STM32 and U8G2 to draw on LCD12864 Using hardware SPI

STM32 bluepill and U8G2 to draw on LCD12864 Using hardware SPI


 

Since there are not a lot of info about this on Internet, so I am making this guide.

We can see some info/guide or example about using Arduino board, or Arduino IDE, or Software SPI to draw using U8g2.

But in this guide, we will use hardware SPI, Stm32 chip, and use Stm32CubeIDE to draw.

Keyword here:

Bluepill, blackpill, u8g2, LCD12864(5V, works with 3.3V stm32 MCU), hardware SPI communication, Stm32CubeIDE. 

Part List in the test:

LCD12864 screen, Bluepill dev board(stm32F103C8T6) or Blackpill dev board( stm32F401CCU6), 5V power supply, stlinkV2 device to flash or debug.

 

Low cost LCD12864 (ST7920), it's large, and good resolution(128X64), cheap. It has a lot of pins for parallel connection, DB0 DB1 DB2...DB7, but who has so many pins to connect?? In fact, this LCD can support SPI connection, it only need 2pins to MCU: MOSI and SCK, CS and RESET is optional, no need to connect to MCU.

 

In SPI mode, pin usage:

Pin 5 RW : MOSI , SPI MOSI. Must connect to MCU.

Pin 6 E : CLK , SPI SCK. Must connect to MCU.

Only above two pins are needed to connect to MCU for SPI connection, so using this LCD with MCU is very easy. We can skip all the DB0 to DB7 pins, MCU can save a ton of GPIO for other higher purposes.

Pin 1 Gnd

Pin2 VCC 5V, but we could or probably should lower the voltage to 4.3V for multiple purposes, more on that later.

Pin 4 RS : CS, SPI CS (Chip select), can always pull high to enable, or connect to MCU, optional. For this test, to make things easier, it can be pulled up. according to the datasheet of the ST7920 page 26

When connecting several ST7920, chip select（CS）..... For a minimal system with only one ST7920 and one MPU, only SCLK and SID pins are necessary. CS pin should pull to high.

Pin 15 PSB : Pull to Gnd to enable SPI mode. ( or, pull high to enable parallel mode, if your MCU has a ton of gpio output to waste.)

Pin 17 Reset : when LCD is first powered on, reset pin must be first pulled low for a while then pull high to do a proper reset, otherwise may have garbage on screen. Can connect to MCU, use software code to pull down then pull high when first power on. or, use hardware solution: resistor and capacitor. To reduce the component we need for this test, we use MCU solution to save the resistor/capacitor and wires.

Pin 19 BLA : back light power,  5V power (or 4.X V)

Pin 20 BLK : back light power, Gnd.



Blue pill STM32F103C8T6 dev board. Once it's very cheap but now the price is getting higher and higher, and full of clone, so a better choice today is to buy Black pill STM32F401CCU6, about the same price and much faster and more ram/flash, but we still use bluepill in this test, yet it should work almost the same for black pill devboard.



Attention before wiring and power on: 5V could damage STM32.

stm32 is a 3.3V device with some pins are 5V tolerant. When it works with LCD12864(5V), it works well, usually, However in some cases stm32 could be damaged by the 5V connection. According to stm's document AN4899 page 19, stm32 VDD 3.3V should be always keep on while the LCD is powered on, so, when power off the system, must power off the LCD12864 first (so to cut off the 5V signal). When we power on the system, must power on the stm32 first, LCD later. 
stm32 AN4899 .

Hardware Connection

with above warning in mind, let's connect the hardware components and power.
5V power supply ( could be from STlink or external power source or usb etc) connect to bluepill 5V.
We will use SPI2 in this test, so we need to connect LCD's Pin5 RW to bluepill PB15 (MOSI2),  LCD's Pin6 E to bluepill PB13(SCK2)
LCD's Pin17 Reset connect to bluepill PA8 (any general gpio works)
Use a diode (ie 1n4007) to lower the 5V to 4.3V, then connect LCD's Pin2 (VCC), Pin4(RS), Pin19(light-A).
Connect LCD's Pin1 (GND) and Pin20(light-K), Pin15(PSB) to GND.
Connect bluepill's GND to LCD's GND so they have a common GND.
Connect bluepill's PA2(TX2) to Stlink's RX. Optional, so we can see debug info via printf() function. I have modified my stlink to have TX and RX. Use other means or ignore if you couldn't use this option.
Connection is done.

