Microwire BUS Electronic Design & Firmware by John Heritage
Microwire is a three-wire synchronous interface developed by National Semiconductor and present on their COP8 processor family.
Similar to SPI, Microwire is a master/slave bus, with serial data out of the master (SO), and serial data in to the master (SI), and signal clock (SK). These correspond to SPI’s MOSI, MISO, and SCK, respectively. There is also a chip select signal, which acts similarly to SPI’s /SS. A full-duplex bus, Microwire is capable of speeds of 625Kbps and faster (capacitance permitting).
Microwire devices from National come with different protocols, based on their data needs. Unlike SPI, which is based on an 8-bit byte, Microwire permits variable length data, and also specifies a “continuous” bitstream mode.
Microwire has the same advantages and disadvantages as SPI with respect to multiple slaves, which require multiple chip select lines. In some instances, an SPI device will work on a Microwire bus, as will a Microwire device work on an SPI bus, although this must be reviewed on a per-device basis.
Both SPI and Microwire are generally limited to on-board communications and traces of no longer than 6 inches, although longer distances (up to 10 feet) can be achieved given proper capacitance and lower bit rates.