I have no idea what you are trying to say, but since you say I am at the very beginning I will try to ask some questions.

One I do think I understand is high level is at 5V, but "low" at 3.7 V ??? You are referring to Vih min and Vih max which basically states that anything between 3.7 and 5 V is seen by the controllse as a '1'.

UART. ....This signal comes from a standard HC00 gate
Are you using a MAX232(equivalent)? a you MUST and b if you have an inverter such as HC00 between the MAX232 and the Rxd the signals will be out of phase and fail.

I've found a post (2. Oct 2001) where it's mentioned that T89... requires a lower source resistance for RXD than Philips that is totally irrelevant, whatever it is, both are CMOS inputs.

1) are you using a MAX232(equivalent) between the PC and your uC. ?
2) have you read "the bible" ?
3) have you read all info about FLIP

Erik