Why use a diode for the LCD, reasons below:

1. lower the voltage to LCD, so it's 4.3V, so 0.7 X 4.3 is 3.01V, it's required for the LCD controller (ST7920) to communicate with STM32, which can only provide 3.3V at most.
if we use 5V for LCD, 5VX0.7= 3.5V, which is above the voltage the stm32 can provide. the st7920 chip may have problem to accept the SPI signal.
2. In case we do something wrong, a lower voltage of 4.3V may have a slightly lower chance of hurting the STM32.
3. the LCD doesn't need to be too bright.

LCD contrast adjustment

Even if we do everything right, there could be still nothing on the LCD, that is because the contrast resistor is at the wrong value. It's a tiny potentiometer, or a trim pot on the back of the LCD PCB, the silk print is VR1, we can easy adjust the value, when the LCD is power on, we can measure the voltage value of Pin3(VO), it should be about 4V, then it should show stuffs on screen.

Take a look at the final hardware setup at the top picture.


Software setup:

Stm32CubeIDE

We are not using Arduino IDE in this test.
Create a new stm32F103C8T6 project, choose clock config first.
We want bluepill work on the maximum clock speed 72Mhz, so must set RCC setting like this first: choose HSE : Crystal/Ceramic Resonator


 

 

then we can choose HSE, PLLMX9, PLLCLK, 72Mhz. Blue pill now can run at maximum speed.



 Then choose below settings for SPI2 and GPIO: SPI2_MOSI, SPI2_SCK, LCD_RESET. ( use USART_TX2  if you want to use usart debug output, or ignore). May add a label for the reset pin - LCD_RESET for easy use.

Note, Do NOTdefine PB13 and PB15 as usual GPIO, that is for software mode (using software to toggle GPIO to send data). We use hardware SPI mode here, so just leave them as is.



 Important, change below settings for SPI2:


Mode: Transmit only master

SPI clock Prescaler 64, speed is 562KBit/s. This speed works well, but it's a bit slow. 
Important: 2 edge, this LCD only works on 2 edge setting.

About SPI Speed: 

If want to make the LCD SPI interface works faster, there is another clock setting: change Stm32 prescaler PLLM X8 (was X9), so the system speed is 8X8=64Mhz (instead of 8X9=72Mhz) ,  then change SPI prescaler to be 32. so SPI works at 1Mbit/s. it seems working well, but bluepill 's speed is a bit slower. Feel free to adjust all settings after the whole system is working well. 
Save to let Stm32CubeIDE to automatically generate all the files. If it doesn't, check the setting in
Windows>Preferences>STM32Cube>Device Configuration Tool.
 

Software and Coding:

Step1 Reset LCD (only when stm32 is just power up)

        bascially it just pull down then pull up the LCD reset pin.

u8g2      u8g2 in github    u8g2 is a GUI API for MCU to use.

We haven't talked about u8g2 yet.
u8g2 works very well with Arduino and in Arduino IDE, but with stm32 in stm32 IDE, we have some additional work to do.
1 define a callback funtion u8x8_stm32_gpio_and_delay_cb() , also define a delay function u8g_hw_port_delay_ns

uint8_t u8x8_stm32_gpio_and_delay_cb(U8X8_UNUSED u8x8_t *u8x8,
U8X8_UNUSED uint8_t msg, U8X8_UNUSED uint8_t arg_int,
U8X8_UNUSED void *arg_ptr) {
    switch (msg) {
    case U8X8_MSG_GPIO_AND_DELAY_INIT:
        break;
    case U8X8_MSG_DELAY_NANO:
        u8g_hw_port_delay_ns(arg_int);
      
