diff --git a/jtag/ChangeLog b/jtag/ChangeLog
index 7584b6c1..5890e352 100644
--- a/jtag/ChangeLog
+++ b/jtag/ChangeLog
@@ -1,3 +1,10 @@
+2008-12-22  Arnim Laeuger  <arniml@users.sourceforge.net>
+
+   * src/tap/usbconn/libftdi.c, src/tap/usbconn/libftd2xx.c,
+     include/usbconn/libftdx.h, src/tap/cable/ft2232.c:
+     increased MPSSE buffer size from 4k to 64k
+     mitigates performance decrease for mid-sized clock operations
+
 2008-12-21  Kolja Waschk <kawk>
 
    * src/svf/svf.c: Fix 2456021 - SVF compare_tdo flaw
diff --git a/jtag/include/usbconn/libftdx.h b/jtag/include/usbconn/libftdx.h
index ba224ac1..97c77824 100644
--- a/jtag/include/usbconn/libftdx.h
+++ b/jtag/include/usbconn/libftdx.h
@@ -24,6 +24,7 @@
 #define _USBCONN_LIBFTDX_H 1
 
 #define FTDX_MAXSEND 4096
+#define FTDX_MAXSEND_MPSSE (64 * 1024)
 
 /* Maximum chunk to receive from ftdi/ftd2xx driver.
    Larger values might speed up comm, but there's an upper limit
diff --git a/jtag/src/tap/cable/ft2232.c b/jtag/src/tap/cable/ft2232.c
index e2630da6..583aaf26 100644
--- a/jtag/src/tap/cable/ft2232.c
+++ b/jtag/src/tap/cable/ft2232.c
@@ -931,8 +931,11 @@ ft2232_clock_schedule( cable_t *cable, int tms, int tdi, int n )
   cx_cmd_queue( cmd_root, 0 );
   while (n > 0)
   {
-    if (cx_cmd_space( cmd_root, 4096 ) < 3)
+    if (cx_cmd_space( cmd_root, FTDX_MAXSEND_MPSSE ) < 4)
     {
+      /* no space left for Clock Data plus Send Immediate
+         transfer queued commands to device and read receive data
+         to internal buffer */
       cx_xfer( cmd_root, &imm_cmd, cable, COMPLETELY );
       cx_cmd_queue( cmd_root, 0 );
     }
@@ -1054,6 +1057,10 @@ ft2232_transfer_schedule( cable_t *cable, int len, char *in, char *out )
     /* reduce chunkbytes to the maximum amount we can receive in one step */
     if (out && chunkbytes > FTDX_MAXRECV)
       chunkbytes = FTDX_MAXRECV;
+    /* reduce chunkbytes to the maximum amount that fits into one buffer
+       for performance reasons */
+    if (chunkbytes > FTDX_MAXSEND_MPSSE - 4)
+      chunkbytes = FTDX_MAXSEND_MPSSE - 4;
     /* restrict chunkbytes to the maximum amount that can be transferred
        for one single operation */
     if (chunkbytes > (1 << 16))
diff --git a/jtag/src/tap/usbconn/libftd2xx.c b/jtag/src/tap/usbconn/libftd2xx.c
index 597bb23e..488de730 100644
--- a/jtag/src/tap/usbconn/libftd2xx.c
+++ b/jtag/src/tap/usbconn/libftd2xx.c
@@ -432,6 +432,9 @@ usbconn_ftd2xx_mpsse_open( usbconn_t *conn )
   if (status == FT_OK) if ((status =  FT_Purge( fc, FT_PURGE_RX )) != FT_OK)
     printf( _("%s(): Can't purge RX buffer.\n"), __FUNCTION__ );
 
+  if (status == FT_OK) if ((status = FT_SetUSBParameters( fc, FTDX_MAXSEND_MPSSE, FTDX_MAXSEND_MPSSE )) != FT_OK)
+    printf( _("%s(): Can't set USB parameters.\n"), __FUNCTION__ );
+
   if (status == FT_OK) if ((status = FT_SetChars( fc, 0, 0, 0, 0 )) != FT_OK)
     printf( _("%s(): Can't set special characters.\n"), __FUNCTION__ );
 
diff --git a/jtag/src/tap/usbconn/libftdi.c b/jtag/src/tap/usbconn/libftdi.c
index e254ab39..0f4efdfb 100644
--- a/jtag/src/tap/usbconn/libftdi.c
+++ b/jtag/src/tap/usbconn/libftdi.c
@@ -461,6 +461,11 @@ usbconn_ftdi_mpsse_open( usbconn_t *conn )
   r = seq_reset( fc );
   if (r >= 0) r = seq_purge( fc, 1, 0 );
 
+  if (r >= 0) if ((r = ftdi_write_data_set_chunksize( fc, FTDX_MAXSEND_MPSSE )) < 0)
+    puts( ftdi_get_error_string( fc ) );
+  if (r >= 0) if ((r = ftdi_read_data_set_chunksize( fc, FTDX_MAXSEND_MPSSE )) < 0)
+    puts( ftdi_get_error_string( fc ) );
+
 #ifdef LIBFTDI_UNIMPLEMENTED
   if (r >= 0) if ((r = ftdi_set_event_char( fc, 0, 0 )) < 0)
     puts( ftdi_get_error_string( fc ) );