Connecting to the serial bus

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Connecting to the serial bus

Figure 3 shows an example of three memory devices connected to an MCU, on a serial bus.

Only one device is selected at a time, so only one device drives the Serial Data output (Q)

line at a time, the other devices are high impedance.

The pull-down resistor R (represented in Figure 3) ensures that no device is selected if the

bus master leaves the S line in the high impedance state.

In applications where the bus master may be in a state where all inputs/outputs are high

impedance at the same time (for example, if the bus master is reset during the transmission

of an instruction), the clock line (C) must be connected to an external pull-down resistor so

that, if all inputs/outputs become high impedance, the C line is pulled low (while the S line is

pulled low): this ensures that C does not become high at the same time as S goes low, and

Figure 3. Bus master and memory devices on the serial bus