DS471 Datasheet, PDF (18/37 Pages) Xilinx, Inc – Channel FIFO

Channel FIFO (CFIFO) (v1.00a)
RFIFO Channel Number Context Switch Operational Description
When either of the channel number inputs to the RFIFO change, an internal context switch occurs in
which the status and state of the previous channel are saved, and the status and state of the new
channel must be retrieved, loaded, and output to the appropriate port. This context switch takes several
clock cycles to complete. During a context switch initiated by a channel number change on the
communications side, the discrete status outputs emitted by the RFIFO on the communications side
become invalid as the previous channel’s status is replaced by the new channel’s status. The channel
valid signal (RFIFO2Comm_) indicates when the context switch has completed and that the new
discrete status outputs are valid. The channel valid signal also indicates that data transfers can take
place on that channel.
An example of a context switch on the Comm Interface is shown in Figure 13. After the Comm Interface
completes a write on channel 1, it then changes Comm2RFIFO_ to channel 2 in clock cycle five. Note
that Comm2RFIFO_ is deasserted in cycle four and then reasserts in cycle 6 (it could have reasserted in
cycle 5, but it must be deasserted in cycle 4). RFIFO2Comm_ negates indicating that the discrete
outputs are currently not valid. It asserts three clocks later, indicating that the context switch is
complete. The RFIFO asserts RFIFO2Comm_ in the same cycle, indicating that the write transaction has
completed.
Figure Top x-ref 13
Clk
Rst
Bus2RFIFO_Ch_Num[0:7]
Bus2RFIFO_Status_RdReq
Bus2RFIFO_Burst
Bus2RFIFO_Data_RdReq
RFIFO2Bus_Ack
RFIFO2Bus_ErrAck
RFIFO2Bus_Ch_Valid
RFIFO2Bus_DBus[0:31]
Comm2RFIFO_Ch_Num[0:7]
RFIFO2Comm_Ch_Valid
Comm2RFIFO_Burst
Comm2RFIFO_WrReq
RFIFO2Comm_Ack
RFIFO2Comm_ErrAck
Comm2RFIFO_DBus[0:31]
RFIFO2Comm_HalfFull
RFIFO2Comm_AlmostFull
RFIFO2Comm_Full
RFIFO2Comm_Vacancy[0:7]
6000
0001
01
0123
4567
FF
FE
01
02
89AB
CDEF
1122
3344
5566
7788
9900
0000
FE
FD
FC
FB
Figure 13: Comm Interface Context Switch
DS471_13_101405
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DS471 April 24, 2009
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