Thisoe Timer

As a C embedded beginner, this project might be a good starter.

You can try build one following my log.

I also noted some background story of mine!

Functionalities

Main features

• Countdown, alarm when time is up;
• Start counting up if is set to 00:00;
• Pause and continue freely;
• Remembers last time set.

Particular details

• Says Hi on boot;
• Debounce buttons with software only;
• Beep on any button press;
• Add 1 minute (1 second) on press;
• Reset time to 00:00 and clear up flags and states when combo of start/stop button and one of the time-setting buttons pressed at the same time;
• Loop back to 00:00 when countup overflow;
• Blink the colon symbol on the display when counting;
• Longpress to add time quickly.

Building log

Hardware Preparation

• An STM32F103C8T6
• An ST-LINK/V2 ICD (in-circuit debugger)
• A TM1637 4-digits 7-seg number display with a colon
• A TMB12A05 buzzer
• Some un-debounced buttons
• Some breadboards and wires

Component Connections & GPIO Settings

Connections to MCU

Component	Signal	STM32 Pin	Notes
TM1637	CLK	PB6	Output (bit-bang)
TM1637	DIO	PB7	Output (open-drain)
BTN_MULTI	OUT	PA1	GPIO_EXTI Rising-edge (GPIO_Input when test), pull-down, active LOW
BTN_MIN	OUT	PA2	GPIO_EXTI Rising-edge (GPIO_Input when test), pull-down, active LOW
BTN_SEC	OUT	PA3	GPIO_EXTI Rising-edge (GPIO_Input when test), pull-down, active LOW
TMB12A05	S	PA8	Output (active HIGH)

Debugging pins within MCU:

.ioc GUI > Pinout & Config > System Core > SYS > Debug: Select Serial Wire

Component	STM32 Pin	Notes
SYS auto	PA13	for debugging
SYS auto	PA14	for debugging
BTN_S2	PA0	Input, pull-up, active LOW
LED_D2	PC13	Output, no pull

Power

Device	3V3	GND
TM1637	VCC	GND
BTNs	VCC	GND
TMB12A05	(mid pin)	-

Hardware tests

Buzzer test

1. Declaration

   In /* USER CODE BEGIN PFP */:

   void alarm_sound(void);

2. Implementation

   In /* USER CODE BEGIN 4 */:

   void alarm_sound(void)
   {
     for(int i = 0; i < 4; i++)
     {
       HAL_GPIO_WritePin(GPIOA, BUZ_Pin, GPIO_PIN_SET);
       HAL_GPIO_WritePin(LED_D2_GPIO_Port, LED_D2_Pin, 0);
       HAL_Delay(70);
       HAL_GPIO_WritePin(GPIOA, BUZ_Pin, GPIO_PIN_RESET);
       HAL_GPIO_WritePin(LED_D2_GPIO_Port, LED_D2_Pin, 1);
       HAL_Delay(70);
     }
     HAL_Delay(700);
   }

3. Call in main while loop

   alarm_sound();

Buttons debugging

I got only 2 buttons but I have a 3-pin switch (that have 2 states: A-B & B-C).

I had to make BTN_MIN and BTN_SEC Pull-down, so that when floating they won't output any signals.

Try out a TM1637 driver lib

nimaltd/tm1637

ChatGPT provided me the seg_map as follow:

static const uint8_t seg_map[11] = {
  0x3F,//0
  0x06,//1
  0x5B,//2
  0x4F,//3
  0x66,//4
  0x6D,//5
  0x7D,//6
  0x07,//7
  0x7F,//8
  0x6F,//9
};

so that I can control the board seg-precisely, with tm1637_raw()from nimaltd's driver.

I added a flag.c to share the current min and sec state across files. Also prepared a place for flags in future coding of the core logic.

Prepare for __WFI()

I want a pure interrupt-driven firmware.

To do this, add __WFI(); inside the main while loop.

Now the HAL_Delay() will ruin the timer's normal functionality, so get rid of them all with their related logics from now on.

Clock configuration

Since there is a 8 MHz crystal on board, let's use it.

• Setup HSE

In .ioc > Pinout Config > System Core > RCC, find High Speed Clock and select Crystal/Ceramic Resonator.

Then in Clock Config:

Setting	Value
HSE Input	8 MHz
PLL Source	HSE
PLLMul	x9
System Clock Mux	PLLCLK
SYSCLK	72 MHz
HCLK	72 MHz
APB1	/ 2 (resulting 36 MHz)

• Setup TIM2

In Pinout Config > Timers > TIM2 > Mode find Clock Source and select Internal Clock.

Configuration contents will appear. Under NVIC Settings, enable TIM2 global interrupt.

Then in Parameter Settings:

Param	Value
Prescaler (PSC)	7200-1
Counter Period (ARR)	5000-1
Counter Mode	Up

Explain of the value:

The goal is to interrupt every 0.5 seconds.
The formula:

f_update = TIM_CLK / ((PSC + 1) * (ARR + 1))

For TIM2, TIM_CLK is 72 MHz;

• Prescaler = 7200 - 1
• Period = 5000 - 1 Result:
• 72 MHz / 7200 = 10 kHz
• 10 kHz / 5000 = 2 Hz

Save .ioc and generate code.

