diff --git a/ntp_io_linux.c b/ntp_io_linux.c
index 3a273aa..9b53d7b 100644
--- a/ntp_io_linux.c
+++ b/ntp_io_linux.c
@@ -220,7 +220,7 @@ add_interface(CNF_HwTsInterface *conf_iface)
 
   SCK_CloseSocket(sock_fd);
 
-  phc_fd = SYS_Linux_OpenPHC(req.ifr_name);
+  phc_fd = SYS_Linux_OpenPHC(req.ifr_name, O_RDONLY);
   if (phc_fd < 0)
     return 0;
 
diff --git a/refclock_phc.c b/refclock_phc.c
index cff75d8..08da75f 100644
--- a/refclock_phc.c
+++ b/refclock_phc.c
@@ -66,7 +66,7 @@ static int phc_initialise(RCL_Instance instance)
 {
   const char *options[] = {"nocrossts", "extpps", "pin", "channel", "clear", NULL};
   struct phc_instance *phc;
-  int phc_fd, rising_edge;
+  int rising_edge;
   struct stat st;
   char *path, *s;
 
@@ -74,19 +74,20 @@ static int phc_initialise(RCL_Instance instance)
 
   path = RCL_GetDriverParameter(instance);
  
-  phc_fd = SYS_Linux_OpenPHC(path);
-  if (phc_fd < 0)
-    LOG_FATAL("Could not open PHC");
-
   phc = MallocNew(struct phc_instance);
-  phc->fd = phc_fd;
-  if (fstat(phc_fd, &st) < 0 || !S_ISCHR(st.st_mode))
-    LOG_FATAL("Could not get PHC index");
-  phc->dev_index = minor(st.st_rdev);
   phc->mode = 0;
   phc->nocrossts = RCL_GetDriverOption(instance, "nocrossts") ? 1 : 0;
   phc->extpps = RCL_GetDriverOption(instance, "extpps") ? 1 : 0;
   UTI_ZeroTimespec(&phc->last_extts);
+
+  phc->fd = SYS_Linux_OpenPHC(path, phc->extpps ? O_RDWR : O_RDONLY);
+  if (phc->fd < 0)
+    LOG_FATAL("Could not open PHC");
+
+  if (fstat(phc->fd, &st) < 0 || !S_ISCHR(st.st_mode))
+    LOG_FATAL("Could not get PHC index");
+  phc->dev_index = minor(st.st_rdev);
+
   phc->clock = HCL_CreateInstance(0, 16, UTI_Log2ToDouble(RCL_GetDriverPoll(instance)),
                                   RCL_GetPrecision(instance));
 
diff --git a/sys_linux.c b/sys_linux.c
index febf0eb..53c49a8 100644
--- a/sys_linux.c
+++ b/sys_linux.c
@@ -879,7 +879,7 @@ verify_fd_is_phc(int phc_fd)
 /* ================================================== */
 
 static int
-open_phc_by_iface_name(const char *iface)
+open_phc_by_iface_name(const char *iface, int flags)
 {
 #ifdef HAVE_LINUX_TIMESTAMPING
   struct ethtool_ts_info ts_info;
@@ -922,7 +922,7 @@ open_phc_by_iface_name(const char *iface)
                "/dev/ptp%d", ts_info.phc_index) >= sizeof (phc_device))
     return -1;
 
-  return open(phc_device, O_RDONLY);
+  return open(phc_device, flags);
 #else
   return -1;
 #endif
@@ -931,18 +931,18 @@ open_phc_by_iface_name(const char *iface)
 /* ================================================== */
 
 int
-SYS_Linux_OpenPHC(const char *device)
+SYS_Linux_OpenPHC(const char *device, int flags)
 {
   int phc_fd = -1;
 
   if (device[0] == '/') {
-    phc_fd = open(device, O_RDONLY);
+    phc_fd = open(device, flags);
     if (phc_fd >= 0)
       phc_fd = verify_fd_is_phc(phc_fd);
   }
 
   if (phc_fd < 0) {
-    phc_fd = open_phc_by_iface_name(device);
+    phc_fd = open_phc_by_iface_name(device, flags);
     if (phc_fd < 0) {
       LOG(LOGS_ERR, "Could not open PHC of iface %s : %s",
           device, strerror(errno));
diff --git a/sys_linux.h b/sys_linux.h
index b7b3ead..b0567a8 100644
--- a/sys_linux.h
+++ b/sys_linux.h
@@ -39,7 +39,7 @@ extern void SYS_Linux_EnableSystemCallFilter(int level, SYS_ProcessContext conte
 
 extern int SYS_Linux_CheckKernelVersion(int req_major, int req_minor);
 
-extern int SYS_Linux_OpenPHC(const char *device);
+extern int SYS_Linux_OpenPHC(const char *device, int flags);
 
 extern int SYS_Linux_GetPHCReadings(int fd, int nocrossts, int *reading_mode, int max_readings,
                                     struct timespec tss[][3]);