
DNA/DNR-IRIG-650 IRIG Timing Layer
Chapter 1 7
Introduction
Tel: 508-921-4600 www.ueidaq.com Vers: 4.6
Date: March 2019 DNx-IRIG-650 Chap1x.fm
© Copyright 2019
United Electronic Industries, Inc.
The IRIG input time code can be received from one of the following sources:
•AM-modulated signal acquired by the 16-bit ADC and processed by the
AM2NRZ module. This module automatically detects the zero crossing,
adjusts the zero level, extracts the high- (mark) and low- (space) ampli-
tude ratio and generates the resulting NRZ-L (i.e. DCLS) code.
•Manchester II encoded signal processed by the MII2NRZ module. You
must select the correct carrier for the NRZ data extraction to work.
•Direct NRZ input; where the input signal is direct-current level shifted
(DCLS) signal which is pulse-width coded but is not AM-modulated.
•A forced initial time setup from the DNA host, immediate, and on 1PPS
edges
NOTE: The encoding “non-return to zero, level (NRZ-L)” is synonymous with
“direct-current level shift (DCLS)” encoded signals
A 1PPx(e.g. 1PPS) input “on-time pulse” can be sourced from NRZ time code.
If an NRZ-encoded time code is not available, an external 1PPS signal can be
delivered from a PowerDNx SYNC line, an external input, or the GPS input.
NRZ information is down-converted by the NRZ2TIME module to create time
characters: “Px” (position identifier), “0” (logic 0), “1” (logic 1), plus idle charac-
ters. In most time codes, the IDLE character is not used, and if present, it usually
indicates a lost carrier signal.
After the input data stream is parsed into time characters, the characters are
processed by the Time Decoder module to: extract an “on-time” pulse, copy
incoming data into double-buffered time messages, validate these messages
using a validation table. Each validated message is further processed to extract
time information, which is then copied into timing registers.
The current time is maintained in the Time Keeper module, which receives time
from the Time Decoder and keeps track of the time and/or date increments. If the
current time does not match the time received from the decoder, an interrupt is
generated. When this occurs, the time received from the decoder is then
accepted as the valid current time.
In hardware the Time Keeper module always maintains a time that is 1 second
ahead of the time just received - but the software does correct the returned time.
The reason for this is that the time code received from the Time Assembler mod-
ule is known to be about 1 time frame behind its own “on-time” pulse.
If an external time source is not available, the Time Keeper module maintains
time based on internal or external 1 PPS pulses and the initial time set by the
host, or based on the last valid time recovered from the time source. The 1PPS
time interval that is used for the flywheel counter should pass strict validation
prior to use, and in case of lost or invalid input data, requires at least two periods
of valid 1 PPS signal prior to use. By default, 1 PPS valid interval is set to ±50µs.
The flywheel takes in the 100MHz clock and synchronizes to it on 1PPS pulse.
The 100MHz base clock of the IRIG-650 is generated by a PLL on the FPGA
that multiplies the output of a very high precision 20MHz voltage controlled,
temperature compensated oscillator with ±50ppb frequency stability and
maximum drift of ±500ppb, the majority of which will happen within the first year
of operation and can be compensated by the in-factory calibration which uses a
Rubidium Standard source.