7.4.13 Cascading Multiple Devices

A GPIO pin can be used to coordinate multiple controllers by using it as a power good-output from one device and connecting it to the PMBUS_CNTRL input pin of another. This imposes a master/slave relationship among multiple devices. During startup, the slave controllers initiate their start sequences after the master has completed its start sequence and all rails have reached regulation voltages. During shutdown, as soon as the master starts to sequence-off, it sends the shut-down signal to its slaves.

A shutdown on one or more of the master rails can initiate shutdowns of the slave devices. The master shutdowns can be initiated intentionally or by a fault condition. This method works to coordinate multiple controllers, but it does not enforce interdependency between rails within a single controller.

The PMBus specification implies that the power-good signal is active when ALL the rails in a controller are regulating at their programmed voltage. The UCD90120A allows GPIOs to be configured to respond to a desired subset of power-good signals.