8-bit microcontroller with accelerated two-clock 80C51 core
external clock input on X1) and if the RTC is not using the crystal oscillator as its clock
source. This allows external devices to synchronize to the P89LPC933/934/935/936. This
output is enabled by the ENCLK bit in the TRIM register.
The frequency of this clock output is 1⁄2 that of the CCLK. If the clock output is not needed
in Idle mode, it may be turned off prior to entering Idle, saving additional power.
8.4 On-chip RC oscillator option
The P89LPC933/934/935/936 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
preprogrammed value to adjust the oscillator frequency to 7.373 MHz ± 1 % at room
temperature. End-user applications can write to the TRIM register to adjust the on-chip
RC oscillator to other frequencies.
8.5 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.
8.6 External clock input option
In this configuration, the processor clock is derived from an external source driving the
P3.1/XTAL1 pin. The rate may be from 0 Hz up to 18 MHz. The P3.0/XTAL2 pin may be
used as a standard port pin or a clock output. When using an oscillator frequency
above 12 MHz, the reset input function of P1.5 must be enabled. An external circuit
is required to hold the device in reset at power-up until VDD has reached its
specified level. When system power is removed VDD will fall below the minimum
specified operating voltage. When using an oscillator frequency above 12 MHz, in
some applications, an external brownout detect circuit may be required to hold the
device in reset when VDD falls below the minimum specified operating voltage.
Product data sheet
All information provided in this document is subject to legal disclaimers.
Rev. 8 — 12 January 2011
© NXP B.V. 2011. All rights reserved.
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