Anatomy of a USB Power Delivery PDO

When you plug a phone into a USB-C charger, before a single milliampere flows beyond the safe 5 V the two devices talk to each other. The charger declares what it can supply, the device chooses, and only then does the voltage rise. That declaration is a list of Power Data Objects (PDOs): 32-bit words, each one a power profile. This tool decodes and builds them; this page explains how they are made.

What a PDO is

A Power Data Object is a 32-bit word that describes one way a source can supply power. A charger does not offer a single profile but a list — its Source Capabilities — typically one to seven PDOs: 5 V, 9 V, 15 V, 20 V, perhaps a programmable profile. The receiving device (the sink) reads the list and asks for what it needs.

The basic rule, set by the specification: the first PDO is always 5 V Fixed, and the subsequent Fixed PDOs are in increasing voltage. This is what guarantees that any device, even the simplest, always finds the 5 V to start from.

The five types

The bits 30–31 of every PDO declare its type. They are the first thing to read, because they determine how to interpret all the other bits:

• Fixed Supply (00) — fixed voltage, maximum current. It is the classic profile: “5 V up to 3 A”, “20 V up to 5 A”.
• Battery (01) — a battery source, expressed as a voltage range and maximum power.
• Variable Supply (10) — unregulated variable voltage, expressed as a voltage range and maximum current.
• Augmented PDO / APDO (11) — the “smart” profiles introduced by the more recent revisions. A further subtype field (bits 28–29) distinguishes:
  • SPR PPS — Programmable Power Supply: the voltage adjusts continuously within a range, for adaptive charging.
  • EPR AVS — Adjustable Voltage Supply: the high-power profile (above 100 W) of the Extended Power Range.

Field scaling: the unit trap

This is the most common mistake when reading a PDO by hand. The fields do not contain volts and amperes: they contain integers, and each PDO type uses a different scale factor. Getting the factor wrong means reading “5 V” where there are “10 V”.

For Fixed, Variable and Battery PDOs:

$$\begin{array}{lcl}\text{Voltage: }50\text{mV/unit}& \to & \text{value}\times 0.05=\text{V}\\ \text{Current: }10\text{mA/unit}& \to & \text{value}\times 0.01=\text{A}\\ \text{Power: }250\text{mW/unit}& \to & \text{value}\times 0.25=\text{W}(\text{Battery only})\end{array}$$

A Fixed PDO of 5 V / 3 A therefore carries the number 100 in the voltage field (100 × 50 mV = 5 V) and 300 in the current field (300 × 10 mA = 3 A).

For APDOs (PPS and AVS) the scaling changes, and this is the second classic stumble:

$$\begin{array}{lcl}\text{Voltage: }100\text{mV/unit}& \to & \text{value}\times 0.1=\text{V}\\ \text{Current: }50\text{mA/unit}& \to & \text{value}\times 0.05=\text{A}\end{array}$$

Same bit, different meaning depending on the type: this is why a decoder must always read the bits 30–31 first, and only then interpret the rest.

The bit-by-bit structure

For each PDO, the tool shows a map of the 32 bits with the fields highlighted. The Fixed PDO is the one you meet most often, and its complete map is below — all thirty-two bits accounted for, with no gaps:

The eight flag bits (29–22) communicate the device’s capabilities, not a power profile: they describe who the device is. They are only to be read on the first Fixed PDO of the list, where the specification makes them meaningful; in subsequent PDOs they are ignored.

The bits people forget: Peak Current

Bits 21–20 are the ones summary descriptions skip, and they say something a designer needs: how much instantaneous overcurrent the source tolerates beyond the declared maximum, and for how long.

The three values in each row correspond to three different time windows (from the very short pulse to the prolonged overload). The practical meaning is that a 3 A PDO with code 11 tolerates peaks up to 6 A for short intervals: decisive information for sizing a sink’s input capacitor, or for understanding why a “3 A” charger does not complain at 4.5 A. With code 00 there is no margin at all, and the declared limit is the real one.

A decoding example

Take the value 0x0002D12C. Bits 30–31 are 00 → it is a Fixed PDO. The voltage field (bits 19–10) holds 180: 180 × 50 mV = 9 V. The current field (bits 9–0) holds 300: 300 × 10 mA = 3 A. The PDO therefore declares “9 V up to 3 A”. Building it is the inverse operation: 9 V / 50 mV = 180 in the voltage field, 3 A / 10 mA = 300 in the current field, type 00.

