SZF2002 datasheet(52/76 Pages) PHILIPS

52 / 76 page

1998 Aug 26

Philips Semiconductors

Product speciﬁcation

Low voltage 8-bit microcontroller with

6-kbyte embedded RAM

SZF2002

19 CLOCK CIRCUITRY

The SZF2002 is clocked with an external digital clock.

The input must be driven with a digital square wave.

Note that the duty cycle influences the timing to the

external components, since both the positive and negative

clock edges are used.

20 RESET

To initialize the SZF2002 a reset is performed by either of

two methods:

• Applying an external signal to the RST pin

• Watchdog Timer overflow.

20.1

External reset using the RST pin

The reset input for the SZF2002 is RST. A reset is

accomplished by holding the RST pin HIGH for at least two

machine cycles (12 clock periods) while the clock is

running. The CPU responds by executing an internal reset.

Port pins adopt their reset state immediately after the RST

goes HIGH. During reset, WE and OE, and CE are held

HIGH.

The external reset is asynchronous to the internal clock.

The RST pin is sampled during state 5, phase 2 of every

machine cycle. After a HIGH is detected at the RST pin, an

internal reset is repeated until RST goes LOW. The reset

circuitry is also affected by the Watchdog Timer as

described in Section 12.5. The internal RAM is not

affected by reset. When VDD is turned on, the RAM

contents are indeterminate.

20.2

Power-on-reset

The device contains on-chip circuitry which switches the

port pins to HIGH as soon as RST goes HIGH. The user

must ensure that the RST pin is held HIGH until the

external clock has stabilised. When RST goes LOW a

further 3 cycles elapse before execution starts.

SZF2002 датащи(PDF) 52 Page - NXP Semiconductors