diff --git a/sys/dev/pcf/pcf.c b/sys/dev/pcf/pcf.c
index e77481260e60..55e03460fb7e 100644
--- a/sys/dev/pcf/pcf.c
+++ b/sys/dev/pcf/pcf.c
@@ -378,7 +378,7 @@ pcf_rst_card(device_t dev, u_char speed, u_char addr, u_char *oldaddr)
 }
 
 int
-pcf_write(device_t dev, char *buf, int len, int *sent, int timeout /* us */)
+pcf_write(device_t dev, const char *buf, int len, int *sent, int timeout /* us */)
 {
 	struct pcf_softc *sc = DEVTOSOFTC(dev);
 	int bytes, error = 0;
diff --git a/sys/dev/pcf/pcfvar.h b/sys/dev/pcf/pcfvar.h
index 8e4d5d82ea2c..33721c9a4331 100644
--- a/sys/dev/pcf/pcfvar.h
+++ b/sys/dev/pcf/pcfvar.h
@@ -135,7 +135,7 @@ pcf_get_S1(struct pcf_softc *sc)
 extern int pcf_repeated_start(device_t, u_char, int);
 extern int pcf_start(device_t, u_char, int);
 extern int pcf_stop(device_t);
-extern int pcf_write(device_t, char *, int, int *, int);
+extern int pcf_write(device_t, const char *, int, int *, int);
 extern int pcf_read(device_t, char *, int, int *, int, int);
 extern int pcf_rst_card(device_t, u_char, u_char, u_char *);
 extern driver_intr_t pcf_intr;