1. Example: Addressing modes
Write code to send 55H to ports P1 and P2, using (a) their names (b) their addresses
Solution :
(a) MOV A,#55H ; A=55H
MOV P1,A ; P1=55H
MOV P2,A ; P2=55H
(b) P1 address=90H; P2 address=A0H
MOV A,#55H ; A=55H
MOV 90H,A ; P1=55H
MOV 0A0H,A ; P2=55H
23-Nov-18
Micro 8051

2. Hardware Structure of I/O Pin
D Q
Clk Q
Vcc
Internal
Pull-Up
Read latch
Read pin
Write to latch
Internal CPU bus M1
P1.X pin
P1.X B1 B2
23-Nov-18
Micro 8051

3. Hardware Structure of I/O Pin
Each pin of I/O ports
Internally connected to CPU bus
A D latch store the value of this pin
Write to latch＝1：write data into the D latch
2 Tri-state buffer：
B1: controlled by “Read pin”
Read pin＝1：really read the data present at the pin
B2: controlled by “Read latch”
Read latch＝1：read value from internal latch
A transistor M1 gate
Gate=0: open
Gate=1: close
23-Nov-18
Micro 8051

4. Port-3 Alternate I/O
23-Nov-18
Micro 8051

5. 23-Nov-18
Micro 8051