Memory and I/O

Buttons, LEDs and sensors as memory

• A sensor node's button, LED and temperature sensor all sit outside the CPU. The CPU talks to every one of them by reading and writing addresses.
• Done well, the firmware is a clean table of names; done badly, a pile of magic numbers. This lesson shows the clean way.

Memory-mapped I/O

• With memory-mapped I/O 内存映射输入输出, each peripheral register appears at an address. Reading a data register receives a value; writing one sends it.
• Each peripheral has a base address, and its registers sit at named offsets from it.

The GPIO port

• A GPIO 通用输入输出 port has a direction register (one bit per pin: input or output) and a data register (the pin's level).
• Set the direction once at start-up, then read or write the data register as often as needed.

Polling

• Polling 轮询 waits for a peripheral by reading its status bit in a loop until the "done" bit is set.
• It is simple, but every lap of the loop is CPU time spent asking. Waiting $T$ seconds costs $T \times f_{\text{clock}}$ cycles.

Worked example: the cost of asking

• At 84 MHz with a 4-cycle loop, the CPU runs 21 million laps a second. A 1 ms wait is 21 000 laps, or 84 000 cycles.
• That is 100% of the CPU for the whole millisecond, doing nothing useful. Interrupts, in the next lesson, remove that cost.

Worked example: the node's map

• GPIO at 0x4000_0000, UART at 0x4001_0000, timer at 0x4002_0000, ADC at 0x4003_0000. Each driver uses its base plus named offsets.
• A typo such as 0x4001_0004 for the UART base shifts every register by four bytes. Reading back each peripheral's ID register after start-up catches it at once.

Three common slips

Recap