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CONNECTION WITH EXTERNAL DEVICES
12.4 Hold function
12.4 Hold function
When composing the external circuit (DMA) which accesses the bus without using the central processing
unit (CPU), the Hold function is used to generate a timing for transferring the right to use the bus from the
CPU to the external circuit.
In the memory expansion or microprocessor mode, the microcomputer enters Hold state by input of "L" level
_____
to the HOLD pin and retains this state while the level of the HOLD pin is at "L." Table 12.4.1 lists the
microcomputer's state in Hold state.
In Hold state, the oscillation of the oscillator does not stop. Accordingly, the internal peripheral devices can
operate. However, Watchdog timer stops operating.
Table 12.4.1 Microcomputer's state in Hold state
Item
Oscillation
φ
CPU
φ
, φ
BIU
_
E
Pins A
to A
, A
/D
to A
0
7
8
8
15
__
A
/D
to A
/D
, R/W, BHE
16
0
23
7
_____
Pins HLDA, ALE
/ φ
Pin P4
2
1
Pins P4
to P4
, P5 to P8 (Note)
3
7
Watchdog timer
Clock φ
output select bit : Bit 7 at address 5E
1
Note: When this functions as a programmable I/O port.
12–18
Operating
Stopped at "L"
Operating
Stopped at "H"
/D
,
Floating
15
___
Outputs "L" level.
In the memory expansion mode:
•When clock φ
•When clock φ
request was accepted.
In the microprocessor mode:
•This pin outputs clock φ
Retains the state when Hold request was accepted.
Stopped
7751 Group User's Manual
_____
State
output select bit = "1," this pin outputs clock φ
1
output select bit = "0," this pin retains the state when Hold
1
.
1
16
.
1
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