mc9s08ac32 Freescale Semiconductor, Inc, mc9s08ac32 Datasheet - Page 188

• Manufacturer Part Number: mc9s08ac32
• Description: Hcs08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Chapter 10 Internal Clock Generator (S08ICGV4)
10.6.5
The internally generated clock source is guaranteed to have a period ± 25% of the nominal value. In some
cases, this may be sufficient accuracy. For other applications that require a tight frequency tolerance, a
trimming procedure is provided that will allow a very accurate source. This section outlines one example
of trimming the internal oscillator. Many other possible trimming procedures are valid and can be used.
In this particular case, the MCU has been attached to a PCB and the entire assembly is undergoing final
test with automated test equipment. A separate signal or message is provided to the MCU operating under
user provided software control. The MCU initiates a trim procedure as outlined in
tester supplies a precision reference signal.
If the intended bus frequency is near the maximum allowed for the device, it is recommended to trim using
a reduction divisor (R) twice the final value. After the trim procedure is complete, the reduction divisor
can be restored. This will prevent accidental overshoot of the maximum clock frequency.
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Initial conditions:
1) Clock supplied from ATE has 500 µsec duty period
2) ICG configured for internal reference with 4 MHz bus
Example #4: Internal Clock Generator Trim
INCREASES THE FREQUENCY)
(DECREASING ICGTRM
ICGTRM - 128 / (2**n)
COUNT < EXPECTED = 500
(RUNNING TOO SLOW)
ICGTRM =
n = n + 1
IS n > 8?
MC9S08AC60 Series Data Sheet, Rev. 2
(COUNT = # OF BUS CLOCKS / 4)
DECREASES THE FREQUENCY)
Figure 10-17. Trim Procedure
NO
START TRIM PROCEDURE
INCOMING CLOCK WIDTH
(INCREASING ICGTRM
ICGTRM = $80, n = 1
ICGTRM + 128 / (2**n)
CASE STATEMENT
MEASURE
ICGTRM =
YES
.
COUNT > EXPECTED = 500
(RUNNING TOO FAST)
COUNT = EXPECTED = 500
Figure 10-17
Freescale Semiconductor
STORE ICGTRM VALUE
IN NON-VOLATILE
CONTINUE
MEMORY
while the