
CPU16
REFERENCE MANUAL
INSTRUCTION TIMING
MOTOROLA
8-1
SECTION 8 INSTRUCTION TIMING
This section gives detailed information concerning calculating the amount of time re-
quired to execute instructions.
8.1 Execution Time Components
CPU16 instruction execution time has three components:
Bus cycles required to prefetch the next instruction.
Bus cycles required for operand accesses.
Clock cycles required for internal operations.
Each bus cycle requires a minimum of two system clock cycles. If the time required to
access an external device exceeds two system clock cycles, bus cycles must be long-
er. However, all bus cycles must be made up of an integer number of clock cycles.
CPU16 internal operations always require an integer multiple of two system clock cy-
cles.
NOTE
To avoid confusion between bus cycles and system clock cycles, this
discussion subsequently refers to the time required by system clock
cycles, or clock periods, rather than to the clock cycles themselves.
Dynamic bus sizing affects bus cycle time. The CPU16 is a component of a modular
microcontroller. Modules in the system communicate via a standardized intermodule
bus and access external devices via an external bus interface. The microcontroller
system integration module manages all accesses in order to make more efficient use
of common resources. See
SECTION 3 SYSTEM RESOURCES
for more information.
The CPU16 does not execute more than one instruction at a time. The total time re-
quired to execute a particular instruction stream can be calculated by summing the in-
dividual execution times of each instruction in the stream.
Total execution time is calculated using the expression:
(CL
T
) = (CL
P
)
+
(CL
O
)
+
(CL
I
)
Where:
(CL
T
) = Total clock periods per instruction
(CL
I
) = Clock periods used for internal operation
(CL
P
) = Clock periods used for program access
(CL
O
) = Clock periods used for operand access
CL
T
is the value provided in the instruction glossary pages.
F
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
.