mimxrt: Fix mp_hal_quiet_timing_enter()/exit() so timer still runs.

The initial code disabled IRQs, which caused the us-ticks timer to stop.
The change here changes the priotity level, such that the timer still runs.

Author: robert-hh

ports/mimxrt/mpconfigport.h

@@ -189,6 +189,23 @@ __attribute__((always_inline)) static inline uint32_t disable_irq(void) {
     return state;
 }
 
+static inline uint32_t raise_irq_pri(uint32_t pri) {
+    uint32_t basepri = __get_BASEPRI();
+    // If non-zero, the processor does not process any exception with a
+    // priority value greater than or equal to BASEPRI.
+    // When writing to BASEPRI_MAX the write goes to BASEPRI only if either:
+    //   - Rn is non-zero and the current BASEPRI value is 0
+    //   - Rn is non-zero and less than the current BASEPRI value
+    pri <<= (8 - __NVIC_PRIO_BITS);
+    __ASM volatile ("msr basepri_max, %0" : : "r" (pri) : "memory");
+    return basepri;
+}
+
+// "basepri" should be the value returned from raise_irq_pri
+static inline void restore_irq_pri(uint32_t basepri) {
+    __set_BASEPRI(basepri);
+}
+
 #define MICROPY_BEGIN_ATOMIC_SECTION()     disable_irq()
 #define MICROPY_END_ATOMIC_SECTION(state)  enable_irq(state)
 

ports/mimxrt/mphalport.h

@@ -53,8 +53,8 @@ extern ringbuf_t stdin_ringbuf;
 #define mp_hal_pin_od_low(p)    mp_hal_pin_low(p)
 #define mp_hal_pin_od_high(p)   mp_hal_pin_high(p)
 
-#define mp_hal_quiet_timing_enter() MICROPY_BEGIN_ATOMIC_SECTION()
-#define mp_hal_quiet_timing_exit(irq_state) MICROPY_END_ATOMIC_SECTION(irq_state)
+#define mp_hal_quiet_timing_enter() raise_irq_pri(1)
+#define mp_hal_quiet_timing_exit(irq_state) restore_irq_pri(irq_state)
 
 void mp_hal_set_interrupt_char(int c);