84
CHAPTER 3 CPU
MB89620 series
3.8 Memory Access Modes
3.8.5 Ready Operation
In external bus mode, the bus cycle can be extended by using the ready operation to
access to the low-speed external memory or peripheral functions.
n
Ready Operation
•
Externally inputting an “L” level to the ready (RDY) pin operates the ready function.
•
If the RDY pin goes to the “L” level when the CPU is performing external access, the latter
half of the bus cycle is extended in CPU operating clock cycle increments (CLK: divide-by-
two source oscillation).
•
As the CPU reads the RDY pin state on the rising edge of the CLK signal, the RDY pin must
be established at the “L” level before this timing.
•
The CPU wakes up from the ready operation by externally setting the RDY pin to the “H”
level and the bus cycle is complete.
•
The CPU ignores the RDY pin state when accessing to an internal area (internal access).
•
Pull-up the RDY pin if not using the ready function in external bus mode.
Figure 3.8.5a shows the ready operation.
Figure 3.8.5a Ready Operation
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Example Ready Input Circuit
Figure 3.8.5b Ready Signal Generator
CLK
RDY
Upper address
Lower
address
Write data
ALE
A08
to A15
AD0
to AD7
WR
Extended cycle
Extended bus cycle
AD0 to AD7
ALE
A08 to A15
RDY
CLK
D
G
A08 to A15
AD0 to AD7
S/L
Q
H
CLK
A
B
C
D
E
F
G
H
Q
8
8
MB89620 series
Chip select signal for the low-speed
external memory or peripheral function
Address latch
Address
decoder
CS
“H” level
Shift register
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