esp-open-rtos/core/include/esp/timer_regs.h

/* esp/timer_regs.h
 *
 * ESP8266 Timer register definitions
 *
 * Not compatible with ESP SDK register access code.
 */

#ifndef _ESP_TIMER_REGS_H
#define _ESP_TIMER_REGS_H

#include "esp/types.h"
#include "common_macros.h"

#define TIMER_BASE 0x60000600
#define TIMER(i) (*(struct TIMER_REGS *)(TIMER_BASE + (i)*0x20))
#define TIMER_FRC1 TIMER(0)
#define TIMER_FRC2 TIMER(1)

/* TIMER registers
 *
 * ESP8266 has two hardware timer counters, FRC1 and FRC2.
 *
 * FRC1 is a 24-bit countdown timer, triggers interrupt when reaches zero.
 * FRC2 is a 32-bit countup timer, can set a variable match value to trigger an interrupt.
 *
 * FreeRTOS tick timer appears to come from XTensa core tick timer0,
 * not either of these.  FRC2 is used in the FreeRTOS SDK however. It
 * is set to free-run, interrupting periodically via updates to the
 * ALARM register. sdk_ets_timer_init configures FRC2 and assigns FRC2
 * interrupt handler at sdk_vApplicationTickHook+0x68
 */

struct TIMER_REGS {            // FRC1  FRC2
    uint32_t volatile LOAD;    // 0x00  0x20
    uint32_t volatile COUNT;   // 0x04  0x24
    uint32_t volatile CTRL;    // 0x08  0x28
    uint32_t volatile STATUS;  // 0x0c  0x2c
    uint32_t volatile ALARM;   //       0x30
};

_Static_assert(sizeof(struct TIMER_REGS) == 0x14, "TIMER_REGS is the wrong size");

#define TIMER_FRC1_MAX_LOAD 0x7fffff

/* Details for LOAD registers */

/* Behavior for FRC1:
 *
 * When TIMER_CTRL_RELOAD is cleared in TIMER(0).CTRL, FRC1 will
 * reload to its max value once underflowed (unless the load
 * value is rewritten in the interrupt handler.)
 *
 * When TIMER_CTRL_RELOAD is set in TIMER(0).CTRL, FRC1 will reload
 * from the load register value once underflowed.
 *
 * Behavior for FRC2:
 *
 * If TIMER_CTRL_RELOAD is cleared in TIMER(1).CTRL, writing to
 * this register will update the FRC2 COUNT value.
 *
 * If TIMER_CTRL_RELOAD is set in TIMER(1).CTRL, the behaviour
 * appears to be the same except that writing 0 to the load register
 * both sets the COUNT register to 0 and disables the timer, even if
 * the TIMER_CTRL_RUN bit is set.
 *
 * Offsets 0x34, 0x38, 0x3c all seem to read back the LOAD_REG value
 * also (but have no known function.)
 */

/* Details for CTRL registers */

/* Observed behaviour is like this:
 *
 *  * When TIMER_CTRL_INT_HOLD is set, the interrupt status bit
 *    TIMER_CTRL_INT_STATUS remains set when the timer interrupt
 *    triggers, unless manually cleared by writing 0 to
 *    TIMER(x).STATUS.  While the interrupt status bit stays set
 *    the timer will continue to run normally, but the interrupt
 *    (INUM_TIMER_FRC1 or INUM_TIMER_FRC2) won't trigger again.
 *
 *  * When TIMER_CTRL_INT_HOLD is cleared (default), there's no need to
 *    manually write to TIMER(x).STATUS. The interrupt status bit
 *    TIMER_CTRL_INT_STATUS automatically clears after the interrupt
 *    triggers, and the interrupt handler will run again
 *    automatically.
 */

/* The values for TIMER_CTRL_CLKDIV control how many CPU clock cycles amount to
 * one timer clock cycle.  For valid values, see the timer_clkdiv_t enum below.
 */

/* TIMER_CTRL_INT_STATUS gets set when interrupt fires, and cleared on a write
 * to TIMER(x).STATUS (or cleared automatically if TIMER_CTRL_INT_HOLD is not
 * set).
 */

#define TIMER_CTRL_INT_HOLD    BIT(0)
#define TIMER_CTRL_CLKDIV_M    0x00000003
#define TIMER_CTRL_CLKDIV_S    2
#define TIMER_CTRL_RELOAD      BIT(6)
#define TIMER_CTRL_RUN         BIT(7)
#define TIMER_CTRL_INT_STATUS  BIT(8)

typedef enum {
    TIMER_CLKDIV_1 = 0,
    TIMER_CLKDIV_16 = 1,
    TIMER_CLKDIV_256 = 2,
} timer_clkdiv_t;

/* Details for STATUS registers */

/* Reading this register always returns the value in
 * TIMER(x).LOAD
 *
 * Writing zero to this register clears the FRC1
 * interrupt status.
 */

/* Details for FRC2.ALARM register */

/* Interrupt match value for FRC2. When COUNT == ALARM,
   the interrupt fires.
*/

#endif /* _ESP_TIMER_REGS_H */