example contains wiring details.
 
-The Battery monitor assumes the use of a Lolin D32 board which uses GPIO-35 as an ADC input, also it has an on-board 100K+100K voltage divider directly connected to the Battery terminals. On other boards, you will need to change the analogRead(35) statement to your board e.g. (39) and attach a voltage divider to the battery terminals. The TTGO T5 and T5S boards already contain the resistor divider on the correct pin.
+The Battery monitor assumes the use of a Lolin D32 board which uses GPIO-35 as an ADC input, also it has an on-board 100K+100K voltage divider directly connected to the Battery terminals. On other boards, you will need to change the analogRead(35) statement to your board e.g. (39) and attach a voltage divider to the battery terminals. The TTGO T5 and T5S boards already contain the resistor divider on the correct pin. The FireBeetle has a battery monitor on GPIO-36.
 
 Compile and upload the code - Enjoy!
 
@@ -56,6 +57,10 @@ Compile and upload the code - Enjoy!
 
 ![alt_text, width="400"](/Waveshare_4_2.jpg)
 
+3.7" 480x280 E-Paper Layout
+
+![alt_text, width="400"](/Waveshare_3_7.jpg)
+
 2.7" 264x176 E-Paper Layout
 
 ![alt_text, width="400"](/Waveshare_2_7.jpg)

examples/Waveshare_3_7/README.md

@@ -0,0 +1,47 @@
+# Waveshare Pi Pico 3.7" wired to FireBeetle
+
+This example is for a
+[Waveshare Pi Pico 3.7" display](https://www.waveshare.com/pico-epaper-3.7.htm) connected
+to a [DFRobot FireBeetle board](https://www.dfrobot.com/product-1590.html).
+
+The display is formally known as *SKU: 20123*, *Part Number: Pico-ePaper-3.7*.
+
+Although the display is designed to operate directly with a
+[Pi Pico](https://www.raspberrypi.com/products/raspberry-pi-pico/), the Pi Pico
+has neither native WiFi support nor super low power modes. The FireBeetle has both.
+
+## Wiring
+
+The below table shows the wiring used in the example. The FireBeetle is wired to the
+Pi Pico header pins on the display. These details can also be found near
+the top of the code.
+
+It should be possible to use the alternative H1 SPI connector on the display board, but I
+didn't have an appropriate plug to try.
+
+The display is wired to SPI0, but uses different pins for the extra control lines than the
+normal Lolin D32 examples in this repository, as some of the normal pins are nominally already
+assigned other tasks on the FireBeetle.
+
+| Function | Display pin | Firebeetle GPIO |
+| -------- | ----------- | --------------- |
+| BUSY     | 17          | D2 (IO25)       |
+| RST      | 16          | D3 (IO26)       |
+| DC       | 11          | D4 (IO27)       |
+| CS       | 12          | D8 (IO5)        |
+| CLK      | 14          | IO18 (SCK)      |
+| DIN      | 15          | IO23 (MOSI)     |
+| GND      | 13          | GND             |
+| VSYS     | 39 (VSYS)   | VCC             |
+
+## ADC and battery monitor
+
+The FireBeetle contains a battery monitor voltage divider consisting of a pair of 1Mohm resistors
+and a smoothing capacitor, but by default it is disabled by two nofit 0ohm resistors. The enable
+the battery monitor function these two missing resistors need to be fitted or bridged. They
+are resistors R10 and R11. Details can be found in the FireBeetle schematic, and the below picture
+shows their location near the 32Khz crystal - the bridged pads are circled in red.
+
+Note, it is much easier to bridge the pads **before** you fit any headers to the board.
+
+![alt_text, width="200"](./firebeetle_adc_links.jpg)