Synchronizing USRP Events Using Timed Commands in UHD

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Application Note Number

AN-TC

Revision History

Date Author Details
2020-02-10 Sam Reiter Initial creation

Abstract

This AN discusses Timed Commands in UHD. We will explore usecases, theory of operation, and examples of timed command use in UHD 3.x.

Timed Commands: Overview and Usecases


Clocking and Timekeeping in the USRP

CHDR Packet Types and Structure

CHDR or "Compressed Header" packets are a derivative of the VITA 49 (VRT) protocol. While the protocol is proprietary to USRP devices, mapping to and from VITA-49 is a trivial task. All packet routing in b2xx and "Gen 3" USRPs uses the CHDR protocol.

There are 4 types of packets used in the USRP:

    • Data
    • Flow Control
    • Command
    • Command Response

VITA Time

PPS (Pulse Per Second)

Radio Core Block Timing

General RFNoC block Timing

Command Queue

Summary

USRP Time Sources

Internal PPS

External PPS

GPSDO

UHD API for Timed Commands

Overview of Relevant Methods

Timed Commands vs. Stream Commands with time_spec (remove this?)

Commands that can be timed

Time Resolution of Timed Commands

Example: Using Timed Commands to Phase Align Channels

Example: Using Timed Commands to Control GPIO

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