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I have checked couple of datasheets for 40109 and although they specify "Supply Voltage Range" as min: 3V, max: 18V, none of the other tables specify values for < 5V.

Can I safely order these chips for use in a (Bluetooth master) 3V3 <=> 5V (ATtiny) level shifter application? And how can I conclude that from the datasheets?

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For unidirectional data flow, although there will be shifters more suited to lower voltages, you should be fine with this chip. On page 6 of the ST datasheet, there is a High voltage vs Low Voltage graph which indicates a minimum combination of ~3V for Vcc and 5V for Vdd:

CD40109

If you are using both Rx and Tx, then you will need a bidirectional level shifter, such as the ADG3304. Or you can use two unidirectional ICs, as discussed in the comments.

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  • \$\begingroup\$ RX and TX are separate pins on the BT master, and I need them both for proper operation. So basically you say: use another level shifter. I'll take the 40109 from my wish list then. \$\endgroup\$ – jippie Apr 24 '13 at 19:24
  • \$\begingroup\$ I'm starting to realize how to read the Vcc and Vdd. Does it make a difference if I use two separate IC's? "- Input Signals can Exceed Both VCC and VDD". \$\endgroup\$ – jippie Apr 24 '13 at 19:25
  • \$\begingroup\$ Yes, you could use two unidirectional ICs in this case, since the individual lines themselves do not change direction. Just arrange them for the correct direction in each case. There are quite a few example UART level shifting circuits out there, the most basic use a couple of transistors. \$\endgroup\$ – Oli Glaser Apr 24 '13 at 19:28
  • \$\begingroup\$ FYI: added an URL to details on the BT module to my question. \$\endgroup\$ – jippie Apr 24 '13 at 19:34
  • \$\begingroup\$ @jipp - thanks, it seems very similar to the HC-05 BT modules I have used in a few designs. \$\endgroup\$ – Oli Glaser Apr 24 '13 at 19:38
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You can use SN74LVCC3245, as it is ideal for converting 3.3V to 5V and vice vesa.

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    \$\begingroup\$ Please tell us why it is ideal. \$\endgroup\$ – pebbles Oct 2 '13 at 10:15

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