ZHCSNB6A April 2021 – February 2022 TPS23882B
PRODUCTION DATA
The Power Allocation settings in register 0x29 sets the maximum power level a port powers on. Settings for each class level from 2-pair 4 W (Class 1) up to 2-pair 30 W (Class 4) have been provided to maximize system design flexibility.
The Power Allocation settings in register 0x29 do not set the power limit for a given port. The port and channel power limiting is configured with the 2P (registers 0x1E- x 21) policing registers
During a turn-on attempt, if a PD presents a classification level greater than the power allocation setting for a port, the TPS23882B limits the number of classification fingers presented to the PD prior to turn on based on the power allocation settings in register 0x29. This behavior is called Power Demotion as it is the number of fingers presented to the PD that sets the maximum level of power the PD is allowed to draw before the PSE is allowed to disable it.
The IEEE 802.3 standard requires PDs that are power demoted by a PSE to limit their total power draw below the Type/class level set by the number of fingers presented by the PSE during mutual identification.
In a 2-pair system, Power demotion is limited to either 30 W (3-fingers) or 15.4 W (1-finger) as there is no other physical means of indicating to a PD over the physical layer that less than 15.4 W is available.
If register 0x29 is configured for either 4 W (class 1) or 7 W (Class 2), and a class 3 or higher device is connected, the port is not powered and a Start Fault is reported along with an "Insufficient Power" indication provided in register 0x24.
| Power Allocation Register 0x29 | Assigned Class Value (Based on the PD Connected at the Port) | ||||
|---|---|---|---|---|---|
| Class 1 PD | Class 2 PD | Class 3 PD | Class 4 PD | Class 5+ PDs | |
| 2-Pair 4 W | Class 1 | Start Fault Insufficient Power | Start Fault Insufficient Power | Start Fault Insufficient Power | Start Fault Insufficient Power |
| 2-Pair 7 W | Class 1 | Class 2 | Start Fault Insufficient Power | Start Fault Insufficient Power | Start Fault Insufficient Power |
| 2-Pair 15.5 W | Class 1 | Class 2 | Class 3 | Class 3 | Class 3 |
| 2-Pair 30 W | Class 1 | Class 2 | Class 3 | Class 4 | Class 4 |