Here's some more on Microwire...
MICROWIRE is a 3Mbps [full-duplex] serial 3-wire interface standard defined by National Semiconductor. The MICROWIRE protocol is essentially a subset of the SPI
interface, CPOL = 0 and CPHA = 0.
Microwire is a serial I/O port on microcontrollers, so the Microwire bus will also be found on EEPROMs and other Peripheral chips. The 3 lines are: SI, the serial input line to the microcontroller, SO is the serial output line, and SK is the serial clock line. Data is shifted out on the falling edge of SK, and is valued on the rising edge. SI is shifted in on the rising edge of SK. In some cases a 4.7k W pull-up
resistor may be used to increase the low-to-high rise time. The voltage level and swings is based on the microcontroller's I/O voltage, normally TTL. The protocol is a '1' state bit followed by a variable length instruction/address word, followed by a variable length data field. Or the device may send no start bit, just a variable length bit stream. The minimum clock cycle time is 2uS, 1uS high and 1uS low. The clock speed is dependent on the microcontroller, typical speeds are 0Hz to 250kHz.
An additional bus enhancement to MICROWIRE is called MICROWIRE/Plus. The main difference between the two buses seems to be that MICROWIRE/Plus architecture within the microcontroller is more complex.