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Introduction

The Trigger LE Hyst core implements a leading-edge discriminator with optional hysteresis, commonly used in nuclear and particle physics data-acquisition systems. The core compares the input signal against a programmable threshold and generates a trigger pulse when the signal crosses the threshold.

A key feature is the optional hysteresis mode (via the DELTA input), which uses two separate threshold levels to prevent false triggers caused by noise when the signal hovers near the threshold level.

Pin Description

DATA_IN Input WordSize bit BIT VECTOR
Input samples to be discriminated. Width is WordSize bits.
Default: Must be connected
POLARITY Input 1 bit BIT

Selects trigger edge polarity.

  • 1 – detect positive excursions (signal rising above threshold)
  • 0 – detect negative excursions (signal falling below threshold) Width = 1 bit.
Default: 1
THRESHOLD Input WordSize bit BIT VECTOR
Threshold level with bit-width equal to WordSize. A trigger is generated when the signal crosses this threshold in the direction specified by POLARITY.
INIBIT Input 16 bit BIT VECTOR
Number of clock cycles during which new triggers are suppressed after each trigger event (dead-time). 16-bit unsigned value. Set to 0 for no inhibition.
Default: 0
CLK Input 1 bit BIT
System sampling clock. All internal operations are synchronous to the rising edge of CLK.
Default: Default Board Clock
RESET Input 1 bit BIT
Synchronous active-high reset. Clears all internal state, trigger flags, and inhibition counters.
Default: Default Board Reset
DATA_OUT Output WordSize bit BIT VECTOR
Pass-through of the input data with 1 clock cycle latency. Width equals WordSize bits.
TRIGGER Output 1 bit BIT
Single-cycle pulse generated when the signal crosses the threshold. Can be used to timestamp the event or to start downstream processing.
TOT Output 1 bit BIT
Time-over-Threshold flag. High when the signal is above THRESHOLD - DELTA (for positive polarity) or below THRESHOLD + DELTA (for negative polarity). Provides a coarse measurement of pulse width.
DELTA WordSize bit

Hysteresis delta value with bit-width equal to WordSize. Defines the gap between the trigger threshold and the re-arm threshold.

  • For positive polarity: re-arm threshold = THRESHOLD - DELTA
  • For negative polarity: re-arm threshold = THRESHOLD + DELTA Only visible when Hysteresis property is enabled.

Properties

Property window

Analog Channel Word Size Wordsize

Set the analog channel word size in bits

Bit-width of the input and output data signals. Applies to DATA_IN, THRESHOLD, DELTA and DATA_OUT signals. Available values: 16, 32, 64, default 16. Note: This property is typically hidden and auto-configured based on the connected signal.

Default: 16

Options: 16 32 64

Hysteresis Hysteresis

Enable hysteresis logic to separate L->H to H->L commutation

Enable hysteresis logic to use separate thresholds for trigger and re-arm. When enabled, the DELTA input becomes available.

  • False (default) – Single threshold mode, trigger re-arms immediately after inhibition
  • True – Hysteresis mode, trigger re-arms only after signal crosses THRESHOLD - DELTA

Default: False

Usage

Functional overview

The core compares the input sample against the programmable threshold and detects transitions:

Without hysteresis (DELTA = 0 or Hysteresis property disabled):

  • Single threshold THRESHOLD is used
  • Trigger occurs when signal crosses THRESHOLD
  • The trigger can re-arm immediately after the inhibition period

With hysteresis (DELTA > 0 and Hysteresis property enabled):

  • Two threshold levels are used: THRESHOLD and THRESHOLD - DELTA
  • For positive polarity:
    • Trigger occurs when signal rises above THRESHOLD
    • The trigger can only re-arm after signal falls below THRESHOLD - DELTA
  • For negative polarity:
    • Trigger occurs when signal falls below THRESHOLD
    • The trigger can only re-arm after signal rises above THRESHOLD + DELTA

What is Hysteresis?

Hysteresis is a technique used to prevent spurious trigger events caused by noise when the signal amplitude is close to the threshold level. Without hysteresis, noise fluctuations can cause the signal to repeatedly cross the threshold, generating multiple false triggers from a single pulse.

With hysteresis enabled, two distinct threshold levels are defined:

  • THRESHOLD: The signal must cross this level to generate a trigger
  • THRESHOLD - DELTA (or + DELTA for negative polarity): The signal must cross this level before the trigger can be re-armed

This creates a “dead zone” between the two thresholds where crossing events are ignored, effectively filtering out noise-induced oscillations.

Hysteresis behavior

In the figure above:

  • The red line shows the threshold level
  • The green region indicates when the trigger is waiting to be re-armed (signal must fall below THRESHOLD - DELTA)
  • TRIGGER REARMED marks when the system is ready to accept a new trigger
  • The TOT signal shows the Time-over-Threshold duration
  • The TRIGGER pulse marks the leading edge detection

Polarity

The POLARITY input allows selecting the trigger edge:

  • POLARITY = 1 (positive): Trigger when signal rises above THRESHOLD
  • POLARITY = 0 (negative): Trigger when signal falls below THRESHOLD

Dead-time Inhibition

After each trigger, new triggers are inhibited for a programmable number of clock cycles using the INIBIT input. This prevents re-triggering on the same pulse or on pile-up events.

Time-over-Threshold (ToT)

The TOT output indicates how long the signal remains above (or below, depending on polarity) the threshold minus delta. This provides a coarse measurement of pulse width or charge.

Timing

  • Threshold comparison is performed on each clock cycle.
  • TRIGGER is issued synchronously with the threshold crossing detection.
  • DATA_OUT provides a pass-through of the input data (1 clock cycle latency).

Typical applications

  • Photon or particle counting with scintillators and SiPM/PMT front-ends
  • Leading-edge discriminator with noise rejection via hysteresis
  • Digital oscilloscope trigger with programmable dead-time
  • Pulse width measurement using ToT output