How to display a graph on a 128x32 COG LCD display?

To display a graph on a 128x32 COG LCD display, you first need to understand the hardware constraints and then write firmware that maps your data points to pixel coordinates. The 128x32 cog lcd display uses a chip-on-glass driver, typically the ST7565 or similar, with a resolution of 128 columns by 32 rows. This means you have 128 pixels horizontally and 32 vertically. For a graph, the horizontal axis can represent time or data index, while the vertical axis must map your data range to fit within 0 to 31 pixels. The SPI interface is common for these displays, requiring at least 4 pins: CS, DC, MOSI, and SCK, plus power and ground. You'll need a microcontroller like an Arduino, ESP32, or STM32 to send commands and pixel data. The display controller expects 8-bit parallel data over SPI, but you send it in a serial stream. The driver IC has internal RAM organized as 128 columns by 32 rows, with each row corresponding to a page of 8 bits. So, to set a pixel at (x, y), you calculate the page (y/8) and the bit position (y%8), then write to that page. For a graph, you'll iterate through your data array, map each value to a y-coordinate, and set the pixel. Keep in mind the display's refresh rate: typical SPI clock speeds of 4 MHz give you a frame rate of around 30 FPS if you redraw the entire screen, but for a graph, you only update changed pixels to save bandwidth. The contrast is controlled via a software command (0x81), with typical values between 0x10 and 0x3F. The display operates at 3.3V logic, but 5V-tolerant pins are common on many microcontrollers. You'll need a level shifter if your MCU runs at 5V. The graph drawing algorithm must handle the 32-pixel height limitation—if your data range exceeds 32 units, you'll need to scale it. For example, if your data ranges from 0 to 100, you map each value to y = (value * 31) / 100. This gives you a linear mapping. For real-time data, you can scroll the graph by shifting pixels left or using a ring buffer. The display's response time is typically 80 ms at 25°C, so fast data changes may appear blurry. The viewing angle is 6 o'clock, meaning optimal visibility when looking from below. The temperature range is -20°C to +70°C, so it's suitable for most indoor environments. Power consumption is around 1.5 mA with the backlight off, and up to 20 mA with the backlight on. The backlight is usually a single LED with a forward voltage of 3.0V and current of 20 mA. You can control brightness via PWM on a transistor. The display module often includes a built-in negative voltage generator for the LCD bias, so you don't need an external negative rail. The contrast voltage is generated internally, but you can adjust it via the software command. The SPI protocol requires a specific sequence: set CS low, send a command byte (DC low), then data bytes (DC high). For graph drawing, you'll use commands like 0xAF (display on), 0xA4 (normal display), and 0x40 (set display start line). The start line command allows you to vertically scroll the entire screen, which is useful for scrolling graphs. You can also use the 0xB0 to 0xB7 commands to set the page address. The column address is set via 0x10 and 0x00 for the high and low nibbles. For a graph, you'll typically set the column address to 0, then write 128 bytes of data for each page. But since you only have 32 rows, you have 4 pages (0 to 3). Each page controls 8 vertical pixels. So, to draw a graph line, you need to set the appropriate bits in each page. For a simple line graph, you can use Bresenham's line algorithm to calculate which pixels to turn on. For a bar graph, you just fill pixels from the bottom to the data point. The display's pixel size is 0.48 mm x 0.48 mm, with a pitch of 0.52 mm. The active area is 66.0 mm x 16.0 mm. The overall module size is 80.0 mm x 36.0 mm. The weight is about 10 grams. The SPI interface runs at up to 10 MHz, but 4 MHz is safe for most MCUs. The display's memory is organized as a 128x32 bitmap, meaning you need 512 bytes of RAM to hold the frame buffer. On a microcontroller with limited RAM, you can store the buffer in the MCU's SRAM and update it incrementally. For example, on an Arduino Uno (2 KB RAM), the 512-byte buffer uses 25% of your RAM. You can reduce this by only storing the current graph data, not the entire screen. But for scrolling, you need a buffer. The typical approach is to have a circular buffer of 128 columns, each column storing 32 bits (4 bytes). Then you update the buffer with new data and redraw the screen. The screen redraw involves sending 512 bytes over SPI, which at 4 MHz takes about 1.3 ms. So you can update the graph at 100 Hz if you only change a few pixels. But the display's response time limits effective updates to around 12 Hz. The contrast setting is critical for readability. The ST7565 driver has a built-in voltage regulator that generates the LCD drive voltage. The command 0x81 followed by a byte sets the contrast. Typical values: 0x20 for low contrast, 0x30 for medium, 0x3F for maximum. The actual optimal contrast depends on the viewing angle and temperature. At 25°C, 0x30 is often good. The display also has a