2001 Serial-Control Multiplexer Expands Chip Selects SPIand
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CONVERSION/SAMPLING CIRCUITS
2001
Serial-Control Multiplexer Expands Chip Selects
SPIand MICROWIREbuses offer popular convenient means minimizing number wires required connecting smart peripherals microcontroller (µC). These synchronous buses comprise serial-clock, data-in, data-out line addition chip-select line each peripheral. scheme chip-select line device, however, quickly precious port pins system. mitigate this situation, dual 4-channel analog multiplexer figure below device itself) multiplexes single port (PC0) provide chip selects four other peripherals system. second port (PC1) selects multiplexer.
dual 4-channel multiplexer expands number input channels chip-select lines that this microcontroller support. switches this multiplexer bidirectional. sections independent, either output (unlike those conventional differential multiplexers) programmed connect any, all, none four input channels. Thus, second (top) section employed independently lower section expand number analog channels available µC's internal multiplexer supports eight channels, this scheme (using four
more) yields total input channels. operating "backwards," lower routes signal CS-bar input selected peripheral. Driving selects that peripheral receiving data, driving high deselects four peripherals. Read write sequences same regular systems, except chip selects must beforehand. then goes low, read/write operation executed, returns high deselect device. This procedure burdensome practice. Typical systems include device that serviced often (such display driver) several others that require service only occasionally (such EEPROMs real-time clocks). Thus, chip-select leave heavily used device selected most time perform update only when selecting device. accommodate other combinations chip-select analog-expansion lines, replace MAX350 with similar device such MAX395, whose eight serially addressed SPST switches configured required.
trademark Motorola, Inc. MICROWIRE trademark National Semiconductor Corp.
similar version this article appeared March 1998 issue Electronic Design magazine. Revision: July 2001
More Information MAX350: QuickView Full (PDF) Data Sheet Free Samples
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