Skip to content

Universal Clock

Jump to bottom
JimGat edited this page Oct 1, 2026 · 2 revisions

Universal Clock and Offline GPS Time Source

CYM's Clock is available from both Classic and Modern navigation on every supported firmware target. It is more than a display: it arbitrates trusted UTC sources, can recover time from public NTP, and can serve local NTP to equipment that has no Internet or cellular service.

Why this matters for amateur radio

Time-synchronized digital modes such as FT4 and FT8 depend on transmit and receive windows lining up. A CYM board with a UART GPS receiver can provide a practical local UTC source at an off-grid station, field deployment, emergency exercise, or any site outside cellular coverage:

  1. Connect and acquire a qualified GPS fix.
  2. Open Clock → AP NTP.
  3. Join the displayed CYM-NTP-xxxxxx Wi-Fi network from the radio computer.
  4. Configure that computer to use the displayed CYM AP address, normally 192.168.4.1, as its NTP server.
  5. Confirm the Clock shows a trusted source rather than UNSYNCED before operating.

This is intended to keep an isolated station comfortably inside normal FT4/FT8 timing needs. With qualified GPS lock, CYM's NTP replies correctly advertise protocol stratum 1 with reference ID GPS; public-NTP-disciplined replies advertise stratum 2 with reference ID SNTP. That protocol classification does not make CYM a PPS-disciplined or laboratory-grade stratum-1 timing instrument: the present firmware timestamps UART NMEA RMC data and does not use a GPS pulse-per-second input. Sub-50-ms performance may be possible with favorable receivers and transport latency, but the current firmware and boards have not been characterized to that level; it is not a guaranteed sub-50-ms source.

The WSJT-X User Guide says the computer clock must be synchronized to UTC within about one second.[3] CYM's displayed uncertainty and trust state should be treated as the operator's go/no-go indication, not the optimistic number from a single trial.

Modes

Display Only

Strictly offline. CYM does not start Wi-Fi station mode, create an AP, or query public NTP. It displays the best source already trusted by the timekeeper: GPS, fitted RTC, validated GPS-module RTC, or disciplined RTOS holdover. If none exists, it reports UNSYNCED / Untrusted.

Client NTP

Joins an existing Wi-Fi network and makes a bounded request to pool.ntp.org. While selected and GPS is not locked, CYM retries every 60 seconds. A successful result disciplines the system clock. On WS-C5-28 it repairs the fitted RTC only when the RTC is absent, invalid, untrusted, or at least five seconds wrong. A failed refresh preserves the last trusted source and its lineage.

AP NTP

Creates a WPA2-protected CYM-NTP-xxxxxx network and serves UDP/123 from the best trusted source, without requiring an upstream network. If time is untrusted, NTP replies set the leap alarm and stratum 16 so clients should reject them.

Source priority and status

  1. GPS LOCK — three consecutive, monotonic active-fix RMC observations.
  2. PUBLIC NTP — accepted network synchronization.
  3. RTC HOLDOVER — trusted fitted RTC, currently WS-C5-28 only.
  4. GPS RTC HOLDOVER — a GPS module's no-fix clock that was previously cross-checked and remains monotonic.
  5. NTP HOLDOVER / RTOS HOLDOVER — disciplined system clock continuing without its original source.
  6. UNSYNCED — no source CYM can honestly trust.

Reliability labels are Excellent, Good, Holdover, Degraded, and Untrusted. Estimated uncertainty grows with source age and the selected board profile. All RTC, GPS, system-clock, public-NTP, and served-NTP values remain UTC; the saved display offset changes presentation only.

With a live GPS fix, Clock also displays a six-character Maidenhead locator such as Grid: EM12ab, calculated locally without Internet access.

Accuracy and suitability grid

These are conservative engineering allowances used by firmware, not measured guarantees.

Source/state Base allowance Growth/holdover allowance Practical guidance
Qualified UART GPS lock ±500 ms Requalified by incoming RMC Served as protocol stratum 1 / GPS; expected to be practical for FT4/FT8 when trusted; no PPS and not guaranteed below 50 ms
Fresh public NTP ±250 ms Becomes NTP/RTOS holdover as it ages Good recovery source when Internet exists; path asymmetry and Wi-Fi load matter
WS-C5-28 PCF85063A RTC holdover Starts from last disciplined UTC 50 ppm, about 4.32 s/day worst-case allowance Battery-backed off-grid holdover; resynchronize before tight timing work after long outages
RTOS holdover on C5/S3 boards Starts from last disciplined UTC 100 ppm, about 8.64 s/day allowance Short outages only for timing-sensitive modes
Classic CYD RTOS holdover Starts from last disciplined UTC 150 ppm, about 12.96 s/day allowance Least predictable profile; no supported GPS UART
Validated GPS-module RTC holdover ±2 s initial 100 ppm, about 8.64 s/day allowance Useful continuity indication, but never shown as GPS lock
UNSYNCED Unknown Unknown Do not use as a timing authority

Physical validation: NM-CYD-C5 v2.15.46

A live LAN test used an NM-CYD-C5 at DHCP address 192.168.50.74 and an NTP-synchronized JARVIS host as the client:

  • Before GPS lock: 3/3 valid 48-byte NTPv4 server replies; leap indicator 0, stratum 2, reference ID SNTP, and matching origin timestamps.
  • After GPS lock: the server changed to stratum 1 and reference ID GPS as designed; 14/15 requests returned valid replies across two batches.
  • The one loss occurred in a rapid 200-ms-spacing batch. A one-second-spacing confirmation returned 5/5 replies, with median RTT 121.889 ms and median measured offset +25.745 ms against the synchronized test host.

This physically validates source-state signaling and basic NTP-client interoperability on the tested board and LAN. It is a point-in-time functional result, not a calibrated accuracy specification. Wi-Fi scheduling, path asymmetry, UART sentence timing, and the lack of PPS still apply. The test address was assigned by DHCP and is not a fixed CYM address.

WS-C5-28 battery-backed RTC reliability

WS-C5-28 includes a PCF85063A and supports battery-backed retention.[4] The RTC IC does not contain an atomic or temperature-compensated frequency reference: it uses an external 32.768 kHz quartz crystal.[2] Real accuracy therefore depends on the fitted crystal's tolerance, temperature curve, PCB capacitance/layout, aging, calibration offset, battery health, and time since the last trusted discipline.

The PCF85063A provides an offset register for calibration, but CYM does not currently characterize each board or automatically calibrate that register. The firmware therefore uses a deliberately conservative 50-ppm holdover allowance—about 4.32 seconds/day—and labels the estimate unqualified. This does not mean every board will drift that much; it means documentation does not promise better until physical soak measurements exist.

Sources: [2] https://www.nxp.com/docs/en/data-sheet/PCF85063A.pdf — NXP PCF85063A data sheet [3] https://wsjt.sourceforge.io/wsjtx-doc/wsjtx-main.html — WSJT-X User Guide [4] https://docs.waveshare.com/ESP32-C5-Touch-LCD-2.8 — Waveshare ESP32-C5-Touch-LCD-2.8 documentation

Clone this wiki locally