diff --git a/unix/mphalport.h b/unix/mphalport.h
index e440eafd4e..9e232c5609 100644
--- a/unix/mphalport.h
+++ b/unix/mphalport.h
@@ -34,6 +34,8 @@ void mp_hal_set_interrupt_char(char c);
 void mp_hal_stdio_mode_raw(void);
 void mp_hal_stdio_mode_orig(void);
 
+// TODO: POSIX et al. define usleep() as guaranteedly capable only of 1s sleep:
+// "The useconds argument shall be less than one million."
 static inline void mp_hal_delay_ms(mp_uint_t ms) { usleep((ms) * 1000); }
 static inline void mp_hal_delay_us(mp_uint_t us) { usleep(us); }