1: Introduction
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4
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Javelin Stamp Manual v1.1
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www.javelinstamp.com
These Virtual Peripherals are built into the Javelin
Stamp’s firmware. Although you can write library classes
that make use of these VPs, the VPs themselves cannot be
modified or rewritten.
How the Javelin Stamp Works
The Javelin Stamp’s hardware architecture is shown in Figure 1.2. Programming and
debugging is done via communication with the serial port. The COM circuit takes care of the
voltage conversions necessary for a TTL device to talk with an RS232 port. The Java
interpreter processes all serial port/COM circuit information. Whether it’s byte codes,
debugging data or serial messages, the interpreter processes the data and decides what to do
with it.
When a program is downloaded, the interpreter buffers the program bytecodes and writes
them to the EEPROM. Upon reset (or a power interruption), all the Javelin Stamp’s I/O pins
are set to input. The interpreter copies the bytecodes to the SRAM, then starts fetching
bytecodes from the SRAM and executing them. The bytecode instructions can be executed
very rapidly because all data is transmitted along parallel data busses instead of synchronous
serial lines. A typical fetch and execute cycle involves a couple of read/write cycles. During
a read/write cycle, the interpreter loads some of the 15 bit address information into an address
latch and writes the other portion directly to the SRAM. When the SRAM address is set, then
the data is read or written by the interpreter as needed.
The Javelin’s internal voltage regulation is done using a switching regulator. The switching
regulator runs cooler and is significantly more efficient than a linear regulator. It accepts
voltages between 6 and 24 V, and makes 5 V available for the Javelin Stamp with a total
current budget of 150 mA. The passive components including the input and output
capacitors, switching diode and inductor are on the top side, and the switching IC is on the
bottom side of the board next to the EEPROM. The switching IC monitors the output voltage
and adjusts the switching duty cycle to the passive components to maintain a constant 5 V
output.
