UART pullup/down + voltage translator

I have two remarks :

  1. On our boards I use TXB0108 to make voltage translation between the modem (HL series) 1.8V UART and my 3.3V MCU. At the beginning (HL6528), the presence of pull ups was not indicated, and the circuit was working fine and I did no go further. But when investigating on the problem I explain at my second point, I noticed that there are pullups (8K) on the UART inputs. This information is not given in all the datasheets (for the HL8518 it is not present, and for HL7692 it has been added later). But using such low pullup with TXB0108 is not good. When driving a 0 from my MCU, I don’t get ~0V on the modem pin but 0.6V. In practice that works, but this is not within the specs (Vil max is 0.35V), and probably that may lead to unexpected comm failure. Curiously, that same king of translator is also used on the MangOH RED (FXMA0108) that prohibit pull up usage !

2.I noticed that on the HL8518 the DSR output does not act as a push pull drive. When driving a “1” it is ok, but when driving a “0” it is in fact in high-Z with pulldown. The voltage translator get lost (the buskeeper logic is not overdriven by the DSR pulldown). I did not seen that on other modules. I guess this is a soft bug on the modem ?

Does anybody have some comments ?


Point 2 has just been confirmed by tech support.