User`s manual

10 rabbit.com Rabbit Hardware Design Overview
2.1.1 Rabbit Programming Connector
The user may be concerned that the requirement for a programming connector places added cost overhead
on the design. The overhead is very small—less than $0.25 for components and board space that could be
eliminated if the programming connector were not made a part of the system.
The programming connector can also be used for a variety of other purposes, including user applications.
A device attached to the programming connector has complete control over the system because it can per-
form a hardware reset and load new software. If this degree of control is not desired for a particular situa-
tion, then certain pins can be left unconnected in the connecting cable, limiting the functionality of the
connector to serial communications. Rabbit develops products and software that assume the presence of
the programming connector.
2.1.2 Memory Chips
Most systems have one static RAM chip and one or two flash memory chips, but more memory chips can
be used when appropriate. Static RAM chips are available in 128K x 8, 256K x 8, and 512K x 8 sizes.
They are all available in 3 V versions. Suggested flash memory chips between 128K x 8 and 512K x 8 are
given in Chapter 10, “Supported Flash Memories.” That chapter also includes instructions for writing your
own flash driver. The list of supported flash memories is in Technical Note 226, “Supported Flash Memo-
ries.”
Dynamic C and a PC are not necessary for the production programming of flash memory since the flash
memory can be copied from one controller to another by cloning. This is done by connecting the system to
be programmed to the same type of system that is already programmed. This connection is made with the
Rabbit Cloning Board. The cloning board connects to the programming ports of both systems. A push of a
button starts the transfer of the program and an LED displays the progress of the transfer.
Please visit www.rabbit.com/store/index.shtml to purchase the Rabbit Cloning Board.
2.1.3 Oscillator Crystals
Generally, a system will have two oscillator crystals:
• A 32.768 kHz crystal oscillator to drive the battery-backable timer,
• A crystal that has a frequency that is a multiple of 614.4 kHz or a multiple of 1.8432 MHz. Typical val-
ues are 7.3728, 11.0592, 14.7456, 22.1184, and 29.4912 MHz.
These crystal frequencies (except 614.4 kHz and 1.8432 MHz) allow generation of standard baud rates up
to at least 115,200 bps. The clock frequency can be doubled by an on-chip clock doubler, but the doubler
should not be used to achieve frequencies higher than about 60 MHz on a 3.3 V system. A quartz crystal
should be used for the 32.768 kHz oscillator. For the main oscillator, a ceramic resonator that is accurate to
0.5% will usually be adequate and less expensive than a quartz crystal for lower frequencies.