Control methods for interface communication

Interfacing connects a processor to a peripheral, and the control method decides how the transfer is controlled. The main methods are:

1. Programmed I/O (polling)

The processor repeatedly reads the status register of the peripheral (busy-wait loop) and transfers data when the device is ready. Simple, but wastes CPU time while polling.

2. Interrupt-driven I/O

The peripheral sends an interrupt request when it is ready; the processor suspends the main program, runs the ISR for the transfer and then resumes. Uses the CPU efficiently. Two forms: fixed ISR location and vectored interrupt (the device supplies the vector).

3. Direct memory access (DMA)

A DMA controller transfers blocks of data between the peripheral and memory without CPU involvement for every byte. Best for large, fast transfers; the CPU only sets up the transfer and handles completion.

Handshaking and timing control

The transfer is also controlled by protocol: strobe (data valid pulse) and handshake (Req/Ack).

Bus access control

When multiple masters exist, arbitration (priority or daisy chain) decides who controls the bus.