AVRDUDE gives 'invalid device signature' message when attempting to flash 16U2

Hi all

I got a second uno r3 to use as an ICSP programmer, in an attempt to rewrite the firmware on my first uno's unresponsive 16U2. I've connected the boards up as specified in http://arduino.cc/en/Hacking/Upgrading16U2Due, then ran avrdude using the following parameters:

avrdude -C ..\etc\avrdude.conf -p at90usb162 -F -P COM13 -c arduino -U flash:w:UNO-dfu_and_usbserial_combined.hex -vvv -b 19200

It gives the following output - looks as though it can't read the target 16U2's signature. Can anyone advise what I'm doing wrong? BTW, the reason I'm taking this approach to update the firmware is because I'm unable to get this 16U2 to enter DFU mode, using the reset/gnd short method.

avrdude: Version 5.11, compiled on Sep  2 2011 at 19:38:36
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2009 Joerg Wunsch

         System wide configuration file is "..\etc\avrdude.conf"

         Using Port                    : COM13
         Using Programmer              : arduino
         Overriding Baud Rate          : 19200
         AVR Part                      : AT90USB162
         Chip Erase delay              : 9000 us
         PAGEL                         : PD7
         BS2                           : PC6
         RESET disposition             : possible i/o
         RETRY pulse                   : SCK
         serial program mode           : yes
         parallel program mode         : yes
         Timeout                       : 200
         StabDelay                     : 100
         CmdexeDelay                   : 25
         SyncLoops                     : 32
         ByteDelay                     : 0
         PollIndex                     : 3
         PollValue                     : 0x53
         Memory Detail                 :

                                  Block Poll               Page
      Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  Max
W   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ---
-- ---------
           eeprom        65    20     4    0 no        512    4    128  9000  90
00 0x00 0x00
                                  Block Poll               Page
      Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  Max
W   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ---
-- ---------
           flash         65     6   128    0 yes     16384  128    128  4500  45
00 0x00 0x00
                                  Block Poll               Page
      Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  Max
W   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ---
-- ---------
           lfuse          0     0     0    0 no          1    0      0  9000  90
00 0x00 0x00
                                  Block Poll               Page
      Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  Max
W   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ---
-- ---------
           hfuse          0     0     0    0 no          1    0      0  9000  90
00 0x00 0x00
                                  Block Poll               Page
      Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  Max
W   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ---
-- ---------
           efuse          0     0     0    0 no          1    0      0  9000  90
00 0x00 0x00
                                  Block Poll               Page
      Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  Max
W   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ---
-- ---------
           lock           0     0     0    0 no          1    0      0  9000  90
00 0x00 0x00
                                  Block Poll               Page
      Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  Max
W   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ---
-- ---------
           calibration    0     0     0    0 no          1    0      0     0
 0 0x00 0x00
                                  Block Poll               Page
      Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  Max
W   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ---
-- ---------
           signature      0     0     0    0 no          3    0      0     0
 0 0x00 0x00

         Programmer Type : Arduino
         Description     : Arduino
         Hardware Version: 2
         Firmware Version: 1.18
         Topcard         : Unknown
         Vtarget         : 0.0 V
         Varef           : 0.0 V
         Oscillator      : Off
         SCK period      : 0.1 us

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.02s

avrdude: Device signature = 0xffffff
avrdude: Yikes!  Invalid device signature.
avrdude: Expected signature for AT90USB162 is 1E 94 82
avrdude: safemode read 1, lfuse value: ff
avrdude: safemode read 2, lfuse value: ff
avrdude: safemode read 3, lfuse value: ff
avrdude: safemode: lfuse reads as FF
avrdude: safemode read 1, hfuse value: ff
avrdude: safemode read 2, hfuse value: ff
avrdude: safemode read 3, hfuse value: ff
avrdude: safemode: hfuse reads as FF
avrdude: safemode read 1, efuse value: ff
avrdude: safemode read 2, efuse value: ff
avrdude: safemode read 3, efuse value: ff
avrdude: safemode: efuse reads as FF
avrdude: NOTE: FLASH memory has been specified, an erase cycle will be performed

         To disable this feature, specify the -D option.
avrdude: erasing chip
avrdude: reading input file "UNO-dfu_and_usbserial_combined.hex"
avrdude: input file UNO-dfu_and_usbserial_combined.hex auto detected as Intel He
x
avrdude: writing flash (7440 bytes):

Writing | ################################################## | 100% 9.09s

avrdude: 7440 bytes of flash written
avrdude: verifying flash memory against UNO-dfu_and_usbserial_combined.hex:
avrdude: load data flash data from input file UNO-dfu_and_usbserial_combined.hex
:
avrdude: input file UNO-dfu_and_usbserial_combined.hex auto detected as Intel He
x
avrdude: input file UNO-dfu_and_usbserial_combined.hex contains 7440 bytes
avrdude: reading on-chip flash data:

Reading | ################################################## | 100% 6.53s

avrdude: verifying ...
avrdude: verification error, first mismatch at byte 0x0000
         0x9c != 0xff
avrdude: verification error; content mismatch

avrdude: safemode read 1, lfuse value: ff
avrdude: safemode read 2, lfuse value: ff
avrdude: safemode read 3, lfuse value: ff
avrdude: safemode: lfuse reads as FF
avrdude: safemode read 1, hfuse value: ff
avrdude: safemode read 2, hfuse value: ff
avrdude: safemode read 3, hfuse value: ff
avrdude: safemode: hfuse reads as FF
avrdude: safemode read 1, efuse value: ff
avrdude: safemode read 2, efuse value: ff
avrdude: safemode read 3, efuse value: ff
avrdude: safemode: efuse reads as FF
avrdude: safemode: Fuses OK

avrdude done.  Thank you.

A signature of FFFFFF would indicate a bad connection.