The ~RESET signal is an open drain. Therefore, a pull-up resistor is needed between this pin and some voltage level. But which voltage level is needed at the pull-up for connecting to the reset signal? Is this 1V8, 2V8 or VBATT? (See section 3.20 of the Technical Specifications & Customer Design Guidelines, rev. 4, July 25th, 2007)
You can use a resistor 4K7 pull up to 2.8V.
I don´t understand?
The section 3.20 of the Technical Specifications (below the figure 39) have the following text:
“The ~RESET time is provided by an internal RC component. To keep the same time, it´s not recommended to plug another R or C component into the ~RESET signal. Only a switch or an open drain is recommended.”
Is really necessary the external pull up resistor in the ~RESET signal?
Look in section 3.20.1 at the “Electrical Characteristics” table on p103 - it tells you that there is an internal pull-up (100k typical);
Also look at the table under “3.20.2 Pin description” - it, too, says that ~RESET has an internal pull-up (but the cross-reference is wrong - it should refer you to section 3.20.1)
The pinout table, on p158, tells you that both ~RESET and ~EXT-RESET are in the VCC_1V8 Voltage Domain
The PTS specifically recommends that you do not add any external pull-up (see p104);
Also, the pinout table, on p158, tells you that both ~RESET and ~EXT-RESET are in the VCC_1V8 Voltage Domain - not 2V8.
The PTS clearly recommends that you should not add any external pull-up!