Philips PTV800, Руководство по обслуживанию

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Clock Calibration
The microcomputer's crystal oscillator frequency of approx. 12MHz is applied to a divider
program (within the micro) in order to supply a frequency of 2403..846153846 Hz. The 12MHz.
oscillator frequency cannot be adjusted, however, a correction factor may be input in to the
divider program in order to adjust for a slow or fast clock reading.
This correction factor is selected in the Service Mode by changing the data value held in Register
20. The data within this register can range from 00 (none) to 7F max. (for a
fast clock ) or FF (for
a slow clock ). Models are shipped from the factory with a default value of 00 or 80 for NO
correction.
One method used for calibration of this clock is accomplished by measuring the frequency at pin
3 on the microcontroller (IC300). This frequency is nominally 2403.846153846 Hz. Using the
calibration tables (refer to the following page), look up the closest frequency to the measured
frequency and then find the corresponding calibration number in hexadecimal. The clock
calibration register should be loaded with this value.
Note: This does not change the oscillator
frequency.
The more practical method is to simply question the customer on the number of minutes per
month the clock is off (assuming the time error has been checked against a known accurate time
source). Calculate using the formula below:
Minutes of error per month X 2 = Seconds of error per day
Is it losing (slow) or gaining (fast)?
Enter the Service Mode and look at register 20. If it is 00 or 80 the task is easy. Look up (use
Table 3-0 on the following page) the seconds of correction needed, fast or slow, and enter it's
"
calibration byte " in for the data held by register 20.
Normal Example:
4 minutes fast a month X 2 = 8 seconds a day fast
The chart says if the clock is 8 seconds a day fast without correction then use 59. Change
register 20 from 00 to 59.
If there is a calibration byte in register 20 other than 00 or 80 then the job gets a bit more
complex.
Complex Example #1:
Using the error from the "normal example", 8 seconds fast a day, you find register 20 the data
value of 16. Look up 16 and you find that there is already a correction for 2 seconds fast a day.
You need 8 seconds fast
more correction (2 + 8 = 10) or a total of 10 seconds fast correction
which is 6E. Enter 6E for the data held by register 20.
Complex Example #2:
If the clock is 8 seconds fast a day and you find register 20 holds the data value of 96 you have 2
seconds a day correction
in the wrong direction (slow). You need to "back up" 8 seconds. First
if you back up to 80 (no correction) you have used 2 of the 8 seconds leaving 6. Then go 6
seconds in the fast column, from no correction to 6 seconds (43). Enter 43 for the data held by
register 20.