Page 124 - Embedded Microprocessor Systems Real World Design

SPuMicrowire in Multichip Designs

Chapter 2 described the SPI/Microwire bus. Many microcontrollers have a
Microwire interface implemented in the hardware. In other cases, you can imple-
ment Microwire using port bits and generating the signals in software. Sometimes
you need to interface a Microwire device to a microprocessor that does not have a
built-in interface and for whatever reason you don’t want to use discrete I/O. Figure
3.4 illustrates a means to interface SPI/Microwire to any processor with the capa-
bility to access external memory.
The interface uses a tristate buffer and a “D” flip-flop. The Microwire SCLOCK
signal is connected to a decoded write strobe, and the enable input of the tristate
buffer is connected to a decoded read strobe (see Chapter 2).
The chip select signal comes from a port bit or other digital output. Before
accessing the Microwire device, the chip select signal is driven low. The width of
the decoded write strobe must be sufficient to meet the SCLOCK low specification
of the Microwire peripheral.
The microprocessor writes data serially to the Microwire device. Each write to
the device address strobes the data on bit DO into the device and clocks the con-

D FLIP-FLOP TRISTATE BUFFER
SPI DEVICE n
MICROPROCESSOR DO , DATA IN DATA OUT
‘CHIP SELECT -cs

- I * -
SCLK
MICROPROCESSOR READS SECOND BIT
JICROPROCESSOR WRITES SECOND BIT
ICROPROCESSOR READS FIRST BIT
ICROPROCESSOR WRITES FIRST BIT

Figure 3.4
Microwire Interface.

106 Embedded Microprocessor Systems