[boards] Fix off-by-one in SysTick cyclesPerTick reload values

SAY-5 · SAY-5 · commit ad47719110de · 2026-05-11T22:24:09.000-07:00
Cortex-M SysTick reloads every (LOAD + 1) cycles, so cyclesPerTick must be (clock / ticks_per_second) - 1 to produce the intended period. All ten board configs were one cycle high, causing systick to run slightly slow. Closes #38
diff --git a/boards/pic32cz_curiosity_ultra/board.c b/boards/pic32cz_curiosity_ultra/board.c
@@ -76,7 +76,7 @@ whal_Timer g_whalTimer = {
     .driver = WHAL_CORTEX_M7_SYSTICK_DRIVER,
 
     .cfg = &(whal_SysTick_Cfg) {
-        .cyclesPerTick = 300000000 / 1000,
+        .cyclesPerTick = (300000000 / 1000) - 1, /* SysTick reloads every LOAD+1 cycles */
         .clkSrc = WHAL_SYSTICK_CLKSRC_SYSCLK,
         .tickInt = WHAL_SYSTICK_TICKINT_ENABLED,
     },
diff --git a/boards/stm32c031_nucleo/board.c b/boards/stm32c031_nucleo/board.c
@@ -92,7 +92,7 @@ whal_Timer g_whalTimer = {
     .driver = WHAL_CORTEX_M0PLUS_SYSTICK_DRIVER,
 
     .cfg = &(whal_SysTick_Cfg) {
-        .cyclesPerTick = 48000000 / 1000, /* 48 MHz / 1 kHz = 1 ms tick */
+        .cyclesPerTick = (48000000 / 1000) - 1, /* 48 MHz / 1 kHz = 1 ms tick (SysTick reloads every LOAD+1 cycles) */
         .clkSrc = WHAL_SYSTICK_CLKSRC_SYSCLK,
         .tickInt = WHAL_SYSTICK_TICKINT_ENABLED,
     },
diff --git a/boards/stm32f091rc_nucleo/board.c b/boards/stm32f091rc_nucleo/board.c
@@ -104,7 +104,7 @@ whal_Timer g_whalTimer = {
     .driver = WHAL_CORTEX_M0_SYSTICK_DRIVER,
 
     .cfg = &(whal_SysTick_Cfg) {
-        .cyclesPerTick = 48000000 / 1000,
+        .cyclesPerTick = (48000000 / 1000) - 1, /* SysTick reloads every LOAD+1 cycles */
         .clkSrc = WHAL_SYSTICK_CLKSRC_SYSCLK,
         .tickInt = WHAL_SYSTICK_TICKINT_ENABLED,
     },
diff --git a/boards/stm32f302r8_nucleo/board.c b/boards/stm32f302r8_nucleo/board.c
@@ -107,7 +107,7 @@ whal_Timer g_whalTimer = {
     .driver = WHAL_CORTEX_M4_SYSTICK_DRIVER,
 
     .cfg = &(whal_SysTick_Cfg) {
-        .cyclesPerTick = 48000000 / 1000,
+        .cyclesPerTick = (48000000 / 1000) - 1, /* SysTick reloads every LOAD+1 cycles */
         .clkSrc = WHAL_SYSTICK_CLKSRC_SYSCLK,
         .tickInt = WHAL_SYSTICK_TICKINT_ENABLED,
     },
diff --git a/boards/stm32f411_blackpill/board.c b/boards/stm32f411_blackpill/board.c
@@ -98,7 +98,7 @@ whal_Timer g_whalTimer = {
     .driver = WHAL_CORTEX_M4_SYSTICK_DRIVER,
 
