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4.1.1 Memory I/O Interface
The Rabbit 3000 address lines (A0–A19) and all the data lines (D0–D7) are routed inter-
nally to the onboard flash memory and SRAM chips. I/0 write (/IOWR) and I/0 read
(/IORD) are available for interfacing to external devices.
Parallel Port A can also be used as an external I/O data bus to isolate external I/O from the
main data bus. Parallel Port B pins PB2–PB7 can also be used as an external address bus.
When using the auxiliary I/O bus, you must add the following line at the beginning of
your program.
#define PORTA_AUX_IO
// required to enable auxiliary I/O bus
The STATUS output has three different programmable functions:
5. It can be driven low on the first op code fetch cycle.
6. It can be driven low during an interrupt acknowledge cycle.
7. It can also serve as a general-purpose output.
4.1.2 Other Inputs and Outputs
Two status mode pins, SMODE0 and SMODE1, are available as inputs. The logic state of
these two pins determines the startup procedure after a reset.
/RESET_IN is an external input used to reset the Rabbit 3000 microprocessor and the
RabbitCore RCM3000 memory. /RES is an output from the reset circuitry that can be used
to reset other peripheral devices.
4.1.3 5 V Tolerant Inputs
The RCM3000 operates over a voltage from 3.15 V to 3.45 V, but most RCM3000 input
pins, except /RESET_IN, VRAM, VBAT_EXT, and the power-supply pins, are 5 V toler-
ant. When a 5 V signal is applied to 5 V tolerant pins, they present a high impedance even if
the Rabbit power is off. The 5 V tolerant feature allows 5 V devices that have a suitable
switching threshold to be connected directly to the RCM3000. This includes HCT family
parts operated at 5 V that have an input threshold between 0.8 and 2 V.
NOTE: CMOS devices operated at 5 V that have a threshold at 2.5 V are not suitable for
direct connection because the Rabbit 3000 outputs do not rise above VDD, and is often
specified as 3.3 V. Although a CMOS input with a 2.5 V threshold may switch at 3.3 V,
it will consume excessive current and switch slowly.
In order to translate between 5 V and 3.3 V, HCT family parts powered from 5 V can be
used, and are often the best solution. There is also the “LVT” family of parts that operate
from 2.0 V to 3.3 V, but that have 5 V tolerant inputs and are available from many suppli-
ers. True level-translating parts are available with separate 3.3 V and 5 V supply pins, but
these parts are not usually needed, and have design traps involving power sequencing.
User ’s Manual
21
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