UART voltage in BC660K-GL

According to the documentation, when VIO_SEL pin is grounded, the maximum UART voltage level is 3.3V and follows VBAT if lower. Our measurements show there is some voltage drop between VBAT and MAIN_TXD and it does not go above approx. 3.2V.

  • Is LDO or something similar used internally to power the UART?
  • Are there any protection diodes used? What type?
  • Our MCU normally runs on 3.6V (sometimes going up to 3.7V). Receiving from 3.2V level is completely fine (3.2V is bigger than 0.7*VBAT), but I’d like to check if we can send VBAT directly to MAIN_RXD. We can add series resistor if needed. When tested there is no additional current flow measured so I assume it should work fine (despite the documentation).
  • Is there any difference in power consumption depending on VIO_SEL mode?

1.There is a circuit inside our module, connected to the UART.
2.Which pin are you referring to that requires protection diodes?According to our documentation, VIO_SEL pins are not needed.
3.Yes,it is OK.
4.The power consumption in different modem modes is different.Are you trying to understand what mode the power consumption differs?Theoretically, it does not depend on VIO_SEL model.

  1. I was referring to MAIN_TXD and MAIN_RXD pins. If RXD pin has a schottky diode to VDD_EXT, feeding more than VDD_EXT+0.3V (as mentioned in bullet 3) would be clamped and resulted in excess current draw.
  2. I expect that VIO_SEL selection has no effect on deep sleep as UART is off during this mode. But when UART is on, I wanted to make sure that VIO_SEL selection does not affect current consumption.
    Thank you for your answers.

1.It is recommended to connect the interface circuit between MCU and MAIN_RXD with 1k resistor in series.
2.When UART is on.VIO_SEL selection does not affect current consumption.