By the simplistic nature of the foreground/background processing, the background loop always resumes at the point it was interrupted, so the background loop would enter the low-power sleep mode while the MCU would have urgent work to do. The MCU will be stopped for a nondeterministic time period until the next interrupt wakes it up. Thus, enabling interrupts before transitioning to a low-power state opens a time window for a race condition between any enabled interrupt and the transition to the low-power mode.
Entering a sleep mode while interrupts are disabled poses a chicken-and-egg problem for waking the system up, because only an interrupt can terminate the low-power sleep mode. To operate in the foreground/background architecture, the MCU must allow entering the low-power sleep mode and enabling the interrupts at the same time, without creating this race condition.
Many MCUs indeed allow such an atomic transition to the sleep mode. Other MCUs support multiple levels of disabling interrupts and can accomplish low-power transitions with interrupts disabled at one level. Yet other MCUs don't provide any way to enter the low-power mode with interrupts disabled and require some different approaches.
HC08
HC08 is an 8-bit MCU family from Freescale Semiconductors. The HC(S)08 instruction set includes two special instructions--WAIT and STOP--for transitioning to the low-power wait and stop modes, respectively.1 The HC08 documentation states very clearly that both WAIT and STOP instructions atomically enable interrupts as a side effect of entering the sleep mode. Clearly, the HC08 designers anticipated that the transition to the low-power mode must happen with interrupts disabled. The code in Listing 1 for the CodeWarrior HC08 C-compiler shows a background loop with the atomic transition to the WAIT mode.
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