Jameco Electronics 3000, 2000 Overview of On-Chip Peripherals and Features, 1 5 V Tolerant Inputs

The Rabbit is an 8-bit processor with an 8-bit external data bus and an 8-bit internal data bus. Because the Rabbit makes the most of its external 8-bit bus and because it has a com- pact instruction set, its performance is as good as many 16-bit processors.

We hesitate to compare the Rabbit to 32-bit processors, but there are undoubtedly occa- sions where the user can use a Rabbit instead of a 32-bit processor and save a vast amount of money. Many Rabbit instructions are 1 byte long. In contrast, the minimum instruction length on most 32-bit RISC processors is 32 bits.

2.2 Overview of On-Chip Peripherals and Features

The on-chip peripherals were chosen based on our experience as to what types of periph- eral devices are most useful in small embedded systems. The major on-chip peripherals are the serial ports, system clock, time/date oscillator, parallel I/O, slave port, motion encoders, pulse width modulators, pulse measurement, and timers. These and other fea- tures are described below.

2.2.1 5 V Tolerant Inputs

The Rabbit 3000 operates on a voltage in the range of 1.8 V to 3.6 V, but most Rabbit 3000 input pins are 5 V tolerant. The exceptions are the power supply pins, and the oscillator buffer pins. 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 directly connected to the Rabbit. 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 outputs do not rise above VDD, which cannot exceed 3.6 V, 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.

Many charge pump chips that perform DC to DC voltage conversion at low cost have been introduced in recent years. These are convenient for systems with dual voltage requirements.

2.2.2 Serial Ports

There are six serial ports designated ports A, B, C, D, E, and F. All six serial ports can operate in an asynchronous mode up to a baud rate equal to the system clock divided by 8. The asynchronous ports use 7-bit or 8-bit data formats, with or without parity. A 9th bit address scheme, where an additional bit is set or cleared to mark the first byte of a mes- sage, is also supported.

The serial port software driver can tell when the last byte of a message has finished trans- mitting from the output shift register - correcting an important defect of the Z180. This is

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