Software avoids interrupt overhead

Hans-Herbert Kirste, WAGO Kontakttechnik GmbH, Minden, Germany -- EDN, 3/2/2000

You can service peripheral ICs connected to a µC by polling or via interrupts. The polling method can be time-consuming, so interrupt handling is often preferable because the µC has to take care of the peripheral only on request. However, each separate interrupt causes the µC to stop normal program execution, save its current state on the system stack, and vector to the interrupt's processing function. The first instructions in the interrupt function normally push some or all registers used onto the stack.

Peripherals, such as the 16550 UART, that have more than one interrupt source, may require that the µC process more than one request at a time. Fortunately, you can write software that allows the µC to process more than one request in one interrupt cycle. Thus, the interrupt overhead occurs only once. The result is improved system performance.

Listing 1 is an example of standard interrupt-handling software. Standard practice involves vectoring, pushing, and popping registers for each interrupt request. A more sophisticated function in Listing 2 tries to handle as many interrupt requests as the peripheral requires. This function processes multiple reads to the identification register, and the process repeats until the µC has serviced all sources. (DI #2489)