Figure 50. Internal Clock Signals
STATE4
INTERNAL
STATE5
P1 P2
STATE6
P1 P2
STATE1
STATE2
P1 P2
STATE3
P1 P2
STATE4
P1 P2
STATE5
P1 P2
CLOCK
P1
P2
P1
P2
XTAL2
ALE
THESE SIGNALS ARE NOT ACTIVATED DURING THE
EXECUTION OF A MOVX INSTRUCTION
EXTERNAL PROGRAM MEMORY FETCH
PSEN
P0
DATA
PCL OUT
DATA
PCL OUT
DATA
PCL OUT
SAMPLED
SAMPLED
SAMPLED
FLOAT
FLOAT
FLOAT
P2 (EXT)
INDICATES ADDRESS TRANSITIONS
READ CYCLE
RD
PCL OUT (IF PROGRAM
MEMORY IS EXTERNAL)
DPL OR Rt OUT
DATA
SAMPLED
P0
FLOAT
P2
INDICATES DPH OR P2 SFR TO PCH TRANSITION
WRITE CYCLE
WR
PCL OUT (EVEN IF PROGRAM
MEMORY IS INTERNAL)
DPL OR Rt OUT
P0
PCL OUT (IF PROGRAM
MEMORY IS EXTERNAL)
DATA OUT
P2
INDICATES DPH OR P2 SFR TO PCH TRANSITION
PORT OPERATION
MOV PORT SRC
OLD DATA
NEW DATA
P0 PINS SAMPLED
P0 PINS SAMPLED
MOV DEST P0
MOV DEST PORT (P1. P2. P3)
P1, P2, P3 PINS SAMPLED
RXD SAMPLED
P1, P2, P3 PINS SAMPLED
(INCLUDES INTO. INT1. TO T1)
SERIAL PORT SHIFT CLOCK
RXD SAMPLED
TXD (MODE 0)
This diagram indicates when signals are clocked internally. The time it takes the signals to propagate to the pins, however,
ranges from 25 to 125 ns. This propagation delay is dependent on variables such as temperature and pin loading. Propaga-
tion also varies from output to output and component. Typically though (TA = 25°C fully loaded) RD and WR propagation
delays are approximately 50 ns. The other signals are typically 85 ns. Propagation delays are incorporated in the AC
specifications.
118
AT89C51RB2/RC2
4180E–8051–10/06
ATMEL [ ATMEL ]
