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Philips Semiconductors
CLOCKS
Therefore when setting or clearing the ENCLK bit, the user should retain the contents of bits 5:0 of the TRIM register. This can
be done by reading the contents of the TRIM register (into the ACC for example), modifying bit 6, and writing this result back into
the TRIM register. Alternatively,the "ANL direct" or "ORL direct" instructions can be used to clear or set bit 6 of the TRIM register.
The oscillator must be configured in
one of the following modes:
- Low Frequency Crystal
- Medium Frequency Crystal
- High Frequency Crystal
* A series resistor may be required to limit
crystal drive levels. This is especially
important for low frequency crystals (see
text).
On-chip RC oscillator option
The P89LPC920/921/922 has a 6-bit TRIM register that can be used to tune the frequency of the RC oscillator. During reset, the
TRIM value is initialized to a factory pre-programmed value to adjust the oscillator frequency to 7.373 MHz, ±1%. (Note: the initial
value is better than 1%; please refer to the datasheet for behavior over temperature). End user applications can write to the TRIM
register to adjust the on-chip RC oscillator to other frequencies. Increasing the TRIM value will decrease the oscillator frequency.
TRIM
Address: 96h
Not bit addressable
Reset Source(s): Power-up only
Reset Value: On power-up reset, ENCLK = 0, and TRIM.5-0 are loaded with the factory programmed value.
BIT
SYMBOL
TRIM.7
-
TRIM.6
ENCLK
TRIM.5-0
Note: on reset, the TRIM SFR is initialized with a factory preprogrammed value. When setting or clearing the ENCLK bit,
the user should retain the contents of bits 5:0 of the TRIM register. This can be done by reading the contents of the TRIM
register (into the ACC for example), modifying bit 6, and writing this result back into the TRIM register. Alternatively, the
"ANL direct" or "ORL direct" instructions can be used to clear or set bit 6 of the TRIM register.
Watchdog oscillator option
The watchdog has a separate oscillator which has a frequency of 400 kHz. This oscillator can be used to save power when a
high clock frequency is not needed.
2003 Dec 8
Quartz crystal or
ceramic resonator
Figure 2-1: Using the crystal oscillator
7
6
-
ENCLK
TRIM.5
FUNCTION
Reserved.
When ENCLK =1, CCLK/ 2 is output on the XTAL2 pin (P3.0) provided that the crystal
oscillator is not being used. When ENCLK=0, no clock output is enabled.
Trim value.
Figure 2-2: On-chip RC oscillator TRIM register
*
5
4
3
TRIM.4
TRIM.3
22
User's Manual - Preliminary -
P89LPC920/921/922
P89LPC920/921/922
XTAL1
XTAL2
2
1
0
TRIM.2
TRIM.1
TRIM.0
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