USB-PD Decoder

What is a PDO?

A Power Data Object (PDO) is a 32-bit word a USB-C source uses to advertise a power profile, the Source Capabilities. Bits 31-30 give its type; the sink selects a PDO and replies with a Request Data Object (RDO). The tool decodes and rebuilds PDOs, but does not handle the packet level nor the physical layer.

A Power Data Object is a 32-bit word a USB-C source uses to advertise a power profile (the Source Capabilities). Bits 31-30 give its type; a sink selects a PDO and replies with a Request Data Object (RDO). This tool decodes and rebuilds PDOs, but does not handle the packet level nor the physical layer of the protocol.

What are the PDO types and how do they scale?

Field scaling depends on the family: Fixed, Battery and Variable encode voltage at 50 mV; Fixed/Variable current at 10 mA, Battery power at 250 mW. The PPS and AVS APDOs use 100 mV, with current at 50 mA or PDP power at 1 W. AVS is expressed in power because current varies with voltage.

Field scaling depends on the family — it is the most common mistake:

Type	bit 31-30	Voltage	Current / Power
Fixed Supply	00	50 mV	10 mA (current)
Battery	01	50 mV	250 mW (power)
Variable	10	50 mV	10 mA (current)
SPR PPS (APDO)	11 · 00	100 mV	50 mA (current)
EPR AVS (APDO)	11 · 01	100 mV	1 W (PDP, power)

The EPR Fixed (28/36/48 V) uses the same encoding as Fixed, with voltage > 20 V. For PPS, note that the runtime adjustment step is 20 mV, but the encoding of the limits in the APDO is at 100 mV — distinct things. For EPR AVS the available current varies with voltage, so the offer is expressed in power (PDP), not current.

Fixed PDO structure

The first Fixed PDO also carries the capability flags (USB-PD R3.1 Source layout):

b31-30  PDO type (00 = Fixed)
b29     Dual-Role Power
b28     USB Suspend Supported
b27     Unconstrained Power
b26     USB Communications Capable
b25     Dual-Role Data
b24     Unchunked Extended Messages
b23     EPR Mode Capable
b22     Reserved
b21-20  Peak Current
b19-10  Voltage      (50 mV/unit)
b9-0    Max Current  (10 mA/unit)

How does PDO → RDO negotiation work?

The negotiator applies a simplified selection: for a requested voltage it prefers the exact Fixed, then a programmable APDO (PPS/AVS) covering the value, finally Variable or Battery; with “maximum power” it picks the PDO with the highest available power. It is not the full Policy Engine state machine. EPR allows up to 13 PDO positions.

The negotiator above applies a simplified selection: for a requested voltage it prefers the exact Fixed, then a programmable APDO (PPS/AVS) covering the value, finally Variable/Battery; with “maximum power” it picks the PDO with the highest available power. It is not the full Policy Engine state machine: it is meant to read which PDO would be selected and the resulting RDO. EPR allows up to 13 PDO positions.

What are the tool’s limitations?

The tool decodes and encodes only the 32-bit PDO/RDO: no CRC, packet header or BMC physical layer. It runs no real electrical sanity check of the source. EPR AVS is expressed in power (PDP) and current at a given voltage is derived as P/V. The negotiator is an educational selection, not the full USB-PD negotiation with messages and timers.

• Decodes/encodes only the 32-bit PDO/RDO: no CRC, packet header or BMC physical layer.
• No real electrical sanity check of the source: values are those declared in the bits.
• EPR AVS is expressed in power (PDP); current at a given voltage is derived as P/V.
• The negotiator is an educational selection, not the full USB-PD negotiation with messages and timers.

References

• USB Power Delivery Specification R3.1 / R3.2 — USB-IF (Source Capabilities, PDO/RDO, APDO).
• Fixed PDO flag layout aligned with the Linux kernel include/linux/usb/pd.h.

Learn more: Anatomy of a USB-PD PDO wiki →