Signal Edge

DIGITAL INPUTS & OUTPUTS

A digital signal uses discrete values (states 0 and 1). These values are transmitted as a function of time.

Rising edge: transition from low to high state (0 to 1)

Falling edge: transition from high to low state (1 to 0)

t_r: Rising time from 10 to 90% of the pulse.

t_f: Falling time from 90 to 10% of the pulse.

Propagation Delay

DIGITAL INPUTS AND OUTPUTS

The propagation delay for digital cards is defined as the amount of time it takes to a signal to travel from the signal source to the destination.

For OPAL-RT digital input card, the designated propagation delay is from the D-SUB connectors to the FPGA pins. In the case of a digital output card, the propagation delay is from the FPGA signal to the D-SUB connector of the simulator.

Minimum Pulse

DIGITAL INPUTS AND OUTPUTS

The minimum pulse t_min is defined as the shortest possible pulse width duration that is required to provide signal reliability.

A reliable signal consists of an input signal that meets the minimum time requirement for which the signal must remain in a specific state (hold time) to allow the digital input card to correctly detect the injected signal to be processed, or, for a digital output card, to allow the customer module correctly read the signal from the OPAL-RT output card.

If a pulse is shorter than the minimum pulse, the signal can be incorrectly interpreted by the system.

Maximum Toggling Rate

DIGITAL INPUTS AND OUTPUTS

The maximum toggling rate is defined as the highest toggling rate that can be supported by the Digital Output or Digital Input card.

The limit can be defined by the unit's minimum pulse width. But it can also be defined by some other simulator limitation (for example, thermal considerations).

In some cases (but not all the time), the maximum toggling rate can be defined as:

Ex. for a minimum pulse of 20 ns … 1/20 ns = 50 Mb/s

Sampling Rate: Maximum Frequency vs. Bit Rate

DIGITAL INPUTS AND OUTPUTS

A sample in digital systems is defined as a signal’s state at a certain point of time.

A sampling rate refers to how many data points (samples) can be taken/generate in a certain amount of time. The sampling rate can be defined in Hertz or bits per second.

MHz: Megahertz

Mb/s: Megabits per second

The relationship between frequency and bitrate can often (but not always) be defined by this figure. For a given signal’s period, two individual bits are present.

Thus, in the case of a clocklike signal like this, the bitrate can be defined as twice the clock signal’s frequency. (ex: 2 Mb/s = 1 MHz.)

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Threshold Voltages

DIGITAL INPUTS ONLY

The threshold voltage is the voltage level that distinguishes a “0 (Low)” from a “1 (High)” in digital circuits. When the voltage at the Integrated circuit (IC) input is in the threshold limit, it is interpreted as “High” or a “Low” according to the device threshold specification in datasheet.

When a voltage close to the threshold value is applied, the state becomes unstable without it being known whether it will be “High” or “Low” due to IC variance, fluctuations in the applied voltage, etc., which could cause the IC to malfunction.