mcf51jf128 Freescale Semiconductor, Inc, mcf51jf128 Datasheet - Page 1180

• Manufacturer Part Number: mcf51jf128
• Description: Mcf51jf128 Reference Manual
• Manufacturer: Freescale Semiconductor, Inc

Application Information
45.6.1 Application 1: Simple Square-Wave Generation
In this example, several different instruction loops are executed, each generating a
square-wave output with a 50% duty cycle. For this analysis, the executed code is
mapped into the processor's RAM. This configuration is selected to remove any jitter
from the output square wave caused by the limitations defined by the two-cycle flash
memory accesses and restrictions on the initiation of a flash access. The following
instruction loops were studied:
The square-wave output frequency was measured and the relative performance results are
presented in the following table. The relative performance is stated as a fraction of the
processor's operating frequency, defined as f MHz. The performance of the BCHG loop
operating on a GPIO output is selected as the reference.
1180
• BCHG_LOOP — In this loop, a bit change instruction was executed using the GPIO
• SET+CLR_LOOP — For this construct, two store instructions are executed: one to
set+clr (+toggle)
data byte as the operand. This instruction performs a read-modify-write operation
and inverts the addressed bit. A pulse counter is decremented until the appropriate
number of square-wave pulses have been generated.
set the GPIO data pin and another to clear it. Single-cycle NOP instructions (the tpf
opcode) are included to maintain the 50% duty cycle of the generated square wave.
The pulse counter is decremented until the appropriate number of square-wave pulse
have been generated.
bchg
Loop
The square-wave frequency is measured from rising-edge to
rising-edge, where the output wave has a 50% duty cycle.
(1/24) × f MHz
(1/12) × f MHz
Table 45-12. Square-Wave Output Performance
Frequency
Sq-Wave
MCF51JF128 Reference Manual, Rev. 2, 03/2011
Peripheral Bus-mapped GPIO
CPU f = 50 MHz
Frequency @
2.083 MHz
4.167 MHz
Preliminary
Note
Relative
Speed
1.00x
2.00x
(1/14) × f MHz
(1/8) × f MHz
Frequency
Sq-Wave
Freescale Semiconductor, Inc.
CPU f = 50 MHz
Frequency @
RGPIO
3.571 MHz
6.250 MHz
Relative
Speed
1.71x
3.00x