NXP was so nice to send me demo board of 74AXP1G57. It is configurable, 3-input digital logic device. Can act as AND, EXNOR, inverter, buffer etc. Power supply is in 0.7-2.75V range, however I checked also voltages below lower limit. Demo board has 4 ICs:
In order to check dynamic behaviour, I connected 3 gates configured as inverters to form ring oscillator. Results are below:
1) supply voltage 0.6V (so lower than stated in datasheet; below stable oscillations were not possible):
Frequency generated was about 600kHz, so transition time of each gate was about 275ns. Wave is not symmetrical due to internal structure of gate.
2) supply voltage 0.65V:
Frequency increased to about 1.8MHz (3 times), so transition time of each gate was about 94ns. Wave is still not symmetrical due to internal structure of gate.
3) supply voltage 0.7V (minimum supply voltage specified for this logic gate):
Frequency increased to about 3.4MHz (2 times), so transition time of each gate was about 49ns. Wave is still not symmetrical due to internal structure of gate.
4) supply voltage 0.8V:
Frequency increased to about 8.6MHz (over 2 times), so transition time of each gate was about 19ns. Symmetricity of wave improved..
5) supply voltage 0.9V:
Frequency increased to about 14.5MHz (nearly 2 times), so transition time of each gate was about 11.5ns. Symmetricity of wave is nearly perfect.
6) supply voltage 1.0V:
Frequency increased to about 21MHz (about 1.5 times), so transition time of each gate was about 8.1ns. Symmetricity of wave is nearly perfect.
7) supply voltage 1.2V:
Frequency increased to about 32MHz (about 1.5 times), so transition time of each gate was about 5.1ns. Symmetricity of wave is nearly perfect.
8) supply voltage 1.5V:
Frequency increased to about 50MHz (about 1.5 times), so transition time of each gate was about 3.3ns. Symmetricity of wave is perfect.
9) supply voltage 1.8V:
Frequency increased to about 61MHz (about 1.2 times), so transition time of each gate was about 2.7ns. Symmetricity of wave is nearly perfect.
10) supply voltage 2.5V:
Frequency increased to about 77MHz (about 1.2 times), so transition time of each gate was about 2.2ns. Symmetricity of wave is nearly perfect.
11) supply voltage 2.75V:
Frequency increased to about 80MHz (about 1.05 times), so transition time of each gate was about 2.1ns. Symmetricity of wave is nearly perfect. Rise and fall times are relatively slow (about 3.1 ns at period time 12.6 ns), so wave looks like sinusoidal. Where is the source of such behaviour? Wave was measured using standard oscilloscope probe:
This probe has long ground connection, which has substantial inductance. In order to improve measurement results it is needed to remove probe tip and use short ground connection:
Rise and fall times improved substantially - from 3.1 ns to about 2 ns:
Output waveforms of all 3 outputs:
You can see easily transitions between gates.
Applications
1. Light sensor
I needed to design light sensor for luminance measurement for low voltage system. As most sensors have supply voltage limit 2.7V or 1.8V and I needed 1.5V, I was in a need to design low voltage sensor. It is also possible to use photodiode, amplifier and ADC input, but this was not possible.
I designed this sensor using 74AXP1G57 configurable logic, photoresistor and capacitor. This simple oscillator generates wave with light-dependent frequency. Variable frequency digital signal can be easily interfaced with low voltage MCU.
Board:
Schematic:
Few results:
| Illuminance [lx] | Output frequency [Hz] |
|---|---|
| 16 | 528 |
| 73 | 1746 |
| 100 | 2215 |
| 1001 | 10220 |
It is versatile, energy-efficient solution. You can change measurement range by selecting other photoresistor type and output frequency range by changing capacitor. For reliable measurements, capacitor should be good quality (C0G ceramic or foil). Output frequency change vs. illuminance is not linear, but can be easily approximated by MCU.
2. PhotoSound
It is designed for fun, I combined two light-dependent generators using Ex-NOR gate to intermodulate output waves. So it is possible to obtain nearly 0 Hz frequency (if both generators have the same frequency) or some beat frequency (up to hearing limit).
Board:
Schematic:
Using demo board, two resistors: R5 and R11 must be removed.
You can have fun playing PhotoSound!
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Problemchild
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