PD780 datasheet(3/24 Pages) NEC

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ttiEC

RD (Read)

This three-state active low output is used to strobe

data from external memory or 1/0 devices onto the data

bus. RD is asserted to indicate the CPU is requesting

data from external memory or 1/0 devices. This line is

three-stated during halt or reset conditions.

MREQ (Memory Request)

This three-state active low output is used to indicate

that the address specified for the memory read or write

is valid.

M1 (Machine Cycle 1)

This active low output is used to indicate that the cur

rent machine cycle is the opcode fetch phase of an

instruction execution.

HALT (Halt State)

This active low input is used with the HALT instruc

tion to initiate a halt state. When HALT is asserted, pro

gram execution stops and does not resume until an in

terrupt is generated. During the halt state, NOPs are

executed in order to continue memory refresh

operations.

WAIT (Wait State)

This active low input is used to indicate that the ex

ternal memory or 1/0 devices addressed by the CPU

are not ready to transfer data. When WAIT is asserted,

the CPU is placed in a wait condition.

�PD780

RESET

This active low input signal is used to initialize the

CPU. When RESET is asserted, the interrupt enable flip

flop is reset, the program counter and the

I and R

registers are cleared, and interrupt response mode 0

is enabled. In a reset condition, the address and data

busses are three-stated and all output control signals

are inactive, after which program execution begins

from address 0000.

The pulse width of RESET must be a minimum of 3

clock cycles in length to reinitialize the CPU and

stabilize operation.

RFSH (Refresh)

This active low output is used in conjunction with the

MREQ signal to initiate a refresh read of all external

dynamic memory. RFSH and MREQ are both asserted

when the least significant 7 bits of the address on the

address bus hold a valid external dynamic memory

address.

� (Clock)

This line is an input for external clock sources.

+5 V

Single +5 V power supply.

GND

Ground.

4-5

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