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Chapter 9. System Interface Operation
9-55
MPX Bus Protocol
DRDY for additional data-only transactions before previous data-only transactions have
completed their data tenures. There is no restriction on the maximum number of
outstanding DRDY assertions. The MPC7400 can assert DRDY for the same number of
times corresponding to the number of outstanding HIT assertions. The MPC7400 supports
six outstanding transactions if no DBGs have been issued for any of the queued data
transactions.
Because DRDY is a pulsed signal, if DRDY is held low for multiple cycles, the system
interprets this as multiple assertions of DRDY. If a device asserts DRDY when the system
is not expecting a DRDY (no pending data-only transaction has been indicated), the system
ignores the DRDY signal. An important example of this would be the cycle after an ARTRY.
See Section 9.6.2.2.6, òRetrying Data-Only Transactions.ó An example of pipelined
data-only transactions is shown in Figure 9-27. (Note that Figure 9-27 assumes that data is
all driven from the same source, so that data streaming is possible.)
Figure 9-27. Pipelined Data-Only Transactions
9.6.2.2.6 Retrying Data-Only Transactions
As described in Section 9.6.1.4.4, òHIT Signal Timing and Data SnarTng,ó it is possible for
the MPC7400 to signal a data-only transaction with the HIT signal while another device
asserts ARTRY. In this case the data-only transaction must be considered retried, and the
system does not expect a corresponding DRDY. If DRDY was asserted simultaneously with
HIT, it is ignored. In general, if DRDY is asserted by the MPC7400 when the system is not
expecting a DRDY, it will be ignored. So if the MPC7400 asserts DRDY after the
D
B
C
D
SYSCLK
Cycles 0, 2, 4, and 6: The device asserts HIT for transactions A, B, C, and D.
Cycles 0 and 2: The device asserts DRDY for the Trst A and B respectively.
Cycle 4: DBG for transaction A issued. DRDY for transaction C is delayed for some
reason.
Cycle 5: Data for transaction A is driven. DRDY for transaction C still delayed.
Cycle 6: Transaction D can be driven even though no DRDY for transaction C and DBG
for transactions B and C have been received.
Cycle 8: DBG for transaction B issued. DRDY for transaction C still delayed.
Cycle 9: Data for transaction B is driven. DRDY for transaction C still delayed.
Cycle 11: DRDY for transaction C is driven.
Cycle 12: DRDY for transaction D is pipelined. DBG for transaction C issued.
B
C
B
C
D
A
B
C
D
DRDY
DBG
TA
A
1
2
3
4
5
6
Cycle
HIT
7
8
9
10
11 12
13
14 15 16
17 18
19 20
21 22
A
A