• Test TIM ISR (Interrupt Service Routine)

Now test the timer interrupt.
In /* USER CODE BEGIN 4 */:

void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim){
  if(htim->Instance==TIM2){
    // Callback when interrupt triggered every 0.5 sec
  }
}

ChatGPT warns that we should keep ISR logic short.

So we better only set a flag in the ISR callback, and move heavy work into the main while loop.

while(1){
  if(flag){
    flag = 0;
    // handling logic...
  }

  __WFI();
}

Setup EXTI: button interruption

EXTI (External Interrupt) is an ISR from a GPIO input.

1. Re-configure Pins

   In .ioc > Pinout & Config > Pinout view, set the 3 button's pins as GPIO_EXTIn.

   Then go to Pinout & Config > System Core > GPIO and set:

   Setting	Value
   GPIO mode	External Interrupt Mode with Rising edge trigger
   GPIO Pull	Pull-up
   User Label	BTN_MULTI_Pin/BTN_MIN_Pin/BTN_SEC_Pin

2. Enable NVIC (Nested Vectored Interrupt Controller)

   In Pinout & Config > System Core > NVIC, check the interrupt of EXTI 1, 2, and 3.

   Save .ioc.

3. ISR callback function

   In /* USER CODE BEGIN 4 */:

   void HAL_GPIO_EXTI_Callback(uint16_t Pin){
     if(pin==BTN_MULTI_Pin){
       B_MULTI.exti = 1u;
     }
     // ...
   }

   Then in main while loop, make a test that toggles led on button press:

   // test
   if(B_MULTI.exti){
     handle();
   }
   
   void handle(){
     // debounce
     if(HAL_GetTick() - BS->useTick < DEBOUNCE_MS) return;
   
     BS->exti = 0;
   
     GF.led ^= 1;
     led(GF.led);
   
   }

Button debouncing

Buttons, physically, are often 2 pieces of thin metal touching. The pressing of the button usually creates one or several bounces of those metals under few milliseconds, which may trigger the EXTI Rising-edge event more than twice with only one button press.

We can fix this just with a few lines of code.

• Begin a 39 ms timer (DEBOUNCE_MS) at the first Rising-edge.

  Inside HAL_GPIO_EXTI_Callback:

  // exti (External Interrupt) Service Routine
  static void eisr(ButtonState *BS){
    if(!BS->exti){
      BS->useTick = HAL_GetTick();
      BS->exti = 1u;
    }
  }

  The BS->exti is a "pending" flag, leaving heavy mission to the main while loop.

• In the main loop's handle_btn():

  • Take a break during debouncing.

      if((HAL_GetTick() - BS->useTick) < DEBOUNCE_MS) return;
      BS->exti = 0;

  • When button still pressed after 39 ms, proceed working the button ought to.

      bool level = (read_btn(BS) == GPIO_PIN_SET);
      if(level){
        GF.led ^= 1;
        led(GF.led);
        countup();
        settime(1u);
      }

Later version took core logic codes out from the if block, also the pending flag resets afterwards.

if(!level) return;
BS->exti = 0;
// main logic...

---

Others

Pure software, the timer core logics, were implemented afterwards.

Buzzer codes

Made beep sound on any button press. See onpress_beep_update().

Also made the classic time up alarm tone using HAL software timestamp. See the new version of alarm_sound() in the current codebase instead of the one showing in README.

Final tests and minor fixes occurred before repo archiving.

Updates after release

Longpress

I totally forgot to code longpress before v1.0 release ㅠ ㅠ

Existing ButtonState->useTick was used to trigger. See longpress().

Also optimized BS->exti resetting in debouncing logic.

Afterword

Before this project

Oh hi! This is Thisoe based in Korea.

As of 2026, I experienced a year (in 2021) coding ESP8266 using MicroPython when I was in college, and another 3+ years of JavaScript (1+ year of TypeScript) web developing as hobby.

I gained enough coding experience. I also curious and eager to learn more about lower-level languages and hardware basics. So I started this new journey of C embedded with teacher OJ Tube aka 오지완 선생님.

Mr. OJ's teaching style was basic enough that middle school students could understand lol and I'm a bit tired following with his tutorial, so I turned to start a project right away.

Holding the freshly purchased STM32 board, I saw my cute little timer sticking on the fridge, and thereupon pulled out my old TM1637 (the time displayer) and TMB12A05 (the buzzer) out of the ESP8266 box.

During coding

I'm attending full-time job, so I coded 1 ~ 3 hrs per day, from the 5th to 19th of April 2026.

During those days, I also learned minimal hardware knowledges and basics on how to wire things up from Ben Eater's early vods.

Random playing Mix Playlist of instrumentals of Suisei's songs. They buff my productivity incredibly. Btw the song «BEEP BEEP» released when I was coding buzzer functions. Looped the album while coding.

Didn't notice my markdown syntax has got THIS natural and as good as fellow AIs lmao ㄷㄷㄷ エグい

Contact

Since I probably won't come back and touch this repo ever again, I'm just gonna archive it... or maybe after adding Korean, Mandarin, and Japanese README later.

Contact me at thisoecode@gmail.com, or DM me on 𝕏 for a chit chat :D