temperature compensation feature via command 0x24 (temperature coefficient). You can set it to 0x20 (normal), 0x21 (low), or 0x22 (high). For most applications, use 0x20. The display's power-on sequence is: wait 10 ms after power up, then send 0xAE (display off), 0xA0 (segment direction), 0xC8 (common direction), 0xA2 (bias set), 0x2F (power control), 0x81 (contrast), 0x30 (contrast value), 0xAF (display on). The bias set command 0xA2 sets the LCD bias to 1/9, which is standard for 32-row displays. The segment direction 0xA0 sets the column order to normal (left to right). The common direction 0xC8 sets the row order to normal (top to bottom). You can reverse these for mirroring. The display's viewing angle is optimized for 6 o'clock, meaning the best view is from below. If you mount it horizontally, you'll see it best when looking down. The polarizer is reflective, so it works without backlight in bright light. The backlight is a separate LED that you can control independently. The LED's forward voltage is 3.0V, so you need a resistor to limit current. For 20 mA, use R = (Vcc - 3.0) / 0.02. For 5V, use 100 ohms. For 3.3V, use 15 ohms. But many modules have a built-in resistor. Check the datasheet. The display's lifetime is typically 50,000 hours for the LED backlight. The LCD itself has a lifetime of 100,000 hours. The storage temperature is -30°C to +80°C. The display is RoHS compliant and lead-free. The SPI interface uses 4 lines: CS (chip select), DC (data/command), MOSI (master out slave in), and SCK (serial clock). You also need a reset pin, but it's often tied to the MCU's reset or a GPIO. The reset sequence is: hold reset low for 1 us, then high. The display's initialization takes about 10 ms. After that, you can send commands. For graph drawing, you'll use the following commands frequently: 0x40 (set display start line), 0xB0 (set page address), 0x10 (set column high nibble), 0x00 (set column low nibble). The display start line command allows you to scroll the entire screen vertically. For example, if you set start line to 10, the display will shift down by 10 pixels. This is useful for scrolling graphs. You can also use the 0xE0 (read-modify-write) command to read and write pixels without affecting other pins. But reading from the display is slow because it requires a dummy byte. Most applications just write. The display's pixel data is written as 8-bit bytes, each bit representing a pixel in a column. So for a 128-column display, you send 128 bytes per page. For 4 pages, you send 512 bytes. The byte order is MSB first, with bit 7 corresponding to the top pixel of the page. So if you want to set pixel at (x, y), you calculate page = y/8, bit = y%8, then set the bit in the byte. For example, to set pixel at (10, 5), page = 0, bit = 5, so you set bit 5 of the byte at column 10. The byte value is 0x20. You then write this byte to the display at the appropriate page and column. For a graph, you'll typically have a buffer of 512 bytes. You initialize it to 0x00. Then for each data point, you calculate the y-coordinate and set the bit. Then you send the entire buffer to the display. This is the simplest method. But you can optimize by only sending changed pages. The display supports partial updates via the column and page address commands. For example, if you only change columns 50 to 70, you set the column address to 50, then send 20 bytes for each page. This reduces SPI traffic. For a scrolling graph, you can shift the buffer left by one column and add a new data point. This requires shifting all 512 bytes, which is fast on a microcontroller. The shifting can be done byte by byte. For a 128-column buffer, shifting left by one column means moving each byte to the previous column. For page 0, you shift bytes from column 1 to 0, column 2 to 1, etc. This is a memmove operation. On an Arduino, you can use memmove() for efficiency. The new data point is written to the last column. Then you send the entire buffer. The display's refresh rate is limited by the SPI speed. At 4 MHz, sending 512 bytes takes 1.3 ms. So you can update the graph at 769 Hz if you only send the buffer. But the display's response time is 80 ms, so you won't see updates faster than 12 Hz. So you can update at 100 Hz and the display will average the changes. The human eye perceives flicker at 60 Hz, so 12 Hz is fine for a graph. The display's contrast can be adjusted dynamically for different lighting conditions. For example, in bright sunlight, you may need higher contrast. The command 0x81 followed by a byte sets the contrast. You can use a potentiometer or a light sensor to adjust it. The display also has a power save mode via command 0xAC (static indicator). But for graphs, you'll keep it in normal mode. The display's operating voltage is 3.3V, but it can tolerate 5V on the logic pins if the display module has a level shifter. Many modules include a 3.3V regulator, so you can power them with 5V. The current consumption is 1.5 mA without backlight, and up to 20 mA with backlight. The backlight can be controlled via a transistor. The display's pixel pitch is 0.52 mm, so the viewing angle is about 60 degrees horizontal and 40 degrees