The MCU CPU communicates with external devices through three main methods: unconditional transfer, query transfer, and interrupt transfer. To illustrate these, we'll use the example of a microcontroller interfacing with a micro printer. Suppose the user wants to print three data values stored in internal memory addresses 10H, 11H, and 12H of the microcontroller. The 8051 uses the parallel port P2 to exchange data with the printer's parallel data port DB.
(1) Unconditional Transfer Method
In this mode, the CPU assumes the printer is always ready, without any handshake signals. Data is sent directly to the printer via P2 without checking its status. While simple, this method is unreliable because the CPU operates much faster than the printer. As a result, data can be lost if the printer isn't ready when the CPU sends it. This method is suitable only for situations where the peripheral device is guaranteed to be ready at all times.
(2) Query Transfer Method
Also known as conditional transfer, this method requires the CPU to check the status of the printer before sending data. The CPU actively queries whether the printer is ready by reading a status pin, such as the BUSY pin. If the pin is high (busy), the CPU waits; if low (ready), it proceeds with data transfer. For instance, the printer could set the BUSY pin high while printing and low once it's ready. The microcontroller then checks this pin before sending each byte. Although more reliable than the unconditional method, this approach still involves waiting, which can reduce CPU efficiency, especially during continuous data transfers.
(3) Interrupt Transfer Method
To address the inefficiencies of the previous methods, the interrupt mode allows the CPU to perform other tasks while waiting for the printer to become ready. When the printer is done processing data, it sends an interrupt signal to the CPU. The CPU then pauses its current task, handles the interrupt, and resumes its original work. This method significantly improves CPU utilization, as it doesn’t waste time polling or waiting unnecessarily. It’s ideal for real-time applications where timely response is critical.
In summary, each transfer method has its own trade-offs. Unconditional transfer is fast but risky, query transfer ensures correctness at the cost of efficiency, and interrupt transfer offers a balance between reliability and performance. Choosing the right method depends on the specific requirements of the system, including speed, reliability, and resource availability.
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