(solved), uart 5v to 3.3v conversion hack needed

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I'ts sunday and all the shops are closed, so no possibility of getting a max232 till monday afternoon.

Not wanting to wait till then i went through my interesting parts box and found a very small usb simmcard reader with a PL2303HX(3.3v) and a 74hc04 in it.

Already did the following:
1. Removed all the unneeded parts(extra crystal, transistors, couple of resistors)
2. Cut the tracks between the two chips
3. Connected the 74hc04 to 5v
4. Buffered the incoming RX and TX both with two 74HC04 ports
5. Connected TX between the chips
6. Connected RX between the chips with a 2k2 resistor

If i connect the external TX-RX i get a nice 5v uart loopback, everything seems great

But the problem is that the 74HC04 TX output is 5v and the 2k2 resistor only lowers it to 4,3v(lower resistors don't work)

The PL2303HX can only handle vcc+0,3=3,6v on it's input

The space i have to work with is very small, does anyone have a good idea for this?

Is a 3,6v Zener to slow?

Last Edited: Sun. Jul 22, 2007 - 02:46 PM
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Maximilian wrote:

Is a 3,6v Zener to slow?

Nope, replaced the 2k2 with a 330 Ohm resistor and connected a zener diode to ground
Tested with loopbackto 921600 baud :D

Problem solved