uint64_t time_us_64() {
   // Need to make sure that the upper 32 bits of the timer
   // don't change, so read that first
   uint32_t hi = timer_hw->timerawh;
   uint32_t lo;
   do {
       // Read the lower 32 bits
       lo = timer_hw->timerawl;
       // Now read the upper 32 bits again and
       // check that it hasn't incremented. If it has loop around
       // and read the lower 32 bits again to get an accurate value
       uint32_t next_hi = timer_hw->timerawh;
       if (hi == next_hi) break;
       hi = next_hi;
   } while (true);
   return ((uint64_t) hi << 32u) | lo;
}

typedef volatile uint32_t io_rw_32;
typedef const volatile uint32_t io_ro_32;
typedef volatile uint32_t io_wo_32;
typedef volatile uint16_t io_rw_16;
typedef const volatile uint16_t io_ro_16;
typedef volatile uint16_t io_wo_16;
typedef volatile uint8_t io_rw_8;
typedef const volatile uint8_t io_ro_8;
typedef volatile uint8_t io_wo_8;

typedef struct {
   _REG_(TIMER_TIMEHW_OFFSET) // TIMER_TIMEHW
   // Write to bits 63:32 of time
   io_wo_32 timehw;

   _REG_(TIMER_TIMELW_OFFSET) // TIMER_TIMELW
   // Write to bits 31:0 of time
   io_wo_32 timelw;

   _REG_(TIMER_TIMEHR_OFFSET) // TIMER_TIMEHR
   // Read from bits 63:32 of time
   io_ro_32 timehr;

   _REG_(TIMER_TIMELR_OFFSET) // TIMER_TIMELR
   // Read from bits 31:0 of time
   io_ro_32 timelr;