
Preliminary W79E225A/227A Data Sheet
Publication Release Date: December 14, 2007
- 84 -
Revision A2.0
OPERAND
OPERAND
OPERAND
OP-CODE
Address A15-8
Address A15-8
Address A15-8
Address A15-8
A7-0
A7-0
A7-0
A7-0
Operand Fetch
Operand Fetch
Operand Fetch
Instruction Fetch
C2
C1
C4
C3
C2
C1
CLK
ALE
PSEN
AD7-0
Port 2
C4
C3
C2
C1
C4
C3
C2
C1
C4
C3
Figure 8-4: Four Cycles Instruction Timing
OPERAND
OPERAND
OPERAND
OP-CODE
Address A15-8
Address A15-8
Address A15-8
Address A15-8
A7-0
A7-0
A7-0
A7-0
Operand Fetch
Operand Fetch
Operand Fetch
Operand Fetch
Instruction Fetch
C2
C1
C4
C3
C2
C1
CLK
ALE
PSEN
AD7-0
PORT 2
C4
C3
C2
C1
C4
C3
C2
C1
C4
C3
C2
C1
C4
C3
OPERAND
A7-0
Address A15-8
Figure 8-5: Five Cycles Instruction Timing
8.1.1 External Data Memory Access Timing
The timing for the MOVX instruction is another feature of the W79E225/227. In the standard 8051/52,
the MOVX instruction has a fixed execution time of 2 machine cycles. However, in W79E225/227, the
duration of the access can be controlled by the user.
The instruction starts off as a normal op-code fetch that takes four clocks. In the next machine cycle,
W79E225/227 puts out the external memory address, and the actual access occurs. The user can
control the duration of this access by setting the stretch value in CKCON, bits 2 – 0. As shown in the
table below, these three bits can range from zero to seven, resulting in MOVX instructions that take
two to nine machine cycles. The default value is one, resulting in a MOVX instruction of three machine
cycles.