SCEA130 April   2023 SN74ACT08 , SN74AHC1G08 , SN74AHC1G09 , SN74LVC125A , SN74LVC1G07 , SN74LVC1G08 , SN74LVC1G125


  1. 11

Functional Block Diagram

For the purpose of this brief, a simplified Optical Network Terminal unit (ONT) block diagram is used to illustrate the logic and translation use cases, see Simplified Block Diagram for Optical Network Terminal Units. Each red block has an associated use-case document. Links are provided in Table 1 and Table 2. For a more complete block diagram, see the interactive online end equipment reference diagram for Optical Network Terminal units.

GUID-20230324-SS0I-HH5F-HTXC-WJBNJW5RZNCR-low.svg Figure 1 Simplified Block Diagram for Optical Network Terminal Units

Logic and Translation Use Cases

Each use case is linked to a separate short document that provides additional details including a block diagram, design tips, and part recommendations. The nearest block and use-case identifiers are listed to match up exactly to the use cases shown in the provided simplified block diagram.

Table 1 Logic Use Cases
Nearest Block Use-Case Identifier Use Case
Non-Isolated DC/DC Power Supply Supply Sequence Power Sequencing With RC Circuits and Schmitt-Trigger Logic
Processing Timed Restart Reset a System for a Short Time
Combine Error Signals Combine Error Signals
Table 2 Translation Use Cases
Nearest Block Use-Case Identifier Use Case
Sensing I2C Translate Voltages for I2C
RS-232, ZigBee® UART Translate Voltages for UART

Power Sequencing With RC Circuits and Schmitt-Trigger Logic

Optical Network Terminal units (ONTs) have multiple internal modules that can be operating at different voltages and require specific power-up sequencing to prevent spurious data from being sent during the system start-up. In many cases, simple logic gates and RC delays can be used to provide a power-up sequence.

The RC delays produce signals that are too slow for standard CMOS inputs, so logic parts with Schmitt-trigger inputs are required for this application. Fortunately, TI has the HCS logic family which has Schmitt-trigger architecture on every input, so any required logic function is available for use.

AND gates are the most common gate used for logic sequencing. Figure 2 shows an example circuit with 2 input power good signals and three delayed output stages. When PG1 goes high, there is a short delay (t1 ⋍ R1 × C1) after which PG2 can directly enable stage one. When PG2 becomes high, stage 1 immediately is activated and the timer for stage two begins (t2 ⋍ 2 × R2 × C2), which then activates stage 2, and finally, after a short delay (t3 ⋍ R3 × C3), stage 3 is activated. The connection of PG1 provides an immediate disable signal to stages two and three, while stage 1 takes a short time (t1) to become disabled. If PG2 becomes low, stage 1 is immediately disabled and stages two and three are disabled after their sequential delays; the same as during the start-up.

Of course, this is just one example. There are infinitely many combinations that can be achieved using discrete logic, and the HCS family enables you to build your own design with reliable operation, low size, and maximum flexibility.

GUID-20230317-SS0I-89MZ-HVMF-KRTJJ5FQTVSN-low.svg Figure 2 Example Logic Circuit for Sequencing Components

See more about a similar use case in the Combine Power Good Signals application report and the Logic Minute video Combining Power Good Signals.

  • Schmitt-trigger input architecture is required for having slow transitioning input signals; a dedicated Schmitt-trigger buffer can be added if a logic gate does not include Schmitt-trigger input architecture
  • The best practice is to power the start-up sequence logic gates directly from the primary power source or from a dedicated low IQ LDO regulator so the logic is powered before all other circuitry
  • All HCS logic family devices include positive input clamp diodes, so be sure to add a series resistor to any inputs that can exceed VCC + 0.5 V. Choose resistor values based on the input voltage (VIN), forward voltage (VF, typically 0.65 V), gate supply voltage (VCC) and maximum clamp diode current (IIK) per the following equation: R > V I N V C C V F I IK
  • Need additional assistance? Ask our engineers a question on the TI E2E™ Logic Support Forum
Table 3 Recommended Parts
Part Number Automotive Qualified VCC Range Type Features
SN74HCS08-Q1 2 V – 6 V Quad AND gate

Schmitt-trigger inputs

Positive input clamp diodes on all pins; add series resistors if input voltage exceeds VCC

SN74LVC1G17-Q1 1.65 V – 5.5 V Schmitt-trigger buffer

Schmitt-trigger inputs

Inputs are over-voltage tolerant; signals can exceed VCC

SN74LVC1G57 1.65 V – 5.5 V Configurable Logic Gate

Schmitt-trigger inputs

Between the '1G57 and '1G58, all 2-input logic gate functions can be produced. See data sheets for details.

SN74LV14A-Q1 2 V – 5.5 V Hex inverter

Schmitt-trigger inputs

Overvoltage tolerant inputs

Partial power-down support (Ioff)

Improved noise characteristics

SN74LV08A-Q1 2 V – 5.5 V Quad AND gate

Over-voltage tolerant inputs

Partial power-down support (Ioff)

Improved noise characteristics

SN74AUP1G57 0.8 V – 3.6 V Configurable Logic Gate

Schmitt-trigger inputs

Ultra low-power operation (0.6 μA max ICC)

Overvoltage tolerant inputs


For more devices with Schmitt-trigger input architecture, browse the online parametric tool which can be sorted by the desired voltage, output current, and other features.