How to use a 1.77 inch TFT with a STM8 MCU
To use a 1.77 inch TFT with a STM8 MCU, you need to wire the display’s SPI interface to the STM8’s GPIO pins, initialize the display controller (typically the ST7735S or similar), and then send pixel data using a minimal framebuffer or direct draw commands. The 1.77 inch TFT commonly has a resolution of 128x160 pixels, with 16-bit RGB565 color depth, meaning each pixel requires 2 bytes of data. For a full frame, that’s 128 * 160 * 2 = 40,960 bytes. The STM8, being an 8-bit MCU with limited RAM (often 2KB to 6KB), cannot hold a full framebuffer, so you must draw graphics line by line or use a partial buffer. The STM8S003F3P6, for example, has only 1KB RAM, which forces you to send data row by row, updating the display in real-time. The SPI clock speed on STM8 can reach up to 8 MHz when using the hardware SPI peripheral, but software bit-banging is also possible if you need more pin flexibility. The TFT module itself usually runs on 3.3V logic, and the STM8 operates at 3.3V or 5V depending on the variant, so you must ensure voltage compatibility—if your STM8 runs at 5V, use a level shifter or a voltage divider on the MOSI, SCK, and CS lines to avoid damaging the TFT. The display’s backlight is typically driven by a separate pin, often controlled via a PWM signal from the STM8 to adjust brightness. The initialization sequence for the ST7735 controller involves sending a series of commands like SWRESET, SLPOUT, COLMOD, DISPON, and others, each with specific parameters. For instance, COLMOD (command 0x3A) set to 0x05 enables 16-bit RGB565 mode. The exact initialization code can be found in datasheets or open-source libraries, but you must adapt it to the STM8’s register-based programming style. The STM8’s SPI registers include SPI_CR1, SPI_CR2, SPI_DR, and SPI_SR. To send a byte, you set the data register, wait for the TXE flag, and then poll the RXNE flag to clear the buffer. A typical SPI transfer function for STM8 looks like this: while(!(SPI_SR & SPI_SR_TXE)); SPI_DR = data; while(!(SPI_SR & SPI_SR_RXNE)); dummy = SPI_DR;. This ensures reliable communication. The TFT’s pixel data is sent in 16-bit chunks, with the high byte first. For example, to draw a red pixel (0xF800 in RGB565), you send 0xF8 then 0x00. The STM8’s limited speed means that filling the entire 128x160 screen with a solid color takes about 0.5 seconds at 8 MHz SPI, but if you need faster updates, you can reduce the SPI clock prescaler to achieve 4 MHz or 2 MHz, though the STM8’s CPU frequency (typically 16 MHz max) becomes the bottleneck. The display’s data sheet specifies the minimum timing for commands and data, such as a 150 ms delay after SWRESET and a 120 ms delay after SLPOUT. These delays are critical—if you skip them, the display may not initialize correctly. The STM8’s timers, like TIM2, can generate precise delays using the SYSCLK, but a simple loop with NOPs is often sufficient for these long delays. For drawing text or shapes, you need a font library, but since the STM8 has limited flash (8KB to 32KB), you must use a compact font format like 5x7 pixels, which consumes about 95 bytes per character for ASCII. A full ASCII set of 95 characters at 5x7 pixels requires 95 * 5 * 7 = 3,325 bits, or about 416 bytes, which is manageable. The STM8’s flash memory can store this, but you must be careful with the linker script to place the font data in the correct section. The TFT’s 1.77 inch diagonal size means the pixel pitch is about 0.22 mm, making it suitable for small displays like handheld meters or sensor readouts. The STM8’s I2C or UART peripherals can be used to receive data from sensors and then display it on the TFT. For example, a temperature sensor like the DS18B20 can be read via OneWire, and the value can be converted to a string and printed on the TFT. The STM8’s printf function is not available in standard libraries, so you must write your own integer-to-string conversion routine. A simple itoa function for 16-bit integers takes about 200 bytes of flash. The display’s SPI interface usually uses 4 pins: CS (chip select), DC (data/command), SCK (serial clock), and MOSI (master out slave in). Some modules also have a RESET pin, which can be tied to the MCU’s reset or a GPIO. The DC pin tells the display whether the incoming byte is a command or data. When DC is low, the next byte is a command; when high, it’s data. The CS pin must be pulled low before any SPI transaction and high after. The STM8’s GPIO configuration for these pins should be set to push-pull output at 10 MHz speed. For the backlight, a PWM signal from a timer output compare channel can be used. For example, TIM2’s channel 1 can generate a 1 kHz PWM with duty cycle from 0 to 100%. The STM8’s timer registers like TIM2_ARR (auto-reload) and TIM2_CCR1 (capture/compare) control the frequency and duty cycle. A typical