Arduino / ESP32

PROGMEM

Store data in flash, not RAM

PROGMEM is a compiler attribute that tells the ESP32 to store a variable in flash memory instead of RAM. The ESP32 has only 320KB of RAM - a 1-second audio clip at 8kHz is already 8KB, and a few clips would overflow it.

Flash is 4MB and is not used at runtime, so storing audio data there leaves RAM free for the program.

Key details

RAM: 320KB - limited, needed at runtime
Flash: 4MB - plenty, read-only during execution
Syntax: const uint8_t data[] PROGMEM = {...}
Read back with: pgm_read_byte(&data[i])
Without PROGMEM: large arrays crash at boot