P80C562 Philips Semiconductors, P80C562 Datasheet - Page 30

• Manufacturer Part Number: P80C562
• Description: 8-bit microcontroller
• Manufacturer: Philips Semiconductors

Philips Semiconductors
16 OSCILLATOR CIRCUITRY
The oscillator circuitry of the P8xC562 is a single-stage
inverting amplifier in a Pierce oscillator configuration.
The circuitry between XTAL1 and XTAL2 is basically an
inverter biased to the transfer point. Either a crystal or
ceramic resonator can be used as the feedback element to
complete the oscillator circuitry. Both are operated in
parallel resonance. XTAL1 (pin 35) is the high gain
amplifier input, and XTAL2 (pin 34) is the output (see
Fig.14). To drive the P8xC562 externally, XTAL1 is driven
from an external source and XTAL2 left open-circuit (see
Fig.15).
17 RESET CIRCUITRY
The reset circuitry for the P8xC562 is connected to the
reset pin RST. A Schmitt trigger is used at the input for
noise rejection. The output of the Schmitt trigger is
sampled by the reset circuitry every machine cycle.
The on-chip Reset circuit is shown in Fig.16.
A reset is accomplished by holding the RST pin HIGH for
at least two machine cycles (24 oscillator periods but at
least 2 s). The CPU responds by executing an internal
reset. During reset both ALE and PSEN output a HIGH
level. In order to perform a correct reset, this level must not
be affected by external elements.
Also with the P8xC562, the RST line can be pulled HIGH
internally by a pull-up transistor activated by the Watchdog
Timer (T3). The length of the output pulse from the
Watchdog Timer is 3 machine cycles. A pulse of such
short duration is necessary in order to recover from a
processor or system fault as fast as possible.
It can be seen that the short reset pulse from T3 cannot
discharge the Power-on reset capacitor (see Fig.17).
Consequently, when the Watchdog Timer is also used to
reset external devices this capacitor arrangement should
not be connected to the RST pin, and an extra circuit
should be used to perform the Power-on-reset operation.
It should be remembered that a T3 overflow, if enabled, will
force a reset condition to the P8xC562 by an internal
connection, whether the output RST is tied LOW or not.
The internal reset is executed during the second cycle in
which RST is HIGH and is repeated every cycle until RST
goes LOW. The internal RAM is not affected by reset.
When V
An internal reset leaves the internal registers as shown in
Table 36.
1997 Apr 08
8-bit microcontroller
DD
is turned on, the RAM content is indeterminate.
30
k, halfpage
ook, halfpage
andbook, halfpage
RST
1) Use fundamental crystals only.
Fig.15 Driving the P8xC562 from an external source.
on-chip
resistor
Fig.14 P8xC562P8xC562 oscillator circuit.
Fig.16 On-chip reset configuration.
external clock
R
compatible)
V DD
RST
(not TTL
20 pF
20 pF
C2
C1
TRIGGER
not connected
SCHMITT
P83C562; P80C562
(1)
CIRCUITRY
MBC751
XTAL1
XTAL2
RESET
Product specification
MGA169
XTAL1
XTAL2
35
34
MBC476 - 1
overflow
timer T3
35
34