diff --git a/board/drivers/gmlan_alt.h b/board/drivers/gmlan_alt.h
index 53f46c2e0..45378747d 100644
--- a/board/drivers/gmlan_alt.h
+++ b/board/drivers/gmlan_alt.h
@@ -128,9 +128,9 @@ void setup_timer(void) {
   REGISTER_INTERRUPT(TIM8_BRK_TIM12_IRQn, TIM12_IRQ_Handler, 40000U, FAULT_INTERRUPT_RATE_GMLAN)
 
   // setup
-  register_set(&(TIM12->PSC), (48-1), 0xFFFFU);    // Tick on 1 us
+  register_set(&(TIM12->PSC), (APB1_TIMER_FREQ-1U), 0xFFFFU);    // Tick on 1 us
   register_set(&(TIM12->CR1), TIM_CR1_CEN, 0x3FU); // Enable
-  register_set(&(TIM12->ARR), (30-1), 0xFFFFU);   // 33.3 kbps
+  register_set(&(TIM12->ARR), (30U-1U), 0xFFFFU);   // 33.3 kbps
 
   // in case it's disabled
   NVIC_EnableIRQ(TIM8_BRK_TIM12_IRQn);