PPS and EPR: two different extensions

The two acronyms are often confused, but they answer opposite needs.

PPS (Programmable Power Supply, from USB-PD 3.0) is used to adjust the voltage precisely and continuously within a range, generally below 21 V. It is meant for adaptive charging: the charger follows the battery’s charging curve instead of imposing fixed steps. A detail that breeds confusion: the runtime adjustment happens in 20 mV steps, but the limits of the range in the PDO are encoded in 100 mV steps. These are two different things — the granularity at which you regulate, and the resolution at which the endpoints are declared.

EPR (Extended Power Range, from USB-PD 3.1) instead serves to go beyond 100 W, up to 240 W, introducing Fixed voltages at 28, 36 and 48 V and the AVS profile. In AVS the current is not fixed: it varies with the voltage, because the constraint is power. This is why, for AVS profiles, the tool shows the power instead of a maximum current — it reflects how the source actually reasons.

The two APDO layouts do not resemble each other

Saying that “for APDOs the scaling changes” covers the units, but is not enough: PPS and AVS have completely different field boundaries, and using one’s layout on the other produces plausible, wrong numbers.

Two asymmetries. AVS does not encode a minimum voltage: the minimum is fixed by the specification at 15 V and does not appear in the bits — a tool has to know this, because there is no field to read. And AVS carries no current but the PDP (Power Delivery Power) in watts: consistently, the allowed current varies with the chosen voltage.

The reserved subtypes

The subtype field is two bits, so it has four possible values, but R3.1 defines two: 00 SPR PPS and 01 EPR AVS. The values 10 and 11 are reserved.

A tool has two ways to behave in front of a reserved subtype, and only one is correct: it can reject the PDO, or decode it best-effort with the nearest layout — but in that second case it must declare the field’s true value in the bit map, marked as reserved. Showing 01 (EPR AVS) over bits that read 10 would be lying about the bits, which is exactly what a decoder must not do. This tool decodes best-effort and flags the field as Reserved.

How much fits in a field

The fields have a fixed number of bits, hence a ceiling:

The Fixed voltage ceiling — 51.15 V — sits comfortably above EPR’s 48 V, while the current one, 10.23 A, is why past 5 A you go up in voltage and not in current. The PPS current field is the narrowest of all (7 bits, maximum 6.35 A): PPS and EPR are not overlapping extensions, and this is one of the points where you can see it.

Negotiation: from PDO to RDO

Decoding the list is only half the work. The living part of the protocol is the negotiation: the sink examines the offered PDOs and chooses one, building a Request Data Object (RDO) that it sends back to the source. The RDO does not contain a voltage: it contains the position of the chosen PDO in the list (the Object Position, from 1 to 13 in EPR) plus the requested operating and maximum current.

The selection logic follows a priority: an exact match on a Fixed PDO takes precedence, then the programmable profiles (PPS/AVS) when the voltage falls within their range, finally the fallbacks. If no profile satisfies the request, the sink stays on the safe 5 V. In negotiation mode the tool shows which PDO would be chosen and why, and builds the corresponding RDO.

The limits of this tool

It is a teaching and working tool, not a conformance validator. It decodes and encodes PDOs and RDOs according to the specification’s scaling, but it does not verify the packets’ CRCs, does not simulate the protocol’s physical layer (BMC, control messages, timers), and covers the commonly used PDO types — not every corner case of the most recent revisions. The values it produces are bit-accurate for the PDO structure; a real conformance check requires a protocol analyzer and the official specification.

References

• USB Implementers Forum (USB-IF) — usb.org, the body that publishes and maintains the specification.
• USB Power Delivery Specification — the normative document (revisions 3.1 / 3.2), available in the documents area of usb.org/documents.
• USBPD-Stack — our open-source USB Power Delivery stack for the PTN5110 controller, which implements the decoding and negotiation described here: github.com/stefanofante/USBPD-Stack.
• Related tool — the PDO decoder/encoder puts all this into practice: paste a PDO and see the fields, or build it from values.