     .cfg = &(whal_SysTick_Cfg) {
-        .cyclesPerTick = 100000000 / 1000, /* 100 MHz / 1 kHz = 1 ms tick */
+        .cyclesPerTick = (100000000 / 1000) - 1, /* 100 MHz / 1 kHz = 1 ms tick (SysTick reloads every LOAD+1 cycles) */
         .clkSrc = WHAL_SYSTICK_CLKSRC_SYSCLK,
         .tickInt = WHAL_SYSTICK_TICKINT_ENABLED,
     },
diff --git a/boards/stm32h563zi_nucleo/board.c b/boards/stm32h563zi_nucleo/board.c
@@ -148,7 +148,7 @@ whal_Timer g_whalTimer = {
     .driver = WHAL_CORTEX_M33_SYSTICK_DRIVER,
 
     .cfg = &(whal_SysTick_Cfg) {
-        .cyclesPerTick = 168000000 / 1000, /* 168 MHz / 1 kHz = 1 ms tick */
+        .cyclesPerTick = (168000000 / 1000) - 1, /* 168 MHz / 1 kHz = 1 ms tick (SysTick reloads every LOAD+1 cycles) */
         .clkSrc = WHAL_SYSTICK_CLKSRC_SYSCLK,
         .tickInt = WHAL_SYSTICK_TICKINT_ENABLED,
     },
diff --git a/boards/stm32l152re_nucleo/board.c b/boards/stm32l152re_nucleo/board.c
@@ -119,7 +119,7 @@ whal_Timer g_whalTimer = {
     .driver = WHAL_CORTEX_M3_SYSTICK_DRIVER,
 
     .cfg = &(whal_SysTick_Cfg) {
-        .cyclesPerTick = 32000000 / 1000,
+        .cyclesPerTick = (32000000 / 1000) - 1, /* SysTick reloads every LOAD+1 cycles */
         .clkSrc = WHAL_SYSTICK_CLKSRC_SYSCLK,
         .tickInt = WHAL_SYSTICK_TICKINT_ENABLED,
     },
diff --git a/boards/stm32n657a0_nucleo/board.c b/boards/stm32n657a0_nucleo/board.c
@@ -195,7 +195,7 @@ whal_Timer g_whalTimer = {
     .driver = WHAL_CORTEX_M55_SYSTICK_DRIVER,
 
     .cfg = &(whal_SysTick_Cfg) {
-        .cyclesPerTick = 64000000 / 1000, /* 64 MHz / 1 kHz = 1 ms tick */
+        .cyclesPerTick = (64000000 / 1000) - 1, /* 64 MHz / 1 kHz = 1 ms tick (SysTick reloads every LOAD+1 cycles) */
         .clkSrc = WHAL_SYSTICK_CLKSRC_SYSCLK,
         .tickInt = WHAL_SYSTICK_TICKINT_ENABLED,
     },
diff --git a/boards/stm32wb55xx_nucleo/board.c b/boards/stm32wb55xx_nucleo/board.c
@@ -136,7 +136,7 @@ whal_Timer g_whalTimer = {
     .driver = WHAL_CORTEX_M4_SYSTICK_DRIVER,
 
     .cfg = &(whal_SysTick_Cfg) {
-        .cyclesPerTick = 64000000 / 1000,
+        .cyclesPerTick = (64000000 / 1000) - 1, /* SysTick reloads every LOAD+1 cycles */
         .clkSrc = WHAL_SYSTICK_CLKSRC_SYSCLK,
         .tickInt = WHAL_SYSTICK_TICKINT_ENABLED,
     },
diff --git a/boards/stm32wba55cg_nucleo/board.c b/boards/stm32wba55cg_nucleo/board.c
@@ -144,7 +144,7 @@ whal_Timer g_whalTimer = {
     .driver = WHAL_CORTEX_M33_SYSTICK_DRIVER,
 
     .cfg = &(whal_SysTick_Cfg) {
-        .cyclesPerTick = 100000000 / 1000,
+        .cyclesPerTick = (100000000 / 1000) - 1, /* SysTick reloads every LOAD+1 cycles */
         .clkSrc = WHAL_SYSTICK_CLKSRC_SYSCLK,
         .tickInt = WHAL_SYSTICK_TICKINT_ENABLED,
     },
