Dual 4-Bit Decade Counter# DM74LS390 Dual Decade and Binary Counters Technical Documentation
Manufacturer : FSC (Fairchild Semiconductor)
## 1. Application Scenarios
### Typical Use Cases
The DM74LS390 is a dual decade and binary counter integrated circuit containing two independent counters, each functioning as:
- Divide-by-2 and divide-by-5 counters (when configured as decade counters)
- Divide-by-2 and divide-by-8 counters (when configured as binary counters)
- Frequency dividers in digital clock circuits
- Event counters in digital instrumentation
- Timing chain elements in sequential logic systems
### Industry Applications
Digital Frequency Counters : Used as prescalers to extend measurement range by dividing high-frequency inputs before counting
- Industrial Automation : Position encoding, motor speed monitoring, and production line event counting
- Telecommunications : Channel selection, frequency synthesis, and clock division in communication equipment
- Consumer Electronics : Digital clocks, timers, and frequency synthesizers in audio/video equipment
- Test and Measurement Equipment : Timebase generation and signal conditioning in oscilloscopes and frequency meters
### Practical Advantages and Limitations
Advantages:
- Low power consumption (typical 45mW power dissipation)
- High noise immunity characteristic of LS-TTL technology
- Wide operating voltage range (4.75V to 5.25V)
- Independent reset functionality for each counter section
- Compact dual-function design reduces board space requirements
Limitations:
- Maximum clock frequency of 35MHz restricts high-speed applications
- TTL-level compatibility requires level shifting for interfacing with CMOS circuits
- Limited drive capability (standard TTL fan-out of 10)
- Temperature sensitivity in extreme environments (-0°C to +70°C operating range)
- No built-in glitch protection on clock inputs
## 2. Design Considerations
### Common Design Pitfalls and Solutions
Clock Signal Integrity
- Pitfall : Clock signal ringing causing false triggering
- Solution : Implement series termination resistors (22-100Ω) close to clock inputs
Reset Timing Issues
- Pitfall : Asynchronous reset causing metastability during counting
- Solution : Synchronize reset signals with system clock or use dedicated reset timing circuits
Power Supply Decoupling
- Pitfall : Inadequate decoupling causing erratic counting behavior
- Solution : Place 100nF ceramic capacitors within 10mm of VCC pins, with bulk 10μF capacitor per board section
### Compatibility Issues with Other Components
CMOS Interface Challenges
- Issue : TTL output levels (0.4V max LOW, 2.4V min HIGH) may not meet CMOS input thresholds
- Resolution : Use pull-up resistors (1-10kΩ) to VCC or dedicated level translators
Mixed Logic Family Integration
- Issue : Different propagation delays when interfacing with other logic families
- Resolution : Add appropriate delay matching or use synchronous design techniques
Load Driving Limitations
- Issue : Limited current sourcing/sinking capability (400μA source, 8mA sink)
- Resolution : Use buffer ICs (74LS244/245) for driving multiple loads or long traces
### PCB Layout Recommendations
Power Distribution
- Use star-point grounding for analog and digital sections
- Implement separate power planes for VCC and GND
- Route power traces wider than signal traces (minimum 20 mil width)
Signal Routing
- Keep clock signals away from output lines to minimize crosstalk
- Route reset signals with minimal length and avoid parallel routing with clock lines
- Use 45° angles instead of 90° for signal trace bends
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