diff --git a/os/hal/ports/STM32/LLD/EXTIv1/stm32_exti.c b/os/hal/ports/STM32/LLD/EXTIv1/stm32_exti.c
index e2877cfe2..3ff2684d8 100644
--- a/os/hal/ports/STM32/LLD/EXTIv1/stm32_exti.c
+++ b/os/hal/ports/STM32/LLD/EXTIv1/stm32_exti.c
@@ -62,36 +62,37 @@
  * @api
  */
 void extiEnableGroup1(uint32_t mask, extimode_t mode) {
+  uint32_t cmask;
 
-  /* Masked out lines must not be touched by this driver.*/
-  osalDbgAssert((mask & STM32_EXTI_IMR1_MASK) == 0U, "fixed lines");
+  /* Mask including only configurable lines.*/
+  cmask = mask & ~STM32_EXTI_IMR1_MASK;
 
   if ((mode & EXTI_MODE_EDGES_MASK) == 0U) {
     /* Disabling channels.*/
     EXTI->IMR1  &= ~mask;
     EXTI->EMR1  &= ~mask;
-    EXTI->RTSR1 &= ~mask;
-    EXTI->FTSR1 &= ~mask;
+    EXTI->RTSR1 &= ~cmask;
+    EXTI->FTSR1 &= ~cmask;
 #if STM32_EXTI_SEPARATE_RF == FALSE
-    EXTI->PR1    =  mask;
+    EXTI->PR1    =  cmask;
 #else
-    EXTI->RPR1   =  mask;
-    EXTI->FPR1   =  mask;
+    EXTI->RPR1   =  cmask;
+    EXTI->FPR1   =  cmask;
 #endif
   }
   else {
     /* Programming edge registers.*/
     if (mode & EXTI_MODE_RISING_EDGE) {
-      EXTI->RTSR1 |= mask;
+      EXTI->RTSR1 |= cmask;
     }
     else {
-      EXTI->RTSR1 &= ~mask;
+      EXTI->RTSR1 &= ~cmask;
     }
     if (mode & EXTI_MODE_FALLING_EDGE) {
-      EXTI->FTSR1 |= mask;
+      EXTI->FTSR1 |= cmask;
     }
     else {
-      EXTI->FTSR1 &= ~mask;
+      EXTI->FTSR1 &= ~cmask;
     }
 
     /* Programming interrupt and event registers.*/
@@ -116,36 +117,37 @@ void extiEnableGroup1(uint32_t mask, extimode_t mode) {
  * @api
  */
 void extiEnableGroup2(uint32_t mask, extimode_t mode) {
+  uint32_t cmask;
 
-  /* Masked out lines must not be touched by this driver.*/
-  osalDbgAssert((mask & STM32_EXTI_IMR2_MASK) == 0U, "fixed lines");
+  /* Mask including only configurable lines.*/
+  cmask = mask & ~STM32_EXTI_IMR1_MASK;
 
   if ((mode & EXTI_MODE_EDGES_MASK) == 0U) {
     /* Disabling channels.*/
     EXTI->IMR2  &= ~mask;
     EXTI->EMR2  &= ~mask;
-    EXTI->RTSR2 &= ~mask;
-    EXTI->FTSR2 &= ~mask;
+    EXTI->RTSR2 &= ~cmask;
+    EXTI->FTSR2 &= ~cmask;
 #if STM32_EXTI_SEPARATE_RF == FALSE
-    EXTI->PR2    =  mask;
+    EXTI->PR2    =  cmask;
 #else
-    EXTI->RPR2   =  mask;
-    EXTI->FPR2   =  mask;
+    EXTI->RPR2   =  cmask;
+    EXTI->FPR2   =  cmask;
 #endif
   }
   else {
     /* Programming edge registers.*/
     if (mode & EXTI_MODE_RISING_EDGE) {
-      EXTI->RTSR2 |= mask;
+      EXTI->RTSR2 |= cmask;
     }
     else {
-      EXTI->RTSR2 &= ~mask;
+      EXTI->RTSR2 &= ~cmask;
     }
     if (mode & EXTI_MODE_FALLING_EDGE) {
-      EXTI->FTSR2 |= mask;
+      EXTI->FTSR2 |= cmask;
     }
     else {
-      EXTI->FTSR2 &= ~mask;
+      EXTI->FTSR2 &= ~cmask;
     }
 
     /* Programming interrupt and event registers.*/
diff --git a/readme.txt b/readme.txt
index 0e57d955a..f92ad066a 100644
--- a/readme.txt
+++ b/readme.txt
@@ -117,6 +117,8 @@
        MEMS Accelerometers.
 - NEW: Safer messages mechanism for sandboxes (to be backported to 20.3.1).
 - NEW: Added latency measurement test application.
+- FIX: Fixed STM32 EXTIv1 driver unable to enable/disable fixed lines
+       (bug #1123)(backported to 20.3.3)(backported to 19.1.5).
 - FIX: Fixed incorrect STM32 iWDG initialization in windowed mode (bug #1122)
        (backported to 20.3.3)(backported to 19.1.5).
 - FIX: Fixed ignored HSIDIV setting on STM32G0xx (bug #1121)