C505_9708 SIEMENS [Siemens Semiconductor Group], C505_9708 Datasheet - Page 54

• Manufacturer Part Number: C505_9708
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

5
5.1
The hardware reset function incorporated in the C505 allows for an easy automatic start-up at a
minimum of additional hardware and forces the controller to a predefined default state. The
hardware reset function can also be used during normal operation in order to restart the device. This
is particularly done when the power-down mode is to be terminated.
Additional to the hardware reset, which is applied externally to the C505, there are two internal reset
sources, the watchdog timer and the oscillator watchdog. This chapter deals only with the external
hardware reset.
The reset input is an active high input. An internal Schmitt trigger is used at the input for noise
rejection. Since the reset is synchronized internally, the RESET pin must be held high for at least
two machine cycles (12 oscillator periods) while the oscillator is running. With the oscillator running
the internal reset is executed during the second machine cycle and is repeated every cycle until
RESET goes low again.
During reset, pins ALE and PSEN are configured as inputs and should not be stimulated externally.
(An external stimulation at these lines during reset activates several test modes which are reserved
for test purposes. This in turn may cause unpredictable output operations at several port pins).
At the reset pin, a pulldown resistor is internally connected to
external capacitor only. An automatic power-up reset can be obtained, when
connecting the reset pin to
hold the voltage level at the reset pin for a specific time to effect a complete reset.
Semiconductor Group
System Reset
Hardware Reset Operation
V
CC
via a capacitor. After
5-1
V
CC
has been turned on, the capacitor must
V
SS
to allow a power-up reset with an
V
System Reset
C505 / C505C
CC
is applied, by
1997-08-01