setup: TIM2_ARR = 1000, TIM2_CCR1 = 500 gives 50% duty. The backlight pin on the TFT is often an LED with a current-limiting resistor, so you can drive it directly from the STM8’s GPIO if the current is below 20 mA, but for higher brightness, use a transistor. The power consumption of the TFT is about 40 mA with backlight on, and the STM8 draws about 10 mA at 16 MHz, so a total of 50 mA is fine for a 3.3V regulator like the AMS1117-3.3. The display’s viewing angle is typically 12 o’clock, meaning the best view is from the top, but the ST7735 supports hardware rotation via the MADCTL command (0x36). By setting bits in the MADCTL register, you can rotate the display 0, 90, 180, or 270 degrees. For example, to rotate 90 degrees, set MADCTL to 0x60. This is useful if your PCB layout requires the display to be mounted sideways. The STM8’s interrupt system can be used to handle SPI transfers in the background, but for simplicity, polling is often used. The SPI interrupt flag is set when a byte is transferred, but the STM8’s interrupt latency is about 10 cycles, so it’s not much faster than polling. The TFT’s response time is about 10 ms, so you don’t need high-speed updates. For a practical project, you might use the STM8S Discovery board, which has an STM8S105C6 with 2KB RAM and 32KB flash. This board has a built-in ST-Link programmer, making it easy to debug. The wiring between the STM8 and the TFT is straightforward: connect CS to PD2, DC to PD3, SCK to PC5, MOSI to PC6, backlight to PD4, and RESET to PD5. The SPI peripheral on the STM8S105 is SPI1, which uses pins PC5 (SCK) and PC6 (MOSI). The STM8’s alternate function registers must be configured to enable the SPI pins. For example, set the AFR (alternate function register) for port C to enable SPI1. The clock for the SPI peripheral is derived from the system clock, which is typically 16 MHz. The SPI baud rate prescaler can be set to 2, 4, 8, 16, 32, 64, or 128. For 8 MHz SPI, use prescaler 2. The STM8’s datasheet specifies that the SPI clock frequency should not exceed 10 MHz, so 8 MHz is safe. The TFT’s datasheet for the ST7735 controller specifies a maximum SPI clock of 15 MHz, so 8 MHz is well within limits. The initialization sequence for the ST7735 is critical. A typical sequence for a 1.77 inch TFT with 128x160 resolution is: send command 0x01 (SWRESET) and wait 150 ms; send command 0x11 (SLPOUT) and wait 120 ms; send command 0x3A (COLMOD) with data 0x05; send command 0x36 (MADCTL) with data 0x00 for normal orientation; send command 0x2A (CASET) with data 0x00, 0x00, 0x00, 0x7F (column start and end); send command 0x2B (RASET) with data 0x00, 0x00, 0x00, 0x9F (row start and end); send command 0x2C (RAMWR) to start writing pixel data; then send pixel data in RGB565 format. The CASET and RASET commands define the window for drawing. If you want to draw only a portion of the screen, you can change these values. For example, to draw a 10x10 pixel square at the top-left corner, set CASET to 0x00, 0x00, 0x00, 0x09 and RASET to 0x00, 0x00, 0x00, 0x09. This reduces the amount of data sent, which is useful for the STM8’s limited bandwidth. The STM8’s flash memory can store multiple images or fonts, but you must be careful with the size. A 128x160 image in 16-bit color takes 40KB, which exceeds the flash of most STM8 devices. So, you typically use compressed images or small icons. For example, a 32x32 icon takes 2KB, which is manageable. You can store the icon in flash as a const array and send it to the TFT using a loop. The STM8’s compiler (like SDCC or Cosmic) supports const arrays in flash, but you must use the __flash keyword. For example, __flash const uint8_t icon[] = { ... };. The STM8’s memory map places flash at address 0x8000, so you can access it directly. The SPI transfer function must be careful with the data type. Since the STM8 is 8-bit, you send bytes one by one. For a 16-bit pixel, you send the high byte first, then the low byte. The TFT’s controller expects the data in big-endian order. If you send the low byte first, the colors will be swapped. The STM8’s endianness is little-endian, so you must manually swap bytes. For example, to send a pixel with value 0xF800, you do: spi_write(0xF8); spi_write(0x00);. The STM8’s SPI peripheral can be configured to send data in 8-bit or 16-bit mode, but 16-bit mode is not commonly used because the STM8’s CPU is 8-bit. So, stick with 8-bit mode. The TFT’s backlight can be controlled with a simple GPIO or PWM. If you use a GPIO, you can turn it on or off, but PWM gives you brightness control. The STM8’s timer PWM output can be set to a frequency above 100 Hz to avoid flicker. A common value is 1 kHz. The duty cycle can be adjusted from 0 to 255 if you use an 8-bit timer. For example, TIM2 is a 16-bit timer, but you can use it in 8-bit mode by setting the ARR to 255. The STM8’s GPIO pins have a maximum