Destination
Source
Destination
Source
Multicast
transfer
mode
Unicast
transfer
mode
(a)
(b)
Unicast transfer mode
240
Network-on-Chip
Figure 8.2
(a) Unicast data transfer; (b) multicast data transfer.
If the underlying NoC routers support multicast mode of transmission, it can
be utilized for testing as well. Otherwise, it is suggested that a multicast wrapper
unit (MWU) be designed. It is transparent to the packets in normal operation
mode. In the test mode, the routing logic block (RLB) of the switch is completely
bypassed. This is achieved by a set of multiplexers and demultiplexers that aid
in bypassing the RLB. As test scheduling is done offline, for each NoC switch,
the subset of input and output ports participating in multicast test data transfer is known beforehand. The MWU design for the switch can be restricted
accordingly. For example, in Figure 8.2, only a few of the switches need multicast facility. However, a proper scheduling of testing the components (i.e.,
switches and links) is necessary. In order to search for this optimal schedule,
it is required to consider the following test times for individual components.
• T l,L : The latency of interswitch link L (generally equal to one clock
cycle if the link is reasonably short)
• T t,L : The time to test the link L
• T l,S : The switch latency (the number of clock cycles required for a flit
to traverse a NoC switch from input to output)
• T t,S : The time required to perform the actual testing of the switch
[Time needed to test the RLB (T RLB ) + Time to test FIFO (T FIFO )]
For example, in a unicast mode of transmission, the total test time T
u
2, 3 to test
both the switches S 2 and S 3 , as shown in Figure 8.3, is given by
T
u
2,3 = 2(T l , L + T l ,S ) + 2T t , S
For a multicast environment, the test time T
m
2, 3 to test the switches S 2 and S 3
is given by
T
m
2, 3 = (T l , L + T l , S ) + T t ,S
The time to test a switch T t,S , or a link T t,L , is determined by the fault model and the
design of these components. However, the test transport time can be controlled