Hi,
However, where a Schottky might be necessary is if there is a large negative voltage. The negative protection diodes are a "substrate diode" and current flowing in the substrate can upset parts of the microcontroller, particularly the ADCs For this reason Rev Ed used to show an "Enhanced Serial Download Circuit" in Manual 1, but have now deemed this to be unnecessary since most USB-serial adapters (including the CH340 "blue" adapters) deliver only 0 - 5 volt levels.
*But there is one "gotcha" if you rely on a clamping diode (either internally or an external Schottky) which is "Phantom Powering". A PICaxe needs only around 600uA to run, so 22k is about as low as you can go before there is a risk that the PICaxe will continue to run from a "clamped" 15 volt external "logic" input, via the protection diode, even when the PICaxe's power supply is disconnected. Not usually a problem but it might cause the PICaxe not to Reset when you think it has/should. As you indicated, zener diodes below 5 volts have a poor "knee" (in their V/I curve) so the simplest solution may be to ensure that there is sufficient current drain (perhaps just a resistor) from the supply rail to ground.
Cheers, Alan.
Before USB to Serial adapters, the PICaxe was designed to be programmed directly from RS232 (serial) levels (e.g. from a PC COM1 port) which can be as high as +/- 15 volts. The 22k resistor limits the current and the on-chip "protection" diodes limit the voltage swing. Microchip rate these diodes to 20 mA so it should work perfectly safely, but you can add an external Schottky diode if you feel more comfortable*.... purposely exceeding the voltage ratings on the pin. ....
Other ideas I've considered are resistor divider, zener clamp, and maybe using an external Schottky
However, where a Schottky might be necessary is if there is a large negative voltage. The negative protection diodes are a "substrate diode" and current flowing in the substrate can upset parts of the microcontroller, particularly the ADCs For this reason Rev Ed used to show an "Enhanced Serial Download Circuit" in Manual 1, but have now deemed this to be unnecessary since most USB-serial adapters (including the CH340 "blue" adapters) deliver only 0 - 5 volt levels.
*But there is one "gotcha" if you rely on a clamping diode (either internally or an external Schottky) which is "Phantom Powering". A PICaxe needs only around 600uA to run, so 22k is about as low as you can go before there is a risk that the PICaxe will continue to run from a "clamped" 15 volt external "logic" input, via the protection diode, even when the PICaxe's power supply is disconnected. Not usually a problem but it might cause the PICaxe not to Reset when you think it has/should. As you indicated, zener diodes below 5 volts have a poor "knee" (in their V/I curve) so the simplest solution may be to ensure that there is sufficient current drain (perhaps just a resistor) from the supply rail to ground.
Cheers, Alan.