Color displays use the fast SPI interface, supporting speeds of 1-80 Mbps with a bus frequency of 10-20 MHz.
Overview
The SPI interface requires significantly more GPIOs than the simpler and slower I2C interface:
- Distinct data lines: SPI comes with two data lines compared to one in I2C:
- MOSI: One line is used to transfer data from the controller to the peripheral.
- MISO: The other one is used for the opposite direction. In displays where communication is typically one-way, MISO is often not used (saving one wire, so-called 3-wire SPI)
- CS Pin: SPI does not use unique device addresses. Instead, the device is selected via the CS pin. Each SPI device therefore needs an exclusive wire and GPIO so the controller can select it.
A few additional GPIOs are required in addition to the SPI pins:
- D/C Pin: displays require a D/C pin. It tells the device whether the incoming data is screen content, or a command for the device control.
- Reset: Many color displays expose a Reset pin. This pin needs to be pulled up for the display to be active.
- Backlight: SPI is typically used with color TFTs. Such displays require an active backlight, so yet another pin is needed to power the backlight LED(s).
When you add the two wires for the power supply, a total of 6 GPIOs and 8 wires may be required to connect a SPI display.
Pins On Display
SPI displays can have these pins:
| Pin | Description |
|---|---|
| SDI/SDA/MOSI | connects to MOSI on the microcontroller. This is where the display receives data |
| SCL/SCK/SCLK | Serial Clock (synchronizes the data speed) |
| CS | Chip Select. Active low: when low, the display is operational |
| DC | Data/Command. Distinguishes between content data and device commands |
| LED/BL/BLK | Backlight LED. Supply appropriate voltage or PWM |
| RES/RESET | Reset pin. Must be pulled high for normal operations |
| VDD | positive voltage, make sure the voltage matches the display requirements |
| GND | Ground |
Sometimes, pins are missing in an effort to save GPIOs. In this case, the missing pins receive standard values directly from the display board:
| Pin | Description | Limitation |
|---|---|---|
| CS | permanently pulled low | SPI can no longer control multiple devices as this device will always receive and process data, even if they were meant for a different device |
| LED/BL/BLK | connected to VDD | Backlight LED always runs at full brightness and cannot be dimmed or turned off to save energy |
| RES/RESET | permanently pulled high | Display controller cannot be resetted manually |
With displays that expose these pins, you can save GPIOs, too, by connecting the pins as indicated in above table.
Pins On Microcontroller
The pins you use for SPI on the microcontroller development board depend on the microcontroller type you use, and sometimes on the board design:
- Mandatory Pins: Older microcontrollers like the one on Arduino Mega have designated SPI pins that you must use.
- Assignable Pins: Microcontrollers like ESP32 have default SPI GPIOs that you should use (because they are more performant), but you can also use any other suitable GPIO instead (which then emulates SPI in software).
- Routable Pins: Modern microcontrollers like ESP32-C3 and ESP32-S3 have routable SPI controllers, so you can assign any suitable GPIO with their hardware-optimized SPI controller(s).
Here is a table with some suggestions:
The development board you are using may use different GPIOs. Modern ESP32 family members such as ESP32-C3 or ESP32-S3 can route any GPIO to its hardware I2C controllers, and development board designers may have designated arbitrary GPIOs.
| Microcontroller | MOSI | MISO | SCK | CS | Remarks |
|---|---|---|---|---|---|
| Arduino Uno/Leonardo/Nano | 11 | 12 | 13 | 10 | |
| Arduino Micro | 16 | 14 | 15 | any | |
| Arduino MKR | 8 | 10 | 9 | any | |
| Arduino Giga R1 | 90 | 89 | 91 | any | |
| Arduino Due | 75 | 74 | 76 | any | |
| Arduino Mega 2560 | 51 | 50 | 52 | any | |
| ESP8266 Wemos D1 Mini | 13/D7 | 12/D6 | 14/D5 | 15/D8 | |
| ESP32 | 23 [13] | 19 [12] | 18 [14] | 5 [15] | VSPI used by default, secondary HSPI in brackets |
| ESP32-S2 | 36 | 35 | 34 | 33 | four SPI interfaces, accessible via GPIO9-GPIO14, and GPIO33-GPIO37. The actual pins you assign from these ranges are free to choose. |
| ESP32-S3 | 11 | 13 | 12 | 10 | default pins run through IOMUX instead of GPIO matrix, IOMUX supports max. 80 MHz |
| ESP32-C3 | 10 | 9 | 8 | any | remappable, may vary greatly between dev boards |
| Teensy | varies | varies | Yvaries | varies | |
| STM32 (Blue Pill) | PB5 | PB4 | PB3 | PA15 | |
| Digispark (ATTiny85) | PB0/pin 5 | PB1/pin 6 | PB2/pin 7 | any other if required | no hardware SPI, emulated using universal serial interface (USI) |
Power Supply
It is crucial to connect the VDD pin on your display with a power source of appropriate voltage. Typically, you must decide whether you connect VDD to the 5V or 3.3V pin on your microcontroller board:
- Multiple Voltages: some displays use internal voltage regulators so you can supply both 5V and 3.3V
- Configurable Voltages: some displays have solder bridges that you can leave open or bridge with some solder. This activates or bypasses internal voltage regulators. Depending on the state of the solder bridge, such displays require either 5V or 3.3V
- Fixed Voltage: some displays support just one fixed voltage, either 3.3V or 5V.
If in doubt, try with the 3.3V pin first. If the display does not work with this lower voltage, you may want to use 5V.
Logical Level Voltage
Most displays internally use 3.3V logic levels for the data pins. When your microcontroller uses 5V (i.e. older Arduinos), you might need a fast level-shifter, or at least some protective restistors for the data pins.
Modern microcontrollers like the ESP32 family run on 3.3V and do not require level shifter.
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(content created Jan 22, 2025)