diff --git a/include/derivative.h b/include/derivative.h
index 301db3d..c0a3312 100644
--- a/include/derivative.h
+++ b/include/derivative.h
@@ -27,8 +27,10 @@
         PORT##port->PCR[pin] |= PORT_PCR_PE(0x1);   \
     }
 
+#define ticks_ms (uint32_t)(ticks_10us / 100)
+
 extern uint32_t CORCLK;
 extern uint32_t BUSCLK;
-extern volatile uint32_t ms_ticks;
+extern volatile uint32_t ticks_10us;
 extern void SysTick_Handler(void);
 extern void UART1_IRQHandler(void);
diff --git a/include/systick.h b/include/systick.h
index f25b60b..ac9b995 100644
--- a/include/systick.h
+++ b/include/systick.h
@@ -3,22 +3,33 @@
 #include "derivative.h"
 #include <stdbool.h>
 
-extern volatile uint32_t ms_ticks;
-
 static inline void spin(volatile uint32_t count) {
     while (count--) asm("nop");
 }
 
 // t: expiration time, prd: period
-static inline bool timer_expired(uint32_t *t, uint32_t prd) {
-    if (ms_ticks + prd < *t) *t = 0;                         // Time wrapped? Reset timer
-    if (*t == 0) *t = ms_ticks + prd;                        // First poll? Set expiration
-    if (*t > ms_ticks) return false;                         // Not expired yet, return
-    *t = (ms_ticks - *t) > prd ? ms_ticks + prd : *t + prd;  // Next expiration time
+static inline bool timer_expired_ms(uint32_t *t, uint32_t prd) {
+    if (ticks_ms + prd < *t) *t = 0;                         // Time wrapped? Reset timer
+    if (*t == 0) *t = ticks_ms + prd;                        // First poll? Set expiration
+    if (*t > ticks_ms) return false;                         // Not expired yet, return
+    *t = (ticks_ms - *t) > prd ? ticks_ms + prd : *t + prd;  // Next expiration time
     return true;                                             // Expired, return true
 }
 
-static inline void delay_ms(uint32_t ms) {
-    uint32_t start = ms_ticks;
-    while (ms_ticks - start < ms) asm("nop");
+static inline bool timer_expired_10us(uint32_t *t, uint32_t prd) {
+    if (ticks_10us + prd < *t) *t = 0;                           // Time wrapped? Reset timer
+    if (*t == 0) *t = ticks_10us + prd;                          // First poll? Set expiration
+    if (*t > ticks_10us) return false;                           // Not expired yet, return
+    *t = (ticks_10us - *t) > prd ? ticks_10us + prd : *t + prd;  // Next expiration time
+    return true;                                                 // Expired, return true
+}
+
+static inline void delay_ms(uint32_t ms) {
+    uint32_t start = ticks_ms;
+    while (ticks_ms - start < ms) asm("nop");
+}
+
+static inline void delay_10us(uint32_t us) {
+    uint32_t start = ticks_10us;
+    while (ticks_10us - start < us) asm("nop");
 }
diff --git a/src/derivative.c b/src/derivative.c
index 518814f..8757269 100644
--- a/src/derivative.c
+++ b/src/derivative.c
@@ -4,10 +4,9 @@
 uint32_t CORCLK = CORCLK_DEFAULT;
 uint32_t BUSCLK = BUSCLK_DEFAULT;
 
-// ms count, volatile is important!!
-volatile uint32_t ms_ticks;
+volatile uint32_t ticks_10us;
 
-void SysTick_Handler(void) { ms_ticks++; }
+void SysTick_Handler(void) { ticks_10us++; }
 
 void UART1_IRQHandler(void) {
     if (UART1->S1 & UART_S1_RDRF_MASK) {
diff --git a/src/startup.c b/src/startup.c
index 369d8cb..1fee318 100644
--- a/src/startup.c
+++ b/src/startup.c
@@ -159,6 +159,8 @@ __attribute__((naked, noreturn)) void _reset(void) {
     copy_data();
     clock_init();
 
+    SysTick_Config(CORCLK / 100000);  // Period of systick timer : 10 us
+
     main();
     for (;;) (void)0;
 }