diff --git a/drivers/isdn/hardware/mISDN/hfc_multi.h b/drivers/isdn/hardware/mISDN/hfc_multi.h
index 4aa6a8b..663b77f 100644
--- a/drivers/isdn/hardware/mISDN/hfc_multi.h
+++ b/drivers/isdn/hardware/mISDN/hfc_multi.h
@@ -197,6 +197,9 @@
 
 	spinlock_t	lock;	/* the lock */
 
+	struct mISDNclock *iclock; /* isdn clock support */
+	int		iclock_on;
+
 	/*
 	 * the channel index is counted from 0, regardless where the channel
 	 * is located on the hfc-channel.
diff --git a/drivers/isdn/hardware/mISDN/hfcmulti.c b/drivers/isdn/hardware/mISDN/hfcmulti.c
index 3fc2e9d..592db93 100644
--- a/drivers/isdn/hardware/mISDN/hfcmulti.c
+++ b/drivers/isdn/hardware/mISDN/hfcmulti.c
@@ -133,6 +133,12 @@
  *	Give the value of the clock control register (A_ST_CLK_DLY)
  *	of the S/T interfaces in TE mode.
  *	This register is needed for the TBR3 certification, so don't change it.
+ *
+ * clock:
+ *	NOTE: only one clockdelay_te value must be given once
+ *	Selects interface with clock source for mISDN and applications.
+ *	Set to card number starting with 1. Set to -1 to disable.
+ *	By default, the first card is used as clock source.
  */
 
 /*
@@ -190,12 +196,13 @@
  */
 
 static uint	type[MAX_CARDS];
-static uint	pcm[MAX_CARDS];
-static uint	dslot[MAX_CARDS];
+static int	pcm[MAX_CARDS];
+static int	dslot[MAX_CARDS];
 static uint	iomode[MAX_CARDS];
 static uint	port[MAX_PORTS];
 static uint	debug;
 static uint	poll;
+static int	clock;
 static uint	timer;
 static uint	clockdelay_te = CLKDEL_TE;
 static uint	clockdelay_nt = CLKDEL_NT;
@@ -207,12 +214,13 @@
 MODULE_VERSION(HFC_MULTI_VERSION);
 module_param(debug, uint, S_IRUGO | S_IWUSR);
 module_param(poll, uint, S_IRUGO | S_IWUSR);
+module_param(clock, int, S_IRUGO | S_IWUSR);
 module_param(timer, uint, S_IRUGO | S_IWUSR);
 module_param(clockdelay_te, uint, S_IRUGO | S_IWUSR);
 module_param(clockdelay_nt, uint, S_IRUGO | S_IWUSR);
 module_param_array(type, uint, NULL, S_IRUGO | S_IWUSR);
-module_param_array(pcm, uint, NULL, S_IRUGO | S_IWUSR);
-module_param_array(dslot, uint, NULL, S_IRUGO | S_IWUSR);
+module_param_array(pcm, int, NULL, S_IRUGO | S_IWUSR);
+module_param_array(dslot, int, NULL, S_IRUGO | S_IWUSR);
 module_param_array(iomode, uint, NULL, S_IRUGO | S_IWUSR);
 module_param_array(port, uint, NULL, S_IRUGO | S_IWUSR);
 
@@ -2629,6 +2637,7 @@
 		iqcnt = 0;
 	}
 #endif
+
 	if (!r_irq_statech &&
 	    !(status & (V_DTMF_STA | V_LOST_STA | V_EXT_IRQSTA |
 	    V_MISC_IRQSTA | V_FR_IRQSTA))) {
@@ -2683,11 +2692,13 @@
 					plxsd_checksync(hc, 0);
 			}
 		}
-		if (r_irq_misc & V_TI_IRQ)
+		if (r_irq_misc & V_TI_IRQ) {
+			if (hc->iclock_on)
+				mISDN_clock_update(hc->iclock, poll, NULL);
 			handle_timer_irq(hc);
+		}
 
 		if (r_irq_misc & V_DTMF_IRQ) {
-			/* -> DTMF IRQ */
 			hfcmulti_dtmf(hc);
 		}
 		if (r_irq_misc & V_IRQ_PROC) {
@@ -4075,6 +4086,15 @@
 	return err;
 }
 
