diff --git a/microbit/adainclude/FreeRTOSConfig.h b/microbit/adainclude/FreeRTOSConfig.h
index 526fe49..76d6966 100644
--- a/microbit/adainclude/FreeRTOSConfig.h
+++ b/microbit/adainclude/FreeRTOSConfig.h
@@ -31,15 +31,15 @@
 #define configUSE_IDLE_HOOK			0
 #define configUSE_TICK_HOOK			0
 #define configCPU_CLOCK_HZ			( 16000000 )
-#define configTICK_RATE_HZ			( ( portTickType ) 1024 )
+#define configTICK_RATE_HZ			( ( TickType_t ) 1024 )
 
 /* Extend the number of priority levels to cope with Ada ceiling
    priorities, which mean we need to raise a task waiting for an ISR
    to a priority corresponding to that interrupt's priority. So we
-   need the additional priorities 8 (interrupt priority 3) to 9
-   (interrupt priority 2, the highest that can use API interrupt-safe
+   need the additional priorities 8 (interrupt priority 3) to 10
+   (interrupt priority 1, the highest that can use API interrupt-safe
    functions). */
-#define configMAX_PRIORITIES			( 9 )
+#define configMAX_PRIORITIES			( 10 )
 
 #define configMINIMAL_STACK_SIZE		( ( unsigned short ) 128 )
 #define configTOTAL_HEAP_SIZE			( ( size_t ) ( 16 * 1024 ) )
@@ -55,6 +55,7 @@
 #define configUSE_APPLICATION_TASK_TAG	        1 /* for Task_Id in TCB */
 #define configUSE_COUNTING_SEMAPHORES	        0
 #define configGENERATE_RUN_TIME_STATS	        0
+#define configUSE_QUEUE_SETS			0
 
 #define configUSE_TIME_SLICING                  0 /* don't want round-robin */
 
@@ -105,6 +106,7 @@ function. */
 routine that makes calls to interrupt safe FreeRTOS API functions.  DO NOT CALL
 INTERRUPT SAFE FREERTOS API FUNCTIONS FROM ANY INTERRUPT THAT HAS A HIGHER
 PRIORITY THAN THIS! (higher priorities are lower numeric values. */
+/* UNCLEAR THAT THIS IS TRUE FOR A CM0 PART, WHICH DOESN'T HAVE BASEPRI */
 #define configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY	1
 
 /* Interrupt priorities used by the kernel port layer itself.  These are generic
@@ -122,7 +124,6 @@ header file. */
 standard names. */
 #define vPortSVCHandler SVC_Handler
 #define xPortPendSVHandler PendSV_Handler
-
 #define xPortSysTickHandler SysTick_Handler
 
 #endif /* FREERTOS_CONFIG_H */
diff --git a/microbit/adainclude/nrf51_clock.adb b/microbit/adainclude/nrf51_clock.adb
index f533ef3..bc1173f 100644
--- a/microbit/adainclude/nrf51_clock.adb
+++ b/microbit/adainclude/nrf51_clock.adb
@@ -31,9 +31,10 @@
 --  below:
 --
 --  priority 0 (highest): FreeRTOS use only
---  priority 1: RTC1_IRQHandler
---  priority 2 .. 3: applications, arranged in this RTS as (interrupt)
---    priorities 9, 8 respectively.
+--  priority 1 .. 3: applications, arranged in this RTS as (interrupt)
+--    priorities 10, 9, 8 respectively.
+
+--  RTC1_IRQHandler runs at ARM priority 3, Interrupt_Priority'First.
 
 with nrf51.CLOCK;
 with nrf51.RTC;
@@ -128,7 +129,7 @@ package body nRF51_Clock is
            Address => System'To_Address (16#E000E100#);
 
          Interrupt : constant := 17;
-         Prio : constant := System.Priority'Last + 1;
+         Prio : constant := System.Interrupt_Priority'First;
       begin
          declare
             Index    : constant Natural := Interrupt / 4;
diff --git a/microbit/adainclude/system.ads b/microbit/adainclude/system.ads
index 7aa02f5..fadf2b4 100644
--- a/microbit/adainclude/system.ads
+++ b/microbit/adainclude/system.ads
@@ -8,7 +8,8 @@
 --                          (ARM Cortex M Version)                          --
 --                 (Derived from GNU-Linux/ARMEL Version)                   --
 --                                                                          --
---    Copyright (C) 1992-2010, 2016-2018, Free Software Foundation, Inc.   --
+--                Copyright (C) 1992-2010, 2016-2018, 2020                  --
+--                      Free Software Foundation, Inc.                      --
 --                                                                          --
 -- This specification is derived from the Ada Reference Manual for use with --
 -- GNAT. The copyright notice above, and the license provisions that follow --
@@ -74,7 +75,7 @@ package System is
    Max_Mantissa          : constant := 63;
    Fine_Delta            : constant := 2.0 ** (-Max_Mantissa);
 
-   Tick                  : constant := 0.01;
+   Tick                  : constant := 1.0 / 1024;
 
    --  Storage-related Declarations
 
@@ -110,12 +111,12 @@ package System is
 
    --  These declarations correspond to FreeRTOS as originally
    --  configured in STM Cube (thread priorities 0 .. 7), with
-   --  priorities 8 to 9 mapped to Cortex interrupt priorities 3
-   --  (lowest) to 2 (highest that can be used by ISRs that call
+   --  priorities 8 to 10 mapped to Cortex interrupt priorities 3
+   --  (lowest) to 1 (highest that can be used by ISRs that call
    --  interrupt-safe FreeRTOS API functions).
 
    Max_Priority           : constant Positive := 7;
-   Max_Interrupt_Priority : constant Positive := 9;
+   Max_Interrupt_Priority : constant Positive := 10;
 
    subtype Any_Priority       is Integer range 0 .. Max_Interrupt_Priority;
    subtype Priority           is Any_Priority