HGSEMI Logic Devices

74HC (High-speed CMOS) has CMOS input levels (0.3×VCC for low, 0.7×VCC for high) and operates from 2V to 6V. 74HCT (High-speed CMOS TTL-compatible) has TTL input levels (0.8V max for low, 2.0V min for high) and operates from 4.5V to 5.5V. Use 74HC in CMOS systems with 3.3V or 5V supplies. Use 74HCT when interfacing with TTL devices or when you need guaranteed TTL-compatible input thresholds at 5V. Both have CMOS output levels (near rail-to-rail) and similar speed and power characteristics.

Use 74HC for CMOS systems. Use 74HCT for TTL compatibility. Contact our FAE team for logic family selection guidance.

Yes, HGSEMI logic devices are designed as direct replacements for industry standard logic families. HG 74HC series is pin-compatible and functionally equivalent to standard 74HC series. HG 74HCT series replaces 74HCT and 74LS TTL. HG 4000 series replaces CD4000 CMOS logic. All devices use identical pinouts, have equivalent or better electrical specifications, and operate over the same voltage ranges. This allows drop-in replacement without PCB changes. HGSEMI devices often offer advantages like lower cost, better availability, and comparable or better performance.

HGSEMI logic provides drop-in replacements. Contact our FAE team for cross-reference assistance.

CMOS logic offers several advantages over TTL: Much lower power consumption - CMOS draws current only during switching, while TTL draws continuous current

Wider supply voltage range - CMOS operates from 2V to 6V (74HC) or 3V to 18V (4000 series), while TTL requires 5V ±5%

Better noise immunity - CMOS has larger noise margins

Higher input impedance - CMOS inputs don't load driving circuits

and Rail-to-rail output swing - CMOS outputs swing nearly to supply rails. TTL's advantage is higher output drive current for some applications, though CMOS drive capability has improved significantly.

Use CMOS for new designs for lower power and better noise immunity. Contact our FAE team for CMOS logic recommendations.

Unused CMOS inputs must never be left floating as they can pick up noise and cause oscillation or excessive power consumption. Options for handling unused inputs: Tie to VCC or GND depending on logic function needed

Connect to used inputs if same signal needed

or Use pull-up or pull-down resistors (10kΩ to 100kΩ) if flexibility needed. For AND/NAND gates, unused inputs should be tied high. For OR/NOR gates, unused inputs should be tied low. For inverters/buffers, inputs can be tied either way or paralleled with used inputs.

Never leave CMOS inputs floating. Tie to appropriate level or used inputs. Our FAE team can provide design guidelines.

Propagation delay is the time for a signal to propagate through a logic gate from input to output. Typical values are 5-15ns for 74HC at 5V. Propagation delay affects: Maximum operating frequency - total delay through signal path determines max speed

Timing margins - in synchronous circuits, delay affects setup and hold times

and Signal integrity - multiple gates in series accumulate delay. When designing high-speed circuits, calculate total propagation delay through the critical path and ensure it meets timing requirements. Temperature and supply voltage affect delay - higher temperature and lower voltage increase delay.

Calculate total propagation delay in critical paths. Select logic family with adequate speed. Contact our FAE team for high-speed design guidance.