diff --git a/hw/opentitan/ot_aon_timer.c b/hw/opentitan/ot_aon_timer.c
index cd89c418a8e8..75abf5cad9c5 100644
--- a/hw/opentitan/ot_aon_timer.c
+++ b/hw/opentitan/ot_aon_timer.c
@@ -497,7 +497,7 @@ static void ot_aon_timer_init(Object *obj)
     ibex_qdev_init_irq(obj, &s->alert, OPENTITAN_DEVICE_ALERT);
 
     memory_region_init_io(&s->mmio, obj, &ot_aon_timer_ops, s,
-                          TYPE_OT_AON_TIMER, 0x400u);
+                          TYPE_OT_AON_TIMER, REGS_SIZE);
     sysbus_init_mmio(SYS_BUS_DEVICE(obj), &s->mmio);
 
     s->wkup_timer =
diff --git a/hw/opentitan/ot_timer.c b/hw/opentitan/ot_timer.c
index 42e8d413dc69..72c4c06b901b 100644
--- a/hw/opentitan/ot_timer.c
+++ b/hw/opentitan/ot_timer.c
@@ -367,7 +367,7 @@ static void ot_timer_init(Object *obj)
     ibex_qdev_init_irq(obj, &s->alert, OPENTITAN_DEVICE_ALERT);
 
     memory_region_init_io(&s->mmio, obj, &ot_timer_ops, s, TYPE_OT_TIMER,
-                          0x400u);
+                          REGS_SIZE);
     sysbus_init_mmio(SYS_BUS_DEVICE(obj), &s->mmio);
 
     s->timer = timer_new_ns(QEMU_CLOCK_VIRTUAL_RT, &ot_timer_cb, s);
diff --git a/hw/opentitan/ot_uart.c b/hw/opentitan/ot_uart.c
index 5b026eeefda6..f2f881ba1eab 100644
--- a/hw/opentitan/ot_uart.c
+++ b/hw/opentitan/ot_uart.c
@@ -599,7 +599,7 @@ static void ot_uart_init(Object *obj)
     }
 
     memory_region_init_io(&s->mmio, obj, &ot_uart_ops, s, TYPE_OT_UART,
-                          REGS_COUNT * sizeof(uint32_t));
+                          REGS_SIZE);
     sysbus_init_mmio(SYS_BUS_DEVICE(obj), &s->mmio);
 }