current of 20 mA, so the backlight LED (which typically draws 20-30 mA) might need a transistor. A simple 2N2222 NPN transistor with a base resistor of 1k ohm can drive the backlight from the STM8’s GPIO. The TFT’s VCC pin should be connected to 3.3V, and the ground to GND. The STM8’s VDD is also 3.3V for most variants, but if you use a 5V STM8, you need a level shifter. The 74LVC245 is a common level shifter for SPI signals. The TFT’s SPI signals are 3.3V tolerant, so if your STM8 outputs 5V, you must reduce the voltage. A simple voltage divider with 1k and 2k resistors can bring 5V down to 3.3V, but it’s not ideal for high-speed SPI. A better solution is to use a 3.3V regulator for the STM8 as well. The STM8S003F3P6 can run at 3.3V with a 16 MHz clock, but the maximum frequency is 16 MHz at 3.3V, so it’s fine. The STM8’s internal RC oscillator is accurate to about 1%, which is sufficient for SPI communication. The TFT’s 1.77 inch size is small, so the viewing angle is not critical. The ST7735 controller supports a 262k color palette, but in 16-bit mode, you have 65k colors. The difference is not noticeable on a small display. The STM8’s code size for a basic TFT driver is about 2KB to 4KB, depending on the features. You can find open-source libraries for STM8 and ST7735, but they often need adaptation. For example, the “stm8-tft” library on GitHub provides a basic driver, but you must modify the pin definitions. The library uses a software SPI implementation, which is slower but more flexible. For a hardware SPI, you need to write your own functions. The STM8’s SPI peripheral has a 2-byte buffer, so you can send two bytes back-to-back without waiting, but for simplicity, wait for each byte. The TFT’s display is 1.77 inch diagonally, which is about 45 mm. The resolution of 128x160 gives a pixel density of about 114 PPI, which is good for text and graphics. The STM8 can draw lines, circles, and rectangles using Bresenham’s algorithm, but the code size increases. A line drawing function takes about 200 bytes. For a simple project, you can use pre-defined shapes. The TFT’s data sheet specifies the power consumption: 40 mA with backlight on, 10 mA with backlight off. The STM8’s power consumption is about 10 mA at 16 MHz. So, a battery-powered project can run for a few hours on a 200 mAh battery. The STM8 has a low-power mode (halt mode) that draws 1 µA, but you must wake it up to update the display. The display update can be triggered by a timer interrupt. For example, update the display every 1 second. The STM8’s watchdog timer can be used to reset the system if it hangs. The TFT’s initialization sequence includes a sleep-out command, which must be sent before any data. If you forget, the display will stay blank. The STM8’s GPIO pins must be configured as outputs for the SPI pins. For example, PC5 and PC6 must be set to push-pull output. The SPI’s MISO pin is not used because the TFT does not send data back. So, you can leave MISO unconnected. The TFT’s CS pin must be pulled low before each transaction and high after. The DC pin must be set low for commands and high for data. The STM8’s code can be structured as a state machine. For example, the main loop reads a sensor, converts the value to a string, clears the display, and draws the string. The clear display function sends a rectangle fill command with a background color. The fill command uses the CASET and RASET commands to set the window, then sends pixel data for the entire window. For a 128x160 screen, this is 40,960 bytes. At 8 MHz SPI, this takes about 5 ms, but the STM8’s CPU overhead adds more time. The total time for a full screen update is about 20 ms, which is fast enough for 50 FPS. But the STM8’s RAM is too small for a framebuffer, so you must send data directly. The STM8’s flash memory can store a lookup table for gamma correction, but the ST7735 has built-in gamma correction, so it’s not needed. The TFT’s contrast and brightness can be adjusted via the command 0xC0 (power control) and 0xC1 (power control 2). The default values are usually fine. The STM8’s code must be compiled with the correct optimization level. SDCC’s -Os option reduces code size. The STM8’s interrupt vector table is at the start of flash, so you must not overwrite it. The TFT’s driver code can be placed in a separate file. The STM8’s linker script must allocate enough RAM for the stack. The stack size is typically 256 bytes, which is enough for a simple project. The TFT’s display can show text, numbers, and simple graphics. The font data can be stored in flash as a bitmap. For example, a 5x7 font for the character ‘A’ is 5 bytes wide, 7 bits high. The font data is stored as a 2D array. The STM8 can read the font data from flash and send it to the TFT. The TFT’s pixel data is sent in row-major order. For a character, you draw each row of the bitmap. The STM8’s code