CONNE CT ION WIT H E X T E R NAL DE VICE S
3.4 Hold function
7721 Group User’s Manual
3–12
3.4 Hold function
When composing the external circuit which accesses the bus without using the central processing unit
(CPU), Hold function is used to generate a timing for transferring the right to use the bus from the CPU to
the external circuit.
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 3.4.1 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 3.4.1 Microcomputer’s state in Hold state
Item
Oscillation
φ
φ
CPU
_
E
7
, A
8
/D
8
to A
15
/D
15
, A
16
/D
0
to A
23
/D
7
, R/W,
___
____
BHE, BLE
Pins ALE, ST1
Pin ST0
Pin
φ
1
Pins P4
3
to P4
7
, P5 to P10
(Note)
Watchdog timer
Note:
This applies when this functions as a programmable I/O port.
__
State
Operating
Operating
Stopped at “L”
Stopped at “H”
Floating
Output “L” level.
Outputs “H” level.
Outputs clock
φ
1
.
Retain the state when Hold request was accepted.
Stopped
3.4.1 Operation description
Judgment of the HOLD pin input level is performed at every falling edge of
φ
1
. When “L” level is detected
at judgment of the input level, bus request (Hold) becomes “1,” when “H” level is detected, bus request
(Hold) becomes “0.”
Bus request (Hold) is sampled within a period when the bus request sampling signal is “1” and bus request
is accepted when there is no bus request (DRAMC). (This is called “Acceptance of Hold request.”) For bus
request, refer to section
“13.2.1 Bus access control circuit.”
When Hold request is accepted,
φ
CPU
stops at “L” level at the next rising edge of
φ
and the ST0 pin’s level
becomes “H,” the ST1 pin’s level becomes “L.” When 1 cycle of
φ
has passed after the levels of the ST0
and ST1 pins are changed, the R/W, BHE, BLE pins and the external bus enter the floating state.
In Hold state, when the HOLD pin’s input level becomes “H,” the ST0 and ST1 pins’ levels are changed
at the next rising edge of
φ
. When 1 cycle of
φ
has passed after the levels of the ST0 and ST1 pins are
changed, the microcomputer terminates Hold state.
Figures 3.4.1 to 3.4.3 show timing of acceptance of Hold request and termination of Hold state.
Note:
φ
has the same polarity and the same frequency as clock
φ
1
.
However,
φ
stops by acceptance of Ready request, or executing the
STP
or
WIT
instruction. Accordingly,
judgment of the input level of the HOLD pin is not performed during Ready state.