C505-2RM SIEMENS [Siemens Semiconductor Group], C505-2RM Datasheet - Page 48

• Manufacturer Part Number: C505-2RM
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

Fail Save Mechanisms
The C505 offers enhanced fail safe mechanisms, which allow an automatic recovery from software
upset or hardware failure :
The watchdog timer in the C505 is a 15-bit timer, which is incremented by a count rate of
upto
divide-by-16 prescaler. For programming of the watchdog timer overflow rate, the upper 7 bits of the
watchdog timer can be written. Figure 24 shows the block diagram of the watchdog timer unit.
Figure 24
Block Diagram of the Programmable Watchdog Timer
The watchdog timer can be started by software (bit SWDT in SFR IEN1) but it cannot be stopped
during active mode of the device. If the software fails to refresh the running watchdog timer an
internal reset will be initiated on watchdog timer overflow. For refreshing of the watchdog timer the
content of the SFR WDTREL is transfered to the upper 7-bit of the watchdog timer. The refresh
sequence consists of two consequtive instructions which set the bits WDT and SWDT each. The
reset cause (external reset or reset caused by the watchdog) can be examined by software (flag
WDTS). It must be noted, however, that the watchdog timer is halted during the idle mode and
power down mode of the processor.
Semiconductor Group
– a programmable watchdog timer (WDT), with variable time-out period from 192 s up to
– an oscillator watchdog (OWD) which monitors the on-chip oscillator and forces the
f
OWDS
OSC
f
External HW Reset
approx. 412.5 ms at 16 MHz.
microcontroller into reset state in case the on-chip oscillator fails; it also provides the clock for
a fast internal reset after power-on.
OSC
/ 6
/192. The system clock of the C505 is divided by two prescalers, a divide-by-two and a
SWDT
WDTS
WDT
2
Control Logic
WDT Reset - Request
16
IP0 (A9 )
48
H
IEN0 (A8 )
IEN1 (B8 )
WDTPSEL
H
H
7
0
6
14
WDTREL (86 )
C505A / C505CA
WDTL
WDTH
C505 / C505C
H
MCB03306
1997-12-01
7
8
0
f
OSC
/12