83C166 Siemens Semiconductor Group, 83C166 Datasheet - Page 22

• Manufacturer Part Number: 83C166
• Description: 16-Bit CMOS Single-Chip Microcontroller
• Manufacturer: Siemens Semiconductor Group

SAB 80C166/83C166
Watchdog Timer
The Watchdog Timer represents one of the fail-safe mechanisms which have been implemented to
prevent the controller from malfunctioning for longer periods of time.
The Watchdog Timer is always enabled after a reset of the chip, and can only be disabled in the time
interval until the EINIT (end of initialization) instruction has been executed. Thus, the chip’s start-up
procedure is always monitored. The software has to be designed to service the Watchdog Timer
before it overflows. If, due to hardware or software related failures, the software fails to do so, the
Watchdog Timer overflows and generates an internal hardware reset and pulls the RSTOUT pin low
in order to allow external hardware components to be reset.
The Watchdog Timer is a 16-bit timer, clocked with the CPU clock divided either by 2 or by 128. The
high byte of the Watchdog Timer register can be set to a prespecified reload value (stored in
WDTREL) in order to allow further variation of the monitored time interval. Each time it is serviced
by the application software, the high byte of the Watchdog Timer is reloaded. Thus, time intervals
between 25 s and 420 ms can be monitored (@ 20 MHz CPU clock). The default Watchdog Timer
interval after reset is 6.55 ms (@ 20 MHz CPU clock).
Bootstrap Loader
The SAB 80C166 provides a built-in bootstrap loader (BSL), which allows to start program
execution out of the SAB 80C166’s internal RAM. The program to be started is loaded via the serial
interface ASC0 and does not require external memory or an internal ROM.
The SAB 80C166 enters BSL mode, when ALE is sampled high at the end of a hardware reset and
if NMI becomes active directly after the end of the internal reset sequence. BSL mode is entered
independent of the bus mode selected via EBC0, EBC1 and BUSACT.
After entering BSL mode the SAB 80C166 scans the RXD0 line to receive a zero byte, i.e. one start
bit, eight ‘0’ data bits and one stop bit. From the duration of this zero byte it calculates the
corresponding baudrate factor with respect to the current CPU clock and initializes ASC0
accordingly. Using this baudrate, an acknowledge byte is returned to the host that provides the
loaded data. The SAB 80C166 returns the value <55
>.
H
The next 32 bytes received via ASC0 are stored sequentially into locations 0FA40
through 0FA5F
H
H
of the internal RAM. To execute the loaded code the BSL then jumps to location 0FA40
. The
H
loaded program may load additional code / data, change modes, etc.
The SAB 80C166 exits BSL mode upon a software reset (ignores the ALE level) or a hardware reset
(remove conditions for entering BSL mode before).
Semiconductor Group
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