Verify That Every Device Releases a Shared SPI MISO Line

Separate chip-select signals do not automatically make several serial outputs safe to connect together. Each device must release the shared return line when another device drives it. Read the output behavior explicitly, especially for parts whose serial port was designed for daisy chaining or readback.

Classify each output

The ADG714 datasheet describes a serial DOUT used for daisy chaining. Analog Devices’ corresponding FAQ explains that deselection does not make this output a conventional released shared MISO. This illustrates why an SPI-compatible label is insufficient evidence of bus-sharing behavior.

For every connected device, record output type, selected-state behavior, deselected-state behavior, power-off behavior and any mode that changes the pin function. Keep the manufacturer’s pin name alongside the schematic’s MISO label so that a shift-register output is not accidentally treated as a generic bus output.

Review the handover interval

Check output-enable and disable timing around chip select. Compare the first device’s release with the next device’s drive time. Include level translators or buffers in that path; their control and direction behavior can create an overlap even when both peripheral devices are compatible individually.

For example, an output that remains low after a completed transaction can corrupt the next device’s high bits. A pull-up resistor may change an idle voltage but cannot make a driven-low output release. Resolve the actual drive behavior rather than tuning the resistor until one transaction happens to pass.

Observe one device at a time

Verify idle, selected and deselected states with a controlled setup that avoids opposing drivers. Then test the planned sharing arrangement after reviewing its electrical limits. Capture chip selects, clock and the return line together so that a malformed bit can be associated with ownership of the bus.

Include reset, initialization and a powered-down peripheral if those states occur in the product. A device may use the expected serial mode after configuration while presenting a different default pin function before that configuration completes.

Choose an explicit sharing solution

If an output does not release as required, consider a separate return input, an appropriately controlled buffer or the intended daisy-chain topology. Check the timing and voltage requirements of the selected approach. Do not describe series resistance as proof that contention is acceptable.

Retain a bus-ownership table with the schematic and firmware interface definition. Each state should identify the permitted driver and all required released outputs. That record helps reviewers catch a later peripheral substitution whose command format matches but whose deselected output behavior does not.