89LPC925 datasheet(27/49 Pages) PHILIPS

27 / 49 page

Philips Semiconductors

P89LPC924/925

8-bit microcontrollers with accelerated two-clock 80C51 core

Product data

Rev. 03 — 15 December 2004

27 of 49

9397 750 14471

Remark: During a power cycle, VDD must fall below VPOR (see Table 8 “DC electrical

characteristics” on page 40) before power is reapplied, in order to ensure a power-on

reset.

Reset can be triggered from the following sources:

• External reset pin (during power-up or if user conﬁgured via UCFG1. This option

must be used for an oscillator frequency above 12 MHz);

• Power-on detect;

• Brownout detect;

• Watchdog Timer;

• Software reset;

• UART break character detect reset.

For every reset source, there is a ﬂag in the Reset Register, RSTSRC. The user can

read this register to determine the most recent reset source. These ﬂag bits can be

cleared in software by writing a ‘0’ to the corresponding bit. More than one ﬂag bit

may be set:

• During a power-on reset, both POF and BOF are set but the other ﬂag bits are

cleared.

• For any other reset, previously set ﬂag bits that have not been cleared will remain

set.

8.16.1

Reset vector

Following reset, the P89LPC924/925 will fetch instructions from either address 0000h

or the Boot address. The Boot address is formed by using the Boot Vector as the high

byte of the address and the low byte of the address = 00h.

The Boot address will be used if a UART break reset occurs, or the non-volatile Boot

Status bit (BOOTSTAT.0) = 1, or the device is forced into ISP mode during power-on

(see

P89LPC924/925 User’s Manual). Otherwise, instructions will be fetched from

address 0000H.

8.17 Timers/counters 0 and 1

The P89LPC924/925 has two general purpose counter/timers which are upward

compatible with the standard 80C51 Timer 0 and Timer 1. Both can be conﬁgured to

operate either as timers or event counter. An option to automatically toggle the T0

and/or T1 pins upon timer overﬂow has been added.

In the ‘Timer’ function, the register is incremented every machine cycle.

In the ‘Counter’ function, the register is incremented in response to a 1-to-0 transition

at its corresponding external input pin, T0 or T1. In this function, the external input is

sampled once during every machine cycle.

Timer 0 and Timer 1 have ﬁve operating modes (modes 0, 1, 2, 3 and 6). Modes 0, 1,

2 and 6 are the same for both Timers/Counters. Mode 3 is different.

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