Page 90 - Embedded Microprocessor Systems Real World Design

Table 2.2
Summary Comparison Between SPI/Microwire and I'C.

SPNMicro wire I'C

Maximum bit rate Into the MHz range About 100 kHz (standard),
400 kHz (fast mode),
3.4MHz (high-speed mode)
Interface pins required Three plus one chip select per Two, regardless of number
peripheral of peripherals
Number of devices As many as there are chip Bus can address up to 127
sharing a bus selects available, as long as peripherals
maximum loading is not
exceeded
Interface method Usually dedicated hardware, Software, but hardware ICs
can be implemented in are available
software

Also note that many SPI/Microwire devices perform operations, such as latch-
ing previously written data, on the rising edge of -CS. Consequently, the -CS signal
must remain stable throughout the access cycle.

Other Serial Interfaces

Some manufacturers sell peripherals with a proprietary serial interface. Analog
Devices, for example, has several ADC and DAC parts with simple serial data/clock
schemes. These devices require three or more signals and can be interfaced to any
general-purpose microcontroller.

DMA

DMA (direct memory access) is a means of having two or more processors share
the same bus. When a secondary processor (or other DMA device) wants control
of the bus, it notifies the first processor, which gives up the bus. The second proces-
sor then drives the address, data, and control lines and accesses the memory and
peripherals just like the first processor. Typical examples of DMA uses are to permit
two processors to communicate through a common memory, to refresh dynamic
RAM, or to transfer data from an 1/0 device (such as a serial port) directly to
processor RAM. Figure 2.22 illustrates a typical DMA transfer.

74 Embedded Micr@-rocessor Systm