SCLA048 May 2022 SN74HCS08

1

Functional Block Diagram

For the purpose of this report, a simplified string inverter block diagram is used to illustrate the logic and translation use cases, see Figure 1-1. Each red block has an associated use-case document. Links are provided in Table 1-1 and Table 1-2. For a more complete block diagram, see the interactive online end equipment reference diagram for string inverters.

Figure 1-1 Simplified Block Diagram for String Inverters

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 1-1 Logic Use Cases

Nearest Block	Use-Case Identifier	Use Case
Self-Diagnostics/Monitoring	Fault Latch	Catch a Digital Pulse Multiple Fault Monitoring
Self-Diagnostics/Monitoring	Fault Combination	Use Fewer Inputs to Monitor Error Signals
Digital Processing	Buffer	Redrive Digital Signals
Output User Interface	LED Driver	Drive Indicator LEDs

Table 1-2 Translation Use Cases

Nearest Block	Use-Case Identifier	Use Case
Wireless Interface	UART	Translate Voltages for UART
Wired Interface	RGMII	Translate Voltages for RGMII

Multiple Fault Monitoring

It is common to see string inverters utilizing multiple sensors that could indicate fault conditions. Each fault signal can be individually monitored utilizing individual pins of a system controller; however, another approach is to utilize a combination of digital latches and a shift register to reduce the number of system controller pins required to monitor for issues. This has the added advantage that the controller can only occasionally poll for errors and will not miss anything due to the added latches.

Figure 1-2 [Left top] Single Negative Pulse Latch, [Left bottom] Single Positive Pulse Latch,
[Right] Monitoring Multiple Latches Using Parallel-Input Shift Register to Reduce Controller Pin Count

See more about similar use cases in the Logic Minute videos Design an Alarm / Tamper Circuit with an S-R Latch and Increase the Number of Inputs on a Microcontroller.

Dedicated logic latches catch even extremely short events that could otherwise be missed by the controller
Positive and negative latches can be combined as needed depending on the output of the sensor
Eight or more sensors can be monitored by utilizing only 4 GPIO pins
Ensure that all input signals are digital; use comparators or Schmitt-trigger buffers to convert analog signals
[FAQ] How does a slow or floating input affect a CMOS device?
Need additional assistance? Ask our engineers a question on the TI E2E™ Logic Support Forum

Table 1-3 Recommended Parts

Part Number	Automotive Qualified	Operating Voltage Range	Features
SN74AUP2G00		0.8 V to 3.6 V	AUP family logic devices are extremely low power; I_CC < 0.9 μA
SN74AUP2G00			One latch per device (2 × 2-input gates)
SN74AUP2G02			('00) NAND-based positive pulse detectors
SN74AUP2G02			('02) NOR-based negative pulse detectors
SN74HCS00-Q1	✓	2 V to 6 V	HCS family logic has integrated Schmitt-trigger inputs allowing for slow input signals
SN74HCS00			Up to two latches per device (4 × 2-input gates)
SN74HCS02-Q1	✓		Low power consumption - I_CC < 2 μA
SN74HCS02			('00) NAND-based positive pulse detectors
SN74HCS165-Q1	✓		('02) NOR-based negative pulse detectors
SN74HCS165			('165) Parallel-input shift registers increase the number of inputs; can be daisy-chained for 16+ inputs from only 4 GPIO pins

For more devices, browse through the online parametric tool where you can sort by desired voltage, channel numbers, and other features.