05 · Real-time polling

How to read live values from an STD module without missing or double-counting a window. Register-level detail: Measurement & energy model and Data-ready output & events.

The measurement window

Samples aggregate into windows of whole mains cycles nearest 100 ms — 5 cycles at 50 Hz, 6 at 60 Hz. Each finished window is published atomically as a complete snapshot (all channels and totals), and WIN_SEQ (0x0016) steps by exactly 1. That is about ten snapshots a second. Polling faster returns the same data.

Without mains the module keeps publishing on a slower fallback tick (500 ms) that refreshes status and clock; on that tick WIN_SEQ steps but there is no DRDY pulse.

Option 1 — poll WIN_SEQ

1. Read WIN_SEQ. If it is unchanged since the last read, there is no new snapshot.
2. If it changed, read the blocks you need — one transaction per block, up to 128 bytes; fields in one transaction come from one publication and cannot tear.

Option 2 — react to the DRDY edge

DRDY is an open-drain output without a pull-up on the module; when you use DRDY, fit a pull-up on the master side — 10 kΩ to the master's 3.3 V logic supply is recommended. It idles high and gives a ~10 µs low pulse for each new window, after the registers are updated — a read triggered by the falling edge always sees the fresh snapshot.

• On each serviced edge, check that WIN_SEQ stepped by exactly 1; a larger step means missed edges.
• Alarm mode: an event bit set in both EVENT_FLAGS and EVENT_MASK holds DRDY low until the flag is cleared or unmasked. Still low after ~100 µs means alarm — read EVENT_FLAGS.

What a snapshot gives you

Per voltage channel: RMS voltage. Per current channel: RMS current, active power (signed; positive = import), reactive power (positive = inductive), apparent power, power factor (signed). Per device: frequency and totals (k = 0 grand, k = 1..3 per phase). Units and scales are in the register map.

Below about 20 mA the published current is also smoothed over a few seconds for a stable reading; power and energy are not affected.

After a restart

Detect a restart with UPTIME (0x08A0, decreasing) or BOOT_COUNT (0x00CE, stepping). Then: re-arm the event mask and thresholds, set the time again, and re-anchor your energy deltas. Right after boot WIN_SEQ reads 0 for about one window, FREQ reads 0 for ~2 s, and the first ~3 windows add no energy; allow ~1 s before judging health.

In the client libraries

readAll() returns one coherent snapshot on both product lines; the libraries pair it with DRDY or with WIN_SEQ for you. See Examples. On a module with snapshot banks and energy counters use Arduino 1.6.2 or later or ESP-IDF 1.7.3 or later.