+static int
+clockctl(void *priv, int enable)
+{
+	struct hfc_multi *hc = priv;
+
+	hc->iclock_on = enable;
+	return 0;
+}
+
 /*
  * initialize the card
  */
@@ -4489,10 +4509,14 @@
 		printk(KERN_WARNING "%s: release card (%d) entered\n",
 		    __func__, hc->id);
 
+	/* unregister clock source */
+	if (hc->iclock)
+		mISDN_unregister_clock(hc->iclock);
+
+	/* disable irq */
 	spin_lock_irqsave(&hc->lock, flags);
 	disable_hwirq(hc);
 	spin_unlock_irqrestore(&hc->lock, flags);
-
 	udelay(1000);
 
 	/* dimm leds */
@@ -5003,6 +5027,10 @@
 	list_add_tail(&hc->list, &HFClist);
 	spin_unlock_irqrestore(&HFClock, flags);
 
+	/* use as clock source */
+	if (clock == HFC_cnt + 1)
+		hc->iclock = mISDN_register_clock("HFCMulti", 0, clockctl, hc);
+
 	/* initialize hardware */
 	ret_err = init_card(hc);
 	if (ret_err) {
@@ -5273,6 +5301,9 @@
 
 	}
 
+	if (!clock)
+		clock = 1;
+
 	err = pci_register_driver(&hfcmultipci_driver);
 	if (err < 0) {
 		printk(KERN_ERR "error registering pci driver: %x\n", err);
diff --git a/drivers/isdn/mISDN/Makefile b/drivers/isdn/mISDN/Makefile
index 1cb5e63..0a6bd2a 100644
--- a/drivers/isdn/mISDN/Makefile
+++ b/drivers/isdn/mISDN/Makefile
@@ -8,6 +8,6 @@
 
