🛠 Comprendre la mémoire : 2 Mo de flash, 1 Mo de RAM TCM/DMA, EEPROM émulée et LittleFS

Objectives

The memory architecture of the Teensy 4.0 is its greatest strength — and its main porting pitfall. The NXP chip has no embedded flash: the program lives in an external Winbond W25Q16JV SPI flash of 2 MB (1984 KB usable — the top 64 KB store the emulated EEPROM and the LED-blink fallback program), executed via a 65536-byte cache at 66 MB/s. On the RAM side: 1024 KB total, split between RAM1 (TCM, single-cycle access via 64-bit ITCM/DTCM bus) and RAM2 (optimized for DMA). You will learn the placement keywords and the emulated EEPROM.

Placement Keywords

• FASTRUN: explicit placement of code in RAM1 (the compiler already places critical code there by default — single-cycle access, the fastest).
• FLASHMEM: force a function into flash (saves RAM, access via cache).
• PROGMEM / F: constant data in flash, outside of RAM.
• DMAMEM: data in RAM2, where DMA peripherals (USB, SDIO, audio) work best.

Complete Code

// Démonstration des zones mémoire + EEPROM émulée.
#include <EEPROM.h>

const char message[] PROGMEM = "constante en flash (F pour les littéraux)";
DMAMEM uint16_t tamponDMA[1024];        // RAM2 : idéal pour le DMA
FASTRUN void cheminCritique {         // code dans RAM1 (TCM)
  for (uint32_t i = 0; i < 1024; i++) tamponDMA[i] = i * 3;
}

void setup {
  Serial.begin(9600);
  Serial.println((const char*)message);

  // EEPROM : 1080 OCTETS émulés dans la flash (pas de vraie EEPROM !)
  uint8_t compteur = EEPROM.read(0);
  compteur++;
  uint32_t t0 = micros;
  EEPROM.update(0, compteur);   // écriture : le code S'ARRÊTE un instant
  uint32_t duree = micros - t0;

  Serial.printf("session %u, écriture EEPROM = %u us\n", compteur, duree);
}

void loop { cheminCritique; delay(1000); }

For files (logs, audio samples), the remaining flash is used via LittleFS: SD.begin(BUILTIN_SDCARD) targets the SDIO connector on the bottom pads, while LittleFS forms the free flash zone — same File API.

Expected Verification

The session counter survives power loss (1080 persistent bytes). Measuring micros around EEPROM.update shows the micro-execution stall documented by PJRC: writing from flash interrupts the code — to be banned from a real-time audio loop. Compare the duration of a cheminCritique loop with and without FASTRUN to feel the difference between TCM and cached flash.

Common Errors

• Writing to the EEPROM on every loop iteration: not only does it stall execution, but it wears out the flash — write only on value change (EEPROM.update does this, EEPROM.put does not).
• Believing you have 2048 KB of program space: it is 1984 KB, the top 64 KB are reserved (EEPROM + restore).
• Putting DMA buffers in RAM1 by reflex: drivers expect RAM2 (DMAMEM) — otherwise hidden copies or weird behaviors occur.
• Porting a Teensy 3.x sketch with floating-point constants: the T4 has a hardware 64-bit double FPU and no longer forces literals to single precision — “this might cause some subtle errors on T4” (output thread); write 1.0f explicitly.
• Storing large files in EEPROM: 1080 bytes, period — for volume, use LittleFS on flash or a 4-bit SD card.
• Forgetting the 16 bytes of RTC RAM (LPGPR0-3 registers) saved by the VBAT battery: this is the ideal place for a small, ultra-persistent critical state.