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Rabbit 2000 Microprocessor User’s Manual
Access to I/O devices is accomplished by using memory access instructions with an I/O
prefix. Access to I/O devices is thus faster and easier compared to processors with a
restricted I/O instruction set.
The hardware design rules are simple. Up to six static memory chips (such as RAM and
flash EPROM) connect directly to the microprocessor with no glue logic. Even larger
amounts of memory can be handled by using parallel I/O lines as high-order address
lines. The Rabbit runs with no wait states at 24 MHz with a memory having an access
time of 70 ns. There are two clocks per memory access. Most I/O devices may be con-
nected without glue logic.
The memory cycle is two clocks long. A clean memory and I/O cycle completely avoid
the possibility of tri-state fights. Peripheral I/O devices can usually be interfaced in a
glueless fashion using pins programmable as I/O chip selects, I/O read strobes or I/O
write strobe pins. A built-in clock doubler allows -frequency crystals to be used to
reduce radiated emissions.
The Rabbit may be cold-booted via a serial port or the parallel access slave port. This
means that flash program memory may be soldered in unprogrammed, and can be
reprogrammed at any time without any assumption of an existing program or BIOS. A
Rabbit that is slaved to a master processor can operate entirely with volatile RAM,
depending on the master for a cold program boot.
There are 40 parallel I/O lines (shared with serial ports). Some I/O lines are timer syn-
chronized, which permits precisely timed edges and pulses to be generated under com-
bined hardware and software control.
There are four serial ports. All four serial ports can operate asynchronously in a variety
of customary operating modes; two of the ports can also be operated synchronously to
interface with serial I/O devices. The baud rates can be very high—1/32 the clock
speed for asynchronous operation, and 1/6 the clock speed externally or 1/4 the clock
speed internally in synchronous mode. In asynchronous mode, the Rabbit, like the
Z180, supports sending flagged bytes to mark the start of a message frame. The flagged
bytes have 9 data bits rather than 8 data bits; the extra bit is located after the first 8 bits,
where the stop bit is normally located, and marks the start of a message frame.
A slave port allows the Rabbit to be used as an intelligent peripheral device slaved to a
master processor. The 8-bit slave port has six 8-bit registers, 3 for each direction of
communication. Independent strobes and interrupts are used to control the slave port in
both directions. Only a Rabbit and a RAM chip are needed to construct a complete
slave system if the clock and reset are shared with the master processor
The built-in battery-backable time/date clock uses an external 32.768 kHz crystal. The
time/date clock can also be used to provide periodic interrupts every 488 s. Typical
battery current consumption is 25 A with the suggested battery circuit. An alternative
circuit provides means for substantially reducing this current.
Numerous timers and counters (six all together) can be used to generate interrupts,
baud rate clocks, and timing for pulse generation.