It’s entirely logical that low-cost miniature microcontrollers have fewer ‘legs’ than their bigger brothers and sisters – sometimes too few. The author has given some consideration to how to economize on pins, making them do the work of several. It occurred that one could exploit the high impedance feature of a tri-state output. In this way, the signal produced by the high impedance state could be used for example as a CS signal of two ICs or else as an RD/ WR signal. All we need are two op-amps or comparators sharing a single operating voltage of 5 V and outputs capable of reaching full Low and High levels in 5-V operation (preferably types with rail-to-rail outputs).
Suitable examples to use are the LM393 or LM311. The resistances in the voltage dividers in this circuit are uniformly 10 kilohms. Consequently input A lies at half the operating voltage (2.5 V), assuming nothing is connected to the input – or the microcontroller pin connected is at high impedance. The non-inverting input of IC1A lies at two-thirds and the inverting input of IC1B at one-third of the operating voltage so that in both cases the outputs are set at a High state. If the microcontroller pin at input A becomes Low, the output of IC1B becomes Low and that of IC1A goes High. If A is High, everything is reversed.
The LM393 series are dual-independent precision voltage comparators capable of single or split supply operation. These devices are designed to permit a common mode range−to−ground level with single supply operation. Input offset voltage specifications as low as 2.0 mV make this device an excellent selection for many applications in consumer, automotive, and industrial electronics.
- Wide Single−Supply Range: 2.0 Vdc to 36 Vdc
- Split−Supply Range: ±1.0 Vdc to ±18 Vdc
- Very Low Current Drain Independent of Supply Voltage: 0.4 mA
- Low Input Bias Current: 25 nA
- Low Input Offset Current: 5.0 nA
- Low Input Offset Voltage: 5.0 mV (max) LM293/393
- Input Common Mode Range to Ground Level
- Differential Input Voltage Range Equal to Power Supply Voltage
- Output Voltage Compatible with DTL, ECL, TTL, MOS, and CMOS Logic Levels
- ESD Clamps on the Inputs Increase the Ruggedness of the Device without Affecting the Performance
- NCV Prefix for Automotive and Other Applications Requiring Unique Site and Control Change Requirements; AEC−Q100 Qualified and PPAP Capable
- These Devices are Pb−Free, Halogen Free/BFR Free and RoHS Compliant