In the following scenario, there is a state where only VBAT is supplied without supplying VCC to RTC.
1.) If VCC is not supplied and only VBAT is supplied, will FOUT and XT operate normally?
2.) When operating RTC under the condition of Q1, what is the value of tvh:fout and txtst?
3.) Even if VCC is not supplied, is BREF Control Register set