Synchronizing USRP Events Using Timed Commands in UHD
Contents
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
- A
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Clocking and Timekeeping in the USRP
In this section, we will cover several key topics relating to USRP synchronization and the use of timed commands in UHD.
CHDR Packet Types and Structure
CHDR or "Compressed Header" packets are a derivative of the VITA 49 (VRT) protocol. While the CHDR 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
The type of packet is determined by the state of bits 63 and 62 in the CHDR header. See UHD Manual: Radio Transport Protocols for more information on CHDR structure.
All of these packet types have a single bit (61) used to denote whether an optional timestamp is included. If present, a timestamp is a 64-bit value representing an absolute time value. In this application note, we’re concerned with the use and functionality of command packets with a timestamp present, also known as “Timed Commands”.
VITA Time
The next topic to cover is the VITA time. This time value (denoted as vita_time) is a 64-bit counter driven by the USRP's FPGA base clock. vita_time is used as an absolute time reference for the radio and is local to the Radio Core of the USRP's FPGA. We'll discuss more about the Radio Core below.
PPS (Pulse Per Second)
PPS is a signal used by USRPs for time synchronization. Within the context of a USRP, a PPS signal is expected to have the following properties:
- 1Hz
- TTL Signal Levels
- 25% duity cycle
A USRP's PPS can be derived from a GPSDO automatically, from an externally supplied PPS signal, or via internal PPS synthesis (not supported in legacy USRPs).
A PPS trigger is used to coordinate time alignment events across multiple devices. For example, the USRPs internal sense of time (the vita_time) can be synchronously set/reset across multiple USRPs via UHD API calls such as
set_time_next_pps
set_time_unknown_pps
For further information on PPS and other common reference signals, see the UHD Manual: Device Synchronization.
