diff --git a/libraries/AP_HAL_ChibiOS/hwdef/common/hrt.c b/libraries/AP_HAL_ChibiOS/hwdef/common/hrt.c
index d724c77e53..21f9cc002f 100644
--- a/libraries/AP_HAL_ChibiOS/hwdef/common/hrt.c
+++ b/libraries/AP_HAL_ChibiOS/hwdef/common/hrt.c
@@ -13,60 +13,84 @@
  * You should have received a copy of the GNU General Public License along
  * with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
-// High Resolution Timer
+/*
+  High Resolution Timer code. This provides support for 32 and 64 bit
+  returns for micros and millis functions
+*/
 
 #include "ch.h"
 #include "hal.h"
 #include "hrt.h"
 #include <stdint.h>
 
-static uint64_t timer_base_us64;
-#if CH_CFG_ST_RESOLUTION == 16
-static uint32_t timer_base_us32;
+#pragma GCC optimize("O2")
+
+/*
+  we have 4 possible configurations of boards, made up of boards that
+  have the following properties:
+
+    CH_CFG_ST_RESOLUTION = 16 or 32
+    CH_CFG_ST_FREQUENCY  = 1000 or 1000000
+
+  To keep as much code in common as possible we create a function
+  system_time_u32_us() for all boards which gives system time since
+  boot in microseconds, and which wraps at 0xFFFFFFFF.
+
+  On top of this base function we build get_systime_us32() which has
+  the same property, but which also maintains timer_base_us64 to allow
+  for micros64()
+*/
+
+#if CH_CFG_ST_FREQUENCY != 1000000U && CH_CFG_ST_FREQUENCY != 1000U
+#error "unsupported tick frequency"
 #endif
-static uint32_t timer_base_ms;
-static volatile systime_t last_systime;
 
 #if CH_CFG_ST_RESOLUTION == 16
-static uint32_t get_systime_us32(void)
+static uint32_t system_time_u32_us(void)
 {
     systime_t now = chVTGetSystemTimeX();
-#if CH_CFG_ST_FREQUENCY != 1000000
-    now *= (1000000UL / CH_CFG_ST_FREQUENCY);
+#if CH_CFG_ST_FREQUENCY == 1000U
+    now *= 1000U;
 #endif
-    if (now < last_systime) {
-        uint32_t last_u32 = timer_base_us32;
-        timer_base_us32 += (uint32_t)TIME_MAX_SYSTIME;
-        if (timer_base_us32 < last_u32) {
-            timer_base_us64 += ((uint32_t)-1);
-            timer_base_ms += ((uint32_t)-1)/1000;
-        }
-    }
+    static systime_t last_systime;
+    static uint32_t timer_base_us32;
+    uint16_t dt = now - last_systime;
     last_systime = now;
-    return timer_base_us32 + (uint32_t)now;
+    timer_base_us32 += dt;
+    return timer_base_us32;
 }
-
 #elif CH_CFG_ST_RESOLUTION == 32
-static uint32_t get_systime_us32(void)
+static uint32_t system_time_u32_us(void)
 {
     systime_t now = chVTGetSystemTimeX();
-#if CH_CFG_ST_FREQUENCY != 1000000
-    now *= (1000000UL / CH_CFG_ST_FREQUENCY);
+#if CH_CFG_ST_FREQUENCY == 1000U
+    now *= 1000U;
 #endif
-    if (now < last_systime) {
-        timer_base_us64 += TIME_MAX_SYSTIME;
-        timer_base_ms += TIME_MAX_SYSTIME/1000;
-    }
-    last_systime = now;
     return now;
 }
 #else
 #error "unsupported timer resolution"
 #endif
 
+// offset for micros64()
+static uint64_t timer_base_us64;
+
+static uint32_t get_systime_us32(void)
+{
+    static uint32_t last_us32;
+    uint32_t now = system_time_u32_us();
+    if (now < last_us32) {
+        const uint64_t dt_us = 0x100000000ULL;
+        timer_base_us64 += dt_us;
+    }
+    last_us32 = now;
+    return now;
+}
+
 /*
-  we use chSysGetStatusAndLockX() to prevent an interrupt while
-  allowing this call from any context
+  for the exposed functions we use chSysGetStatusAndLockX() to prevent
+  an interrupt changing the globals while allowing this call from any
+  context
 */
 
 uint64_t hrt_micros64()
@@ -88,9 +112,5 @@ uint32_t hrt_micros32()
 
 uint32_t hrt_millis32()
 {
-    syssts_t sts = chSysGetStatusAndLockX();
-    uint32_t now = get_systime_us32();
-    uint32_t ret = (now / 1000U) + timer_base_ms;
-    chSysRestoreStatusX(sts);
-    return ret;
+    return (uint32_t)(hrt_micros64() / 1000U);
 }