🛠 Timers matériels, interruptions imbriquées et FASTRUN : la vitesse Cortex-M4

Objectives

Move away from delay and polling: the Teensy 3.2 hides a true real-time powerhouse — a Nested Vectored Interrupt Controller (NVIC) with priorities, 16 general-purpose DMA channels, a suite of timers (PDB, CMT, LPTMR, 4 PIT counters, PWM FTM timers), the FASTRUN keyword for executing functions from RAM, and a dual 32-bit bus (simultaneous instruction fetch + data). This tutorial blinks LED 13 using a hardware IntervalTimer and measures the gains of a tight toggle loop.

Hardware

• 1 Teensy 3.2 (the orange LED on pin 13 is sufficient)
• Optional: oscilloscope or logic analyzer on a free pin to measure frequencies

Complete Code

digitalToggle and the IntervalTimer class are documented on the PJRC shop page (sections Digital Output Pins / Timing):

#include <IntervalTimer.h>

IntervalTimer t1;
volatile bool flashing = false;

void flash {           // execute depuis un timer materiel, pas la boucle
  digitalToggle(13);     // broche 13 = LED orange integrée
}

void setup {
  pinMode(13, OUTPUT);
  t1.priority(0);        // attention : sur Teensy, 0 = priorite LA PLUS haute
  t1.begin(flash, 500000); // appelle flash toutes les 500 ms (en us)
}

void loop {
  // Pour la vitesse brute, digitalToggle en boucle serrée bat les paires
  // digitalWrite ; pour une forme d'onde précise, pilotez une broche PWM.
}

Speeding Up the Code

• digitalToggle in a tight loop: the fastest digital output in the Arduino API; for absolute maximum speed, direct access to K20 registers is possible — the pin → port bit mapping must then be taken from the MK20DX256 manual (K20P64M72SF1RM.pdf) and the card7a pinout card.
• FASTRUN: FASTRUN void myFunction { ... } places the function in the 64 KB of RAM (faster than flash, 256 KB at 192 MB/s with a 256-byte cache).
• Overclocking: the core is rated at 72 MHz but is officially overclockable to 96 MHz (techspecs table) — the clock comes from a 16 MHz crystal multiplied by a PLL.
• Interrupt priorities: the NVIC supports nesting (a higher-priority interrupt interrupts a lower-priority one) — t1.priority(0) is the highest here, following Teensy convention.
• DMA (16 channels): move blocks (SPI, ADC, UART, memory) without CPU — OctoWS2811 and the Audio library are prime examples.

Expected Verification

LED 13 blinks at exactly 1 Hz, even if loop is busy with a calculation loop — proof that the hardware timer works independently. Comment out flash in the code: the LED stays fixed, and your main code continues. On an oscilloscope connected to a freely toggling pin, measure the maximum frequency, then compare it with digitalToggle driven by an IntervalTimer at 1 µs.

Common Errors

• Getting the priority direction wrong: on Teensy, 0 = highest (contrary to the intuition "0 = none").
• Doing long operations inside an interrupt or timer routine: delay, heavy Serial.print → cascading blocks; keep ISRs short and use a volatile flag.
• Forgetting volatile on variables shared between ISR and loop: the compiler optimizes them away, and your flag never changes "magically".
• Putting ALL code in FASTRUN: RAM is 64 KB, flash is 256 KB — only place critical functions in RAM.
• Confusing timer groups: the 4 PITs, LPTMR, PDB, CMT, and FTMs do not share the same pins or uses (the LPTMR is optimized for ultra-low power).
• Expecting identical behavior after overclocking to 96 MHz: check libraries that depend on timing (bit-banging, calibrated delays).