Introduction

ToTDigital implements a Time Over Threshold measurement for digital signals.

Time Over Threshold (ToT) is a technique widely used in particle physics and radiation detection where the duration of a pulse above a threshold is measured. This duration is often correlated to the energy or amplitude of the original signal, providing a simple way to estimate pulse charge without a full ADC.

The measurement resolution depends on the selected clock frequency:

$$ \Delta t = \frac{1}{f_{\text{CLK}}} $$

Clock Resolution
200 MHz 5 ns
500 MHz 2 ns

Pin Description

IN
Digital input signal to be measured. The pulse width of this signal is measured. The active edge is configurable via the Signal Edge property.
RUN
Enable signal. When high, the block is active and can perform measurements. When low, measurements are inhibited. Default value: 1 (always enabled).
CLK
Output clock. The ToT output and DV signal are synchronous to this clock (acquisition clock domain).
RESET
Global reset. Resets the internal state machine and counters.
ToT
48-bit Time Over Threshold output. Contains the measured pulse width in clock cycles. Valid only when DV = 1.
DV
Data Valid strobe. High for one CLK cycle when a new ToT measurement is available on the output.

Properties

Clock Source ClkSource
Selects the clock frequency for the ToT counter. Higher frequencies provide better resolution. Options: Acquisition, 200MHz, 500MHz.

Default: 200MHz

Signal Edge EdgeSignal

Selects which edge of the input signal starts the measurement.

  • Rising: Measure time from rising edge to falling edge (positive pulse)
  • Falling: Measure time from falling edge to rising edge (negative pulse)

Default: Rising

Usage

Functional overview

ToTDigital measures the width of digital pulses by counting clock cycles while the input signal is active.

Feature Value
Output Width 48 bits
Clock Options Acquisition, 200MHz, 500MHz
Edge Selection Rising or Falling

Operation Principle

The Time Over Threshold measurement works as follows:

  1. Leading Edge Detection: The block detects the configured edge (rising or falling) of the input signal
  2. Counter Start: A high-frequency counter begins counting
  3. Trailing Edge Detection: When the signal returns to its original state, the counter stops
  4. Output: The counter value (pulse width) is output with a DV strobe

Timing Diagram

Use Cases

Time Over Threshold is commonly used in:

  • SiPM/PMT readout: Pulse width correlates with the number of photons detected
  • Scintillator detectors: ToT provides energy information without waveform digitization
  • ASIC readout: Many front-end ASICs provide ToT outputs
  • High-rate applications: ToT requires less bandwidth than full waveform recording

Energy Correlation

For many detector types, the ToT value has a relationship with the deposited energy or charge:

$$ E \propto f(\text{ToT}) $$

The exact relationship depends on the detector and front-end electronics. Typically, a calibration curve is established to convert ToT values to energy.

Supported Boards

This component is available on:

  • DT5550
  • DT5560
  • DT5550SE
  • R5560

Clock Source Options

Option Frequency Resolution
Acquisition Board-dependent Variable
200MHz 200 MHz 5 ns
500MHz 500 MHz 2 ns

Maximum Measurable Pulse Width

With 48-bit counter at different clock frequencies:

Clock Max ToT Value Max Pulse Width
200 MHz 2^48 - 1 ~1.4 million seconds
500 MHz 2^48 - 1 ~562,000 seconds

In practice, typical ToT values are much smaller (ns to µs range).