 # multi objects
 
-mISDN_core-objs := core.o fsm.o socket.o hwchannel.o stack.o layer1.o layer2.o tei.o timerdev.o
+mISDN_core-objs := core.o fsm.o socket.o clock.o hwchannel.o stack.o layer1.o layer2.o tei.o timerdev.o
 mISDN_dsp-objs := dsp_core.o dsp_cmx.o dsp_tones.o dsp_dtmf.o dsp_audio.o dsp_blowfish.o dsp_pipeline.o dsp_hwec.o
 l1oip-objs := l1oip_core.o l1oip_codec.o
diff --git a/drivers/isdn/mISDN/clock.c b/drivers/isdn/mISDN/clock.c
new file mode 100644
index 0000000..44d9c3d
--- /dev/null
+++ b/drivers/isdn/mISDN/clock.c
@@ -0,0 +1,216 @@
+/*
+ * Copyright 2008  by Andreas Eversberg <andreas@eversberg.eu>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * Quick API description:
+ *
+ * A clock source registers using mISDN_register_clock:
+ * 	name = text string to name clock source
+ *	priority = value to priorize clock sources (0 = default)
+ *	ctl = callback function to enable/disable clock source
+ *	priv = private pointer of clock source
+ * 	return = pointer to clock source structure;
+ *
+ * Note: Callback 'ctl' can be called before mISDN_register_clock returns!
+ *       Also it can be called during mISDN_unregister_clock.
+ *
+ * A clock source calls mISDN_clock_update with given samples elapsed, if
+ * enabled. If function call is delayed, tv must be set with the timestamp
+ * of the actual event.
+ *
+ * A clock source unregisters using mISDN_unregister_clock.
+ *
+ * To get current clock, call mISDN_clock_get. The signed short value
+ * counts the number of samples since. Time since last clock event is added.
+ *
+ */
+
+#include <linux/types.h>
+#include <linux/stddef.h>
+#include <linux/spinlock.h>
+#include <linux/mISDNif.h>
+#include "core.h"
+
+static u_int *debug;
+static LIST_HEAD(iclock_list);
+DEFINE_RWLOCK(iclock_lock);
+u16	iclock_count;		/* counter of last clock */
+struct	timeval iclock_tv;	/* time stamp of last clock */
+int	iclock_tv_valid;	/* already received one timestamp */
+struct	mISDNclock *iclock_current;
+
+void
+mISDN_init_clock(u_int *dp)
+{
+	debug = dp;
+	do_gettimeofday(&iclock_tv);
+}
+
+static void
+select_iclock(void)
+{
+	struct mISDNclock *iclock, *bestclock = NULL, *lastclock = NULL;
+	int pri = -128;
+
+	list_for_each_entry(iclock, &iclock_list, list) {
+		if (iclock->pri > pri) {
+			pri = iclock->pri;
+			bestclock = iclock;
+		}
+		if (iclock_current == iclock)
+			lastclock = iclock;
+	}
+	if (lastclock && bestclock != lastclock) {
+		/* last used clock source still exists but changes, disable */
+		if (*debug & DEBUG_CLOCK)
+			printk(KERN_DEBUG "Old clock source '%s' disable.\n",
+				lastclock->name);
+		lastclock->ctl(lastclock->priv, 0);
+	}
+	if (bestclock && bestclock != iclock_current) {
+		/* new clock source selected, enable */
+		if (*debug & DEBUG_CLOCK)
+			printk(KERN_DEBUG "New clock source '%s' enable.\n",
+				bestclock->name);
+		bestclock->ctl(bestclock->priv, 1);
+	}
+	if (bestclock != iclock_current) {
+		/* no clock received yet */
+		iclock_tv_valid = 0;
+	}
+	iclock_current = bestclock;
+}
+
+struct mISDNclock
+*mISDN_register_clock(char *name, int pri, clockctl_func_t *ctl, void *priv)
+{
+	u_long			flags;
+	struct mISDNclock	*iclock;
+
+	if (*debug & (DEBUG_CORE | DEBUG_CLOCK))
+		printk(KERN_DEBUG "%s: %s %d\n", __func__, name, pri);
+	iclock = kzalloc(sizeof(struct mISDNclock), GFP_ATOMIC);
+	if (!iclock) {
+		printk(KERN_ERR "%s: No memory for clock entry.\n", __func__);
+		return NULL;
+	}
+	strncpy(iclock->name, name, sizeof(iclock->name)-1);
+	iclock->pri = pri;
+	iclock->priv = priv;
+	iclock->ctl = ctl;
+	write_lock_irqsave(&iclock_lock, flags);
+	list_add_tail(&iclock->list, &iclock_list);
+	select_iclock();
+	write_unlock_irqrestore(&iclock_lock, flags);
+	return iclock;
+}
+EXPORT_SYMBOL(mISDN_register_clock);
+
+void
+mISDN_unregister_clock(struct mISDNclock *iclock)
+{
+	u_long	flags;
+
+	if (*debug & (DEBUG_CORE | DEBUG_CLOCK))
+		printk(KERN_DEBUG "%s: %s %d\n", __func__, iclock->name,
+			iclock->pri);
+	write_lock_irqsave(&iclock_lock, flags);
+	if (iclock_current == iclock) {
+		if (*debug & DEBUG_CLOCK)
+			printk(KERN_DEBUG
+				"Current clock source '%s' unregisters.\n",
+				iclock->name);
+		iclock->ctl(iclock->priv, 0);
+	}
+	list_del(&iclock->list);
+	select_iclock();
+	write_unlock_irqrestore(&iclock_lock, flags);
+}
+EXPORT_SYMBOL(mISDN_unregister_clock);
+
+void
+mISDN_clock_update(struct mISDNclock *iclock, int samples, struct timeval *tv)
+{
+	u_long		flags;
+	struct timeval	tv_now;
+	time_t		elapsed_sec;
+	int		elapsed_8000th;
+
+	write_lock_irqsave(&iclock_lock, flags);
+	if (iclock_current != iclock) {
+		printk(KERN_ERR "%s: '%s' sends us clock updates, but we do "
+			"listen to '%s'. This is a bug!\n", __func__,
+			iclock->name,
+			iclock_current ? iclock_current->name : "nothing");
+		iclock->ctl(iclock->priv, 0);
+		write_unlock_irqrestore(&iclock_lock, flags);
+		return;
+	}
+	if (iclock_tv_valid) {
+		/* increment sample counter by given samples */
+		iclock_count += samples;
+		if (tv) { /* tv must be set, if function call is delayed */
+			iclock_tv.tv_sec = tv->tv_sec;
+			iclock_tv.tv_usec = tv->tv_usec;
+		} else
+			do_gettimeofday(&iclock_tv);
+	} else {
+		/* calc elapsed time by system clock */
+		if (tv) { /* tv must be set, if function call is delayed */
+			tv_now.tv_sec = tv->tv_sec;
+			tv_now.tv_usec = tv->tv_usec;
+		} else
+			do_gettimeofday(&tv_now);
+		elapsed_sec = tv_now.tv_sec - iclock_tv.tv_sec;
+		elapsed_8000th = (tv_now.tv_usec / 125)
+			- (iclock_tv.tv_usec / 125);
+		if (elapsed_8000th < 0) {
+			elapsed_sec -= 1;
+			elapsed_8000th += 8000;
+		}
+		/* add elapsed time to counter and set new timestamp */
+		iclock_count += elapsed_sec * 8000 + elapsed_8000th;
+		iclock_tv.tv_sec = tv_now.tv_sec;
+		iclock_tv.tv_usec = tv_now.tv_usec;
+		iclock_tv_valid = 1;
+		if (*debug & DEBUG_CLOCK)
+			printk("Received first clock from source '%s'.\n",
+			    iclock_current ? iclock_current->name : "nothing");
+	}
+	write_unlock_irqrestore(&iclock_lock, flags);
+}
+EXPORT_SYMBOL(mISDN_clock_update);
+
+unsigned short
+mISDN_clock_get(void)
+{
+	u_long		flags;
+	struct timeval	tv_now;
+	time_t		elapsed_sec;
+	int		elapsed_8000th;
+	u16		count;
+
+	read_lock_irqsave(&iclock_lock, flags);
+	/* calc elapsed time by system clock */
+	do_gettimeofday(&tv_now);
+	elapsed_sec = tv_now.tv_sec - iclock_tv.tv_sec;
+	elapsed_8000th = (tv_now.tv_usec / 125) - (iclock_tv.tv_usec / 125);
+	if (elapsed_8000th < 0) {
+		elapsed_sec -= 1;
+		elapsed_8000th += 8000;
+	}
+	/* add elapsed time to counter */
+	count =	iclock_count + elapsed_sec * 8000 + elapsed_8000th;
+	read_unlock_irqrestore(&iclock_lock, flags);
+	return count;
+}
+EXPORT_SYMBOL(mISDN_clock_get);
+
diff --git a/drivers/isdn/mISDN/core.c b/drivers/isdn/mISDN/core.c
index 43fd97b..9116f54 100644
--- a/drivers/isdn/mISDN/core.c
+++ b/drivers/isdn/mISDN/core.c
@@ -199,6 +199,7 @@
 
