W79E225A Datasheet, PDF (84/200 Pages) Winbond – 8-bit Microcontroller

Preliminary W79E225A/227A Data Sheet
Instruction Fetch
Operand Fetch
Operand Fetch
Operand Fetch
C1 C2 C3 C4 C1 C2 C3 C4 C1 C2 C3 C4 C1 C2 C3 C4
CLK
ALE
PSEN
AD7-0
A7-0 ^OP-CODE
A7-0 OPERAND
A7-0 OPERAND
A7-0 OPERAND
Port 2
Address A15-8
Address A15-8
Address A15-8
Address A15-8
Figure 8-4: Four Cycles Instruction Timing
Instruction Fetch Operand Fetch
Operand Fetch
Operand Fetch
Operand Fetch
C1 C2 C3 C4 C1 C2 C3 C4 C1 C2 C3 C4 C1 C2 C3 C4 C1 C2 C3 C4
CLK
ALE
PSEN
AD7-0
A7-0 OP-CODE
A7-0 ^OPERAND
A7-0 OPERAND
A7-0 OPERAND A7-0 OPERAND
PORT 2
Address A15-8
Address A15-8
Address A15-8
Address A15-8
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.
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Publication Release Date: December 14, 2007
Revision A2.0