 

2 Very important: define the 2nd callback function, this is for spi data transmit: 

we call the STM HAL SPI transmit function HAL_SPI_Transmit here.

 uint8_t u8x8_byte_3wire_hw_spi(u8x8_t *u8x8, uint8_t msg, uint8_t arg_int, void *arg_ptr) {
    switch (msg) {
    case U8X8_MSG_BYTE_SEND:
         HAL_SPI_Transmit(&LCD_SPI, (uint8_t*) arg_ptr, arg_int, 10000);
        break;
 
With this HAL_SPI_Transmit function for the message MSG_BYTE_SEND, we can use hardware SPI method to send data to the LCD screen.
With above two call back functions defined for u8g2 to use, we now can let stm32 to use u8g2 in stm32cubeIDE.

 3 Define a user class using above two call back functions

class U8G2_ST7920_128X64_F_STM32_HW_SPI: public U8G2 {
public:
    U8G2_ST7920_128X64_F_STM32_HW_SPI(const u8g2_cb_t *rotation = U8G2_R0) : U8G2() {
        u8g2_Setup_st7920_s_128x64_f(&u8g2, rotation, u8x8_byte_3wire_hw_spi, u8x8_stm32_gpio_and_delay_cb);
    }
};

Explanation about some key points:

u8g2_Setup_st7920_s_128x64_f : the LCD12864 is using st7920 ic, since stm32 has a ton of ram, bluepill has 20KB sram, so we can easily give 1K to u8g2 ( _f function takes 1kb, there are also _1 and _2 simular functions use less RAM, probably arduino should use it).   128 Dot X 64 rows, one byte takes care of 8dots, so it uses 128/8X64=1024Bytes =1K.

u8x8_byte_3wire_hw_spi : the call back function we defined above, it's responsible for transmitting data to SPI.

u8x8_stm32_gpio_and_delay_cb: the other call back function we defined above, mostly for delay for our platform.

Then define our special u8g2 for stm32 hw spi instance

U8G2_ST7920_128X64_F_STM32_HW_SPI  u8g2;

 Then we can use our u8g2 to init and draw stuffs, and finally send to LCD.

Init steps:
    u8g2.initDisplay();
    u8g2.setPowerSave(0);
Drawing steps
    u8g2.clearBuffer();
    u8g2.setFont(u8g2_font_ncenB08_tr);
    u8g2.drawStr(0, 12, "LCD12864 ST7920 u8g2");
Finally send to LCD
    u8g2.sendBuffer();

We need to put the drawing steps and sendBuffer() steps inside the main while(1) loop.



Go to here to see the full source code in github.

bluepill_LCD12864_u8g2 hardware SPI

files worth noting: 

stm32_u8g2_lcd12864.cpp , 

stm32_u8g2_lcd12864.h

my_main.cpp

I use my_main.cpp to use C++, and my_main.cpp won't be overwritten by stm32 IDE automatically everytime I change setting.


More setting about stm32cubeIDe and u8g2.

we also need to copy the whole u8g2 folder inside the project's folder, since the IDE need u8g2's source code to compile, and it cannot choose a source folder outside of the project.

also the include path need to point to the u8g2.csrc is needed for header files, and cppsrc is needed for cpp project,

see below setting screenshot.




Please check the below video see how to actually works.


https://youtu.be/fgs0LIRNCDI

 

About speed and screen FPS

I have measure each sendBuffer() call takes 43ms, so one second can have at most 23frames, with some simple drawing and counter, the refresh rate dropped to 16frame per second as shwon in the video.

Is there a way to improve the speed? 

the bottleneck is at the SPI speed, as mentioned above, if we lower the system speed then we can use a higher SPI speed (about 1Mbit), but the LCD doesn't work well at higher speed (1.1+ Mbit/s   etc).

I have tested with SPI prescaler X32 (instead of 64) and with slower system speed 64Mhz, the FPS can be above 20 ( instead of 16).

We will explore more with Stm32's DMA feature to help with the speed.

 


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