volatile and const in Embedded C

volatile tells the compiler a value can change outside the program's control; const promises it won't be modified.

Intermediate

volatile is essential for:

hardware registers (*(volatile uint32_t *)0x40020014),
variables shared with an interrupt service routine,
variables changed by another thread or DMA.

volatile bool data_ready = false;   // set inside an ISR

ISR(USART_RX_vect) { data_ready = true; }

while (!data_ready) { }             // without volatile this could loop forever

const marks read-only data; on microcontrollers const tables are often placed in flash instead of RAM. A register can even be const volatile (read-only, but changes on its own).

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