The Roving Networks RN42 is a 3.3V device so some level shifting is needed. I will design my own gadget with the RN42 so I looked how it was done on Sparkfun Bluetooth mate device. But it’s quite strange when you look at the Sparkfun schematic, see the pdf http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Wireless/Bluetooth/BlueSMiRF-Gold-ChipAnt-v1_rotat2.pdf. I would expect a voltage divider at the rx pin, however, there is only a 10 kOhm resistance in serie but no voltage divider!
I connected my Arduino (5V version) to the Sparkfun module I have. Then, measured pin 14 on the RN42 (UART RX) with a oscillosopce and there certainly is 5V and not 3.3V .
According the the RN42 datasheet http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Wireless/Bluetooth/Bluetooth-RN-42-DS.pdf max pin voltage should be VDD+0,4 i.e. 3.7 volt.
Is that perfectly ok anyway? Do I not need the voltage divider? Or is just bad design by Sparkfun? :~
(Last edit: Updated and clarified subject)