W79E225A_08 Datasheet, PDF (84/203 Pages) Winbond – 8-bit Microcontroller

Preliminary W79E225A/226A/227A Data Sheet
8.1 Instruction Timing
This section is important because some applications use software instructions to generate timing
delays. It also provides more information about timing differences between the W79E22X SERIES and
the standard 8051/52.
In W79E22X SERIES, each machine cycle is four clock periods long. Each clock period is called a
state, and each machine cycle consists of four states: C1, C2 C3 and C4, in order. Both clock edges
are used for internal timing, so the duty cycle of the clock should be as close to 50% as possible.
The W79E22X SERIES does one op-code fetch per machine cycle, so, in most instructions, the
number of machine cycles required is equal to the number of bytes in the instruction. There are 256
available op-codes. 128 of them are single-cycle instructions, so many op-codes are executed in just
four clock periods. Some of the other op-codes are two-cycle instructions, and most of these have
two-byte op-codes. However, there are some instructions that have one-byte instructions yet take two
cycles to execute. One important example is the MOVX instruction.
In the standard 8051/52, the MOVX instruction is always two machine cycles long. However, in the
W79E22X SERIES, the duration of this instruction is controlled by the software. It can vary from two to
nine machine cycles long, and, RD and WR strobe lines are elongated proportionally. This is called
stretching, and it gives a lot of flexibility when accessing fast and slow peripherals. It also reduces the
amount of external circuitry and software overhead.
The rest of the instructions are three-, four- or five-cycle instructions. At the end of this section, there
are timing diagrams that provide an example of each type of instruction (single-cycle, two-cycle, …).
In summary, there are five types of instructions in the W79E22X SERIES, based on the number of
machine cycles, and each machine cycle is four clock periods long. The standard 8051/52 has only
three types of instructions, based on the number of machine cycles, but each machine cycle is twelve
clock periods long. As a result, even though the number of categories is higher, each instruction in the
W79E22X SERIES runs 1.5 to 3 times faster, based on the number of clock periods, than it does in
the standard 8051/52.
CLK
ALE
PSEN
AD7-0
PORT 2
Single Cycle
C1
C2
C3
C4
A7-0
Data_ in D7-0
Address A15-8


Figure 8-1: Single Cycle Instruction Timing
- 84 -
Publication Release Date: April 15, 2008
Revision A4.0