+----------------------------------------------------------------------------+
|  ESP32-DevKitC-V4  ---  led-display                                        |
+----------------------------------------------------------------------------+
|  Rails: 5V, 3V3, GND                                                       |
|                                                                            |
|  matrix  [MAX7219 7-segment / 8x8 LED matrix driver (SPI)]  -- LED matrix  |
|    VCC   -> rail 5V                                                        |
|    GND   -> rail GND                                                       |
|    CLK   -> spi0 (GPIO18)                                                  |
|    MOSI  -> spi0 (GPIO23)                                                  |
|    CS    -> GPIO5                                                          |
|                                                                            |
|  panel  [TM1638 8-digit 7-segment + 8 LEDs + 8 buttons]  -- LED&KEY        |
|    VCC   -> rail 5V                                                        |
|    GND   -> rail GND                                                       |
|    STB   -> GPIO16                                                         |
|    CLK   -> GPIO17                                                         |
|    DIO   -> GPIO4                                                          |
|                                                                            |
|  Passives:                                                                 |
|    c1: 100nF capacitor, matrix.VCC  <->  GND   (decoupling matrix)         |
|                                                                            |
|  BOM:                                                                      |
|    - ESP32-DevKitC-V4                                                      |
|    - MAX7219 7-segment / 8x8 LED matrix driver (SPI)  (matrix)             |
|    - TM1638 8-digit 7-segment + 8 LEDs + 8 buttons  (panel)                |
|    - 1x 100nF capacitor                                                    |
|                                                                            |
|  Power: ~110mA typical, ~400mA peak (budget 500mA)  OK                     |
|                                                                            |
|  Warnings: none                                                            |
+----------------------------------------------------------------------------+
