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Rabbit 6000 User’s Manual
digi.com
42
4. S
YSTEM
M
ANAGEMENT
4.1 Overview
There are a number of basic system peripherals in the Rabbit 6000 processor, some of which are covered in
later chapters. The peripherals covered in this chapter are the periodic interrupt, the real-time clock, the
watchdog timers, the battery-backed onchip-encryption RAM, and some of the miscellaneous output pins
and their control and processor registers that provide the processor ID and revision numbers.
The periodic interrupt, when enabled, is generated every 16 clocks of the 32 kHz clock (every 488 µs, or
2.048 kHz). This interrupt can be used to perform periodic tasks.
The real-time clock (RTC) consists of a 48-bit counter that is clocked by the 32 kHz clock. It is powered
by the VBAT pin, and so can be battery-backed. The value in the counter is not affected by reset, and can
only be set to zero by writing to the RTC control register. The 48-bit width provides a 272-year span before
rollover occurs.
There are two watchdog timers in the Rabbit 6000, both clocked by the 32 kHz clock. The main watchdog
timer can be set to time out from 250 ms to 2 seconds, and resets the processor if not reloaded within that
time. Its purpose is to restart the processor when it detects that a program gets stuck or disabled.
The secondary watchdog timer can time out from 30.5 µs up to 7.8 ms, and generates a Priority 3 second-
ary watchdog interrupt when it is not reset within that time. The primary use for the secondary watchdog is
to act as a safety net for the periodic interrupt — if the secondary watchdog is reloaded in the periodic
interrupt, it will count down to zero if the periodic interrupt stops occurring. In addition, it can be used as a
periodic interrupt on its own.
The battery-backed onchip-encryption RAM consists of 32 bytes of memory that are powered by the
VBAT pin (note that this RAM is separate from the battery-backed 32 KB SRAM). Their values are not
affected by a reset, but are erased if the state of the SMODE pins changes. These 32 bytes are intended for
storing sensitive data (such as an encryption key) somewhere other than an external memory device. The
“tamper-protection” erase feature erases these bytes if an attempt is made to load a program into the
onchip RAM to read out the bytes.
A feature new to the Rabbit 6000 is a 14-bit CPU clock cycle counter. This counter counts the number of
CPU cycles that occur during one 32 kHz clock period. This is useful for determining the frequency of the
main CPU oscillator which can be used in baud rate calculations as well as other CPU clock dependant
features.
The following other registers are also described in this chapter.
•
Global Output Control Register (GOCR), which controls the behavior of the CLK, STATUS, /WDT,
and /BUFEN pins
•
Global CPU Register (GCPU), which holds the identification number of the processor.
•
Global Revision Register (GREV), which hold the revision number of the processor.
Summary of Contents for 6000
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