GPTIMER

1. GPTIMER Timer Overview

GPTIMER directly operates ESP32 hardware timers, executing callbacks in ISR with deterministic delay and no RTOS scheduling jitter. This experiment configures a 1-second period hardware timer and prints the trigger count in the ISR. Compared to esp_timer in the previous section, GPTIMER trades convenience (ISR cannot use Serial, this experiment is for teaching only) for nanosecond-level timing determinism.

Typical Application Scenarios:

2. Core Concepts

2.1 Hardware Timer Architecture

ESP32 has 4 groups of 64-bit hardware timers (TIMG0/1, 2 per group). The clock source is APB_CLK = 80MHz, divided to obtain the counting frequency.

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timerBegin(divider) Parameter Description:
  APB_CLK = 80,000,000 Hz
  Post-divider frequency = 80,000,000 / divider
  If divider = 80 → post-divider = 1MHz → each tick = 1μs

timerAlarm(timer, 1000000, true, 0) Parameter Description:
  1000000 = alarm threshold (tick count)
  At 1MHz, 1,000,000 ticks = 1,000,000 μs = 1 second
  true = auto-reload, counter resets to zero after trigger and restarts counting

2.2 IRAM_ATTR Attribute

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void IRAM_ATTR Timer_ISR(void) { ... }

IRAM_ATTR places function code in SRAM instead of Flash. Flash reads have the risk of cache misses, and a cache miss in ISR can cause unpredictable delays or even watchdog reset. IRAM_ATTR ensures ISR code is always in RAM with deterministic execution time.

This experiment calls Serial.printf in the ISR - this is strictly prohibited in production code. It is only used here for teaching convenience to observe timer behavior.

2.3 esp_timer vs GPTIMER Comparison

3. Hardware Dependencies

This project is a pure software experiment with no external hardware required. Only USB cable needed to connect serial port for output viewing.

4. Project Architecture and Flow

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setup()
  ├─ Serial.begin(115200)              // Initialize serial port
  ├─ timer = timerBegin(1000000)       // Initialize timer, divide to 1MHz
  ├─ timerAttachInterrupt(timer, ISR)  // Bind ISR function
  ├─ timerAlarm(timer, 1000000, true, 0) // Alarm threshold = 1s, auto-reload
  └─ timerStart(timer)                 // Start counting

loop()                                 // Empty loop

[Hardware Interrupt]
  └─ Timer_ISR()                       // Triggers once per second
       ├─ trigger_count++              // Increment counter
       └─ Serial.printf(...)           // Print (teaching only! Forbidden in production)

5. Source Code Details

Full source code: Source Code → Arduino → 1.Basic Peripherals → 5.GP_TIMER → 5.GP_TIMER.ino

5.1 Global Variables and Timer Handle

hw_timer_t* is a pointer to the hardware timer control block, volatile variables are used for sharing between ISR and main loop.

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hw_timer_t *timer = NULL;
volatile uint32_t trigger_count = 0;

timer does not need volatile - it is initialized in setup() and never changes afterward, ISR does not write to it.

5.2 Timer ISR

IRAM_ATTR places the function in RAM ensuring deterministic ISR execution time.

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void IRAM_ATTR Timer_ISR(void) {
  trigger_count++;
  Serial.printf("Trigger Count: %u | Current Time: %lu ms\n", trigger_count, millis());
}

Warning: Serial.printf is unsafe in ISR, this experiment uses it only for teaching convenience. In real projects, ISR should only set flags, printing should be done in loop().

5.3 setup() — Four Steps of Timer Initialization

Hardware timer initialization has more steps than esp_timer, each step corresponds to an independent API.

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void setup() {
  Serial.begin(115200);
  Serial.println("GPTIMER Pure Timer Test Started");

  timer = timerBegin(1000000);                     // ① Initialize, divide to 1MHz
  timerAttachInterrupt(timer, Timer_ISR);          // ② Bind ISR
  timerAlarm(timer, 1000000, true, 0);             // ③ Alarm threshold 1s, auto-reload, infinite
  timerStart(timer);                               // ④ Start
}

void loop() {}  // Empty loop - all work done by timer ISR

The four steps must not be reordered: timer must be created first, then bind ISR, set alarm threshold, finally start.

6. Operation Flow

6.1 Build and Flash

Arduino flash process: See Arduino → 1. Before Development → 2. Build and Flash.

6.2 Normal Behavior

Open Serial Monitor (115200 bps), print one line of trigger count every second.

6.3 Verification Method

7. Common Issues