Rev. 3.0, 03/01, page 47 of 390
5.4.2
High Speed Timing
With the programmable bit (SLS) contained in System Configuration Register (SCONFR), the
HD64465 is able to increase the system performance, depending on different bus clock (CKIO)
rates. When SLS bit is programmed with 0, the internal bus timing is switched to the basic cycle
composed of three wait states. This is because the internal H/W latency related to such a high
speed clock rate. Though the High Speed Timing has to go through 3 wait states, which is one
more wait state than the 2 wait states for Low Speed Timing, the performance of the High Speed
Timing is still higher. This is because the High Speed Timing has higher CKIO rate. For example
25Mhz basic timing (4 clock cycles) needs 160 ns but 66Mhz basic timing (5 clock cycles) only
requires 75 ns to complete.
High-Speed Basic Internal Peripheral Bus Access Timing is shown in the Figure 5-4. These
basic cycles are T1, TWs1, TWs2, and TWs3 and T2 phases. Note that three wait states (TWs1,
TWs2, and TWs3) are in command cycle. In this case, no external peripheral hardware wait is
asserted. The *WAIT# signal is kept high before T2 stage. This means that peripheral module does
not need external cycles to accomplish the command. Therefore, the command cycle enters the T2
phase after TWs2. At the end of T2 phase, the question that either T1 or T_idle phase is followed is
determined by the configuration of the host CPU bus idle state. If host CPU configures at least one
idle state, the corresponding T_idle phase is followed. If host CPU configures no idle state, the T1
phase is followed after T2.
T1
T2
TWs3
T_idle(T1)
TWs1
t
AD
t
MSD
t
MSD
t
RWD
t
RDD
t
WED
t
WDD
t
WDYS
t
WDYH
t
RDD
t
RDH
t
WED
t
RDS
TWs2
CKIO
A20-A1
A24,A25
IMADDR
*
MS#
*
WAIT#
IMRDWR#
IMRD#
(READ)
D31-D0
MODATA
D31-D0
MIDATA
IMWE0/1/2/3#
(WRITE)
Figure 5.4 High-Speed Basic Internal Peripheral Bus Access Timing
Содержание HD64465
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