Introduction
When using asynchronous serial protocols, such as ‘RS-232’, the two devices that are communicating must agree on a communication frequency (the baud rate) and each device then uses (say) an independent crystal-based clock to ensure that it operates at the required rate. One consequence of this approach is that, if the clocks in the transmitter and receiver devices vary by more than a few per cent, the receiving device will be unable to decode the incoming data correctly.
By contrast, the serial peripheral interface (SPI) uses a synchronous communication protocol: this means that both the transmitter and receiver devices share a common clock. The transitions of this common clock determine when to send and receive the various bits. For example, in a simple synchronous protocol, the transmitter device may write a bit on the rising edge of the clock and the receiving device will then read this bit when it detects the falling clock edge. Note that the clock frequency does not need to be held constant.
Such synchronous interfaces are primarily intended for use over distances meas- ured in centimetres rather than in metres and we will restrict our discussions here to the use of SPI to link the 8051 with compatible peripherals.