SPX Reference Manual 1
Introduction
Description
The VersaLogic SPX-2 is a 16-channel digital I/O expansion module designed to be used with
any SPX™ enabled base board. Its features include:
Microchip MCP23S17 16-bit I/O
Expander
Compatible with any SPX
enabled base board
VersaLogic SPX boards are a line of I/O expansion boards using the industry standard Serial
Peripheral Interface (SPI) bus. These are small 1.2” x 3.775” that can mount in either “user
connector” areas of a PC/104-Plus stack using the normal PC/104 stand-offs. They can also
mount up to two feet away from the base board using custom cabling.
SPX boards are electrically connected to a base board via a 14-pin 2mm cable. Up to four boards
can be daisy-chained together. Four is the maximum number of SPI chips that can be driven by
the 14-pin interface. The SPI bus requires each chip to have a discrete chip-select signal, and the
14-pin interface supplies four chip-select signals. The maximum clock rate is 8 MHz.
Power for SPX boards is supplied through the interface cable. I/O connections on SPX boards are
provided through screw terminal/wire connections.
All SPX boards are RoHS compliant and industrial temperature rated.
ABOUT SPI
The SPI bus specifies four logic signals: SCLK – Serial clock (output from master); MOSI –
Master output, slave input (output from master); MISO – Master input, slave output (output from
slave); and SS – Slave select (output from master).
The SPI implementation on VersaLogic CPU boards adds additional features, such as hardware
interrupt input to the master. The master initiates all SPI transactions. A slave device responds
when its slave select is asserted and it receives clock pulses from the master.
Slave selects are controlled in one of two modes: manual or automatic. In automatic mode, the
slave select is asserted by the SPI controller when the most significant data byte is written. This
initiates a transaction to the specified slave device. In manual mode, the slave select is controlled
by the user and any number of data frames can be sent. The user must command the slave select
high to complete the transaction.
The SPI clock rate can be software configured to operate at speeds between 1 MHz and 8 MHz.
All four common SPI modes are supported through the use of clock polarity and clock phase
controls.
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