Philips Semiconductors

Product specification

80C451/83C451/87C451

80C51 8-bit microcontroller family

4K/128 OTP/ROM/ROMless, expanded I/O

1998 May 01

13

EXPLANATION OF THE AC SYMBOLS

Each timing symbol has five characters. The first character is always

‘t’ (= time). The other characters, depending on their positions,

indicate the name of a signal or the logical status of that signal. The

designations are:

A – Address

C – Clock

D – Input data

H – Logic level high

I – Instruction (program memory contents)

L – Logic level low, or ALE

P – PSEN

Q – Output data

R– RD signal

t – Time

V – Valid

W– WR signal

X – No longer a valid logic level

Z – Float

ALE

PSEN

PORT 0

PORT 2

A0–A15

A8–A15

A0–A7

A0–A7

INSTR IN

SU00006

Figure 2. External Program Memory Read Cycle

ALE

PSEN

PORT 0

PORT 2

RD

A0–A7

FROM RI OR DPL

DATA IN

A0–A7 FROM PCL

INSTR IN

P2.0–P2.7 OR A8–A15 FROM DPH

A0–A15 FROM PCH

SU00007

Figure 3. External Data Memory Read Cycle