Do using BAT42 Schottky diodes will limit voltage to 3.3V on the data lines?

[Los Frijoles]

I am using a chip (Wireless USB chip) which requires a 3.3V max source on all pins. I am using a PIC to control it which runs at 5V. Will the following arrangement using BAT42 Schottky diodes work to limit the voltage to 3.3V on the data lines going from the MCU to the WUSB? The lines from the MCU usually are at about 4.5V.

**broken link removed**

 

[_i386sx25_]

Re: Diodes

Should limit at 3,3V+Vdiode. But without any current limiting serial resistance, you might burn out either diodes or PIC's output ports. Is it possible to run PIC at 3,3V too?

 

[Los Frijoles]

Diodes

Unless you get industrial PICs, you can't run them at 3.3v. The internal oscillator breaks down and the instructions stop being executed. It also causes ram data loss, resulting in a reset.

Would a 1K resistor work to add resistance or is that too big? I have a friend who used 100Ω resistors in series w/o schottkeys and his wusb did not burn up. I don't this his solution was reliable though as it depended on having a constant resistance and voltage from the pic and wusb. Resistors also slow down bus speed.

 

[_i386sx25_]

Re: Diodes

BAT42's max forward current 200mA, PIC's should be 10mA~20mA.
Resistors could be down to (5-3,3-0,4)/10mA=130 ohm.

Does it disturbs the communication? I don't know. Depends on frequency. Either try to make a simulation using IBIS models of PIC and WUSB, or build up the circuit and check the waveforms by a oscilloscope. Second is easier.

 

[hdrzaman]

Diodes

BAT42's max forward current 200mA, PIC's should be 10mA~20mA.
Resistors could be down to (5-3,3-0,4)/10mA=130 ohm.

Does it disturbs the communication? I don't know. Depends on frequency. Either try to make a simulation using IBIS models of PIC and WUSB, or build up the circuit and check the waveforms by a oscilloscope. Second is easier.