W79E225A_08 Datasheet, PDF (86/203 Pages) Winbond – 8-bit Microcontroller

Preliminary W79E225A/226A/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 W79E22X SERIES. In the standard
8051/52, the MOVX instruction has a fixed execution time of 2 machine cycles. However, in W79E22X
SERIES, 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,
W79E22X SERIES 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: April 15, 2008
Revision A4.0