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XC2200 Derivatives
System Units (Vol. 1 of 2)
The External Bus Controller EBC
User’s Manual
9-4
V2.1, 2008-08
EBC_X8, V1.0d1
9.2
Timing Principles
The external bus timing is subdivided into six different timing phases (A-F).
9.2.1
Basic Bus Cycle Protocols
The phases A-F define the flow of all control signals needed for any access sequence to
external devices. At the beginning of a phase, the output signals may change within a
given output delay time. After the output delay time, the values of the control output
signals are stable within this phase. The output delay times are specified in the AC
characteristics (see Data Sheet). Each phase can occupy a programmable number of
clock cycles. The number of clock cycles is programmed in the TCONCSx register
selected via the related address range and CSx.
Figure 9-1
Phases of a Sequence of Several Accesses
Phase A is used for tristating databus drivers from the previous cycle (tristate wait states
after CS switch). Phase A cycles are not inserted at every access cycle but only when
changing the CS. If an access using one CS (CSx) was finished and the next access with
a different CS (CSy) is started then Phase A cycle(s) are performed according to the
control bits as set in the
first
CS (CSx).
The A Phase cycles are inserted while the addresses and ALE of the next cycle are
already applied.
The following diagrams show the 6 timing phases for read and write accesses on the
demultiplexed bus and the multiplexed bus.
Table 9-2
Write Configurations (see
Written Byte
General Write Configuration
Separated Byte Low/High Writes
Low
High
WR
BHE
ADDR[0] WRL
WRH
ADDR[0]
–
–
inactive
don’t care 0/1
inactive
inactive
0/1
write
–
active
inactive
0
active
inactive
0/1
–
write
active
active
1
inactive
active
0/1
write
write
active
active
0
active
active
0/1
B C D E F
MCA05373
Phases
A
A
B C D E F
A
B C D E F
A
B C D E F
A
Access n
Access n + 1
Access n + 2
Access n + 3
Address n
Address n + 1
Address n + 2
Address n + 3
Address
FCON of n
FCON of n + 1
FCON of n + 2
FCON of n + 3
FCONCSx