 	printk(KERN_INFO "Modular ISDN core version %d.%d.%d\n",
 		MISDN_MAJOR_VERSION, MISDN_MINOR_VERSION, MISDN_RELEASE);
+	mISDN_init_clock(&debug);
 	mISDN_initstack(&debug);
 	err = mISDN_inittimer(&debug);
 	if (err)
diff --git a/drivers/isdn/mISDN/core.h b/drivers/isdn/mISDN/core.h
index 7da7233..7ac2f81 100644
--- a/drivers/isdn/mISDN/core.h
+++ b/drivers/isdn/mISDN/core.h
@@ -74,4 +74,6 @@
 extern int 	Isdnl2_Init(u_int *);
 extern void	Isdnl2_cleanup(void);
 
+extern void	mISDN_init_clock(u_int *);
+
 #endif
diff --git a/drivers/isdn/mISDN/dsp_cmx.c b/drivers/isdn/mISDN/dsp_cmx.c
index 04dbb40..efe4c74 100644
--- a/drivers/isdn/mISDN/dsp_cmx.c
+++ b/drivers/isdn/mISDN/dsp_cmx.c
@@ -1557,13 +1557,11 @@
 	schedule_work(&dsp->workq);
 }
 
-static u32	samplecount;
+static u32	jittercount; /* counter for jitter check */;
 struct timer_list dsp_spl_tl;
 u32	dsp_spl_jiffies; /* calculate the next time to fire */
-#ifdef UNUSED
-static u32	dsp_start_jiffies; /* jiffies at the time, the calculation begins */
-#endif /* UNUSED */
-static struct timeval dsp_start_tv; /* time at start of calculation */
+static u16	dsp_count; /* last sample count */
+static int	dsp_count_valid ; /* if we have last sample count */
 
 void
 dsp_cmx_send(void *arg)
@@ -1577,38 +1575,32 @@
 	int r, rr;
 	int jittercheck = 0, delay, i;
 	u_long flags;
-	struct timeval tv;
-	u32 elapsed;
-	s16 length;
+	u16 length, count;
 