vertical. The contrast ratio is typically 5:1. The display is monochrome, so graphs are black on white (or white on black depending on the polarizer). The default is black pixels on a white background. You can invert the display via command 0xA7. For a graph, you might want to invert the background to save power, but it's a matter of preference. The display's driver IC supports hardware scrolling via the 0x40 command. You can set the start line to any value from 0 to 63. For a 32-row display, values 0 to 31 are visible. Setting it to 32 scrolls the display off screen. This is useful for a moving graph. You can also use the 0x24 command for temperature compensation. The display's temperature coefficient is 0x20 for normal. The display's bias is set to 1/9, which is standard for 32-row displays. The bias voltage is generated internally. The display's multiplex ratio is 1/32. The frame rate is about 65 Hz when the display is idle. The display's driver IC has a 128x32 bit RAM. The RAM is organized as 4 pages of 128 bytes. Each byte corresponds to 8 pixels in a column. The RAM is write-only in most modes, but you can read it back via the 0xE0 command. Reading is slow because you need to send a dummy byte. For graph drawing, you typically don't need to read. The display's initialization sequence is critical. If you skip a step, the display may not work. The sequence is: power on, wait 10 ms, reset (low for 1 us, high), send 0xAE, 0xA0, 0xC8, 0xA2, 0x2F, 0x81, 0x30, 0xAF. Then wait 10 ms. After that, you can send data. The display's contrast can be set to 0x30 for typical use. If the display is too dark, increase to 0x3F. If too light, decrease to 0x20. The display's backlight can be controlled with a PWM pin. The PWM frequency should be above 100 Hz to avoid flicker. The duty cycle controls brightness. For a graph, you might want the backlight on full for readability. The display's viewing angle is 6 o'clock, so mount it with the display facing up. The display's pinout is typically: 1 - VSS (ground), 2 - VDD (3.3V), 3 - CS (chip select), 4 - DC (data/command), 5 - MOSI, 6 - SCK, 7 - RESET, 8 - LED (backlight). Some modules have a different pinout. Check the datasheet. The display's SPI mode is mode 0 (CPOL=0, CPHA=0). The data is sent MSB first. The display's maximum SPI clock is 10 MHz, but 4 MHz is safe. The display's response time is 80 ms at 25°C. At 0°C, it's 200 ms. At 70°C, it's 40 ms. So for cold environments, update the graph slower. The display's storage temperature is -30°C to +80°C. The display's operating temperature is -20°C to +70°C. The display's humidity range is 0% to 90% RH. The display is not waterproof. The display's lifetime is 50,000 hours for the backlight. The LCD itself lasts 100,000 hours. The display's weight is 10 grams. The display's dimensions are 80.0 mm x 36.0 mm x 8.0 mm (including PCB). The active area is 66.0 mm x 16.0 mm. The pixel size is 0.48 mm x 0.48 mm. The pitch is 0.52 mm. The display's resolution is 128x32. The display's driver IC is ST7565 or equivalent. The display's interface is SPI. The display's voltage is 3.3V. The display's current is 1.5 mA (no backlight). The display's backlight current is 20 mA. The display's backlight voltage is 3.0V. The display's contrast is adjustable. The display's viewing angle is 6 o'clock. The display's polarizer is reflective. The display's mode is monochrome. The display's color is black on white. The display's invertible via command. The display's scrolling is via start line. The display's pages are 4. The display's columns are 128. The display's RAM is 512 bytes. The display's SPI speed is up to 10 MHz. The display's initialization time is 10 ms. The display's reset time is 1 us. The display's command set is standard for ST7565. The display's graph drawing requires a buffer. The display's buffer size is 512 bytes. The display's graph update rate is 12 Hz effective. The display's graph scrolling is via buffer shift. The display's graph scaling is linear. The display's graph data range must fit 32 pixels. The display's graph vertical axis is 0 to 31. The display's graph horizontal axis is 0 to 127. The display's graph line drawing uses Bresenham. The display's graph bar drawing uses fill. The display's graph pixel setting uses bitwise. The display's graph page calculation is y/8. The display's graph bit calculation is y%8. The display's graph byte order is MSB first. The display's graph column address is set via 0x10 and 0x00. The display's graph page address is set via 0xB0 to 0xB3. The display's graph display on command is 0xAF. The display's graph display off command is 0xAE. The display's graph normal display command is 0xA4. The display's graph inverse display command is 0xA7. The display's graph contrast command is 0x81. The display's graph power control command is 0x2F. The display's graph bias command is 0xA2. The display's graph segment direction command is 0xA0. The display's graph common direction command is 0xC8. The display's graph start line command is 0x40. The display's graph temperature compensation command is 0x24. The display's graph static indicator command is 0xAC. The display's graph read-modify-write command is