Electronic – the diode used for in this circuit

The diode improves the stability of the current source with temperature changes.

Imagine the Zener voltage is stable with temperature, then without the diode, the current will increase by about 2mV/Re = 4uA for every degree C. The diode forward voltage drops by about 2mV every degree C, so it mostly compensates for the transistor Vbe drop.

They are operated at different currents, so the compensation is not perfect, and the Zener will have some tempco (not much because it will be close to the optimum voltage for Zener tempco) but this will improve overall stability by perhaps 5:1 or 10:1. Over a range of, say, -20'C to +80'C it might improve the stability from 5% to 1%, where the capacitor tempco will dominate. It's a reasonable thing to do if you care about the frequency. I once used a similar circuit without the compensation in a dual role to generate an ADC ramp (homemade 15 bit ADC in this product) and to measure board temperature.

It will work if the higher voltage side is driving the lower voltage side.

The diode should face "from" the lower voltage side "to" the higher voltage side with pullups at the lower voltage side (ie. block current flow from the higher voltage signal to the lower voltage signal).

What happens:

• Higher voltage side is logic high - lower voltage side is pulled high (high=high)
• Higher voltage side is logic low - current flows through the diode and pulls the line low (low=low).

There is also a simpler hack - just place a resistor in series (eg. 2k2) and rely on the lower voltage side chip ESD protection diodes to clamp the higher voltage (eg. 5V to 3,3V, difference is 1,7V, resistor is 2k2, current flowing through the ESD protection diodes is less than 1mA, the maximum allowed clamping current is specified in the datasheet).