 	/* lock */
 	spin_lock_irqsave(&dsp_lock, flags);
 
-	if (!dsp_start_tv.tv_sec) {
-		do_gettimeofday(&dsp_start_tv);
+	if (!dsp_count_valid) {
+		dsp_count = mISDN_clock_get();
 		length = dsp_poll;
+		dsp_count_valid = 1;
 	} else {
-		do_gettimeofday(&tv);
-		elapsed = ((tv.tv_sec - dsp_start_tv.tv_sec) * 8000)
-		    + ((s32)(tv.tv_usec / 125) - (dsp_start_tv.tv_usec / 125));
-		dsp_start_tv.tv_sec = tv.tv_sec;
-		dsp_start_tv.tv_usec = tv.tv_usec;
-		length = elapsed;
+		count = mISDN_clock_get();
+		length = count - dsp_count;
+		dsp_count = count;
 	}
 	if (length > MAX_POLL + 100)
 		length = MAX_POLL + 100;
-/* printk(KERN_DEBUG "len=%d dsp_count=0x%x.%04x dsp_poll_diff=0x%x.%04x\n",
- length, dsp_count >> 16, dsp_count & 0xffff, dsp_poll_diff >> 16,
- dsp_poll_diff & 0xffff);
- */
+	/* printk(KERN_DEBUG "len=%d dsp_count=0x%x\n", length, dsp_count); */
 
 	/*
-	 * check if jitter needs to be checked
-	 * (this is about every second = 8192 samples)
+	 * check if jitter needs to be checked (this is every second)
 	 */
-	samplecount += length;
-	if ((samplecount & 8191) < length)
+	jittercount += length;
+	if (jittercount >= 8000) {
+		jittercount -= 8000;
 		jittercheck = 1;
+	}
 
 	/* loop all members that do not require conference mixing */
 	list_for_each_entry(dsp, &dsp_ilist, list) {
diff --git a/include/linux/mISDNif.h b/include/linux/mISDNif.h
index f75d596..364f101 100644
--- a/include/linux/mISDNif.h
+++ b/include/linux/mISDNif.h
@@ -371,6 +371,7 @@
 #define DEBUG_L2_TEI		0x00100000
 #define DEBUG_L2_TEIFSM		0x00200000
 #define DEBUG_TIMER		0x01000000
+#define DEBUG_CLOCK		0x02000000
 
 #define mISDN_HEAD_P(s)		((struct mISDNhead *)&s->cb[0])
 #define mISDN_HEAD_PRIM(s)	(((struct mISDNhead *)&s->cb[0])->prim)
@@ -384,6 +385,7 @@
 struct mISDNchannel;
 struct mISDNdevice;
 struct mISDNstack;
+struct mISDNclock;
 
 struct channel_req {
 	u_int			protocol;
@@ -460,6 +462,16 @@
 #endif
 };
 
+typedef	int	(clockctl_func_t)(void *, int);
+
+struct	mISDNclock {
+	struct list_head	list;
+	char			name[64];
+	int			pri;
+	clockctl_func_t		*ctl;
+	void			*priv;
+};
+
 /* global alloc/queue functions */
 
 static inline struct sk_buff *
@@ -510,8 +522,13 @@
 extern void	mISDN_unregister_device(struct mISDNdevice *);
 extern int	mISDN_register_Bprotocol(struct Bprotocol *);
 extern void	mISDN_unregister_Bprotocol(struct Bprotocol *);
+extern struct mISDNclock *mISDN_register_clock(char *, int, clockctl_func_t *,
+						void *);
+extern void	mISDN_unregister_clock(struct mISDNclock *);
 
 extern void	set_channel_address(struct mISDNchannel *, u_int, u_int);
+extern void	mISDN_clock_update(struct mISDNclock *, int, struct timeval *);
+extern unsigned short mISDN_clock_get(void);
 
 #endif /* __KERNEL__ */
 #endif /* mISDNIF_H */