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.

Another method to cascade multiple devices is to connect the power-good output of the first device to a MON pin of the second device; connect the power-good output of the second device to a MON pin of the third device, and so on. Optionally, connect the power-good output of the last device to a MON pin of the first device. The rails controlled by a device have dependency on the previous device’s power-good output. This way, the rails controlled by multiple devices can be sequenced. Also, the de-assertion of a power-good output can trigger a UV fault of the next device. The UV fault response can be configured to shut down other rails controlled by the same device. This way, when one rail has fault shutdown, other rails controlled by other devices can be shut down accordingly.

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 UCD9090A allows GPIOs to be configured to respond to a desired subset of power-good signals.

Multiple UCD9090A devices can also work together and coordinate when faults happen with a fault pin connection. One GPI pin can be configured as a Fault pin. The Fault pin is connected to a Fault Bus. Each Fault Bus is pulled up to 3.3 V by a 10-kΩ resistor. All the UCD9090A devices on the same Fault Bus are informed of the same fault condition. An example of Fault Pin connections is shown in Figure 7-19.

Figure 7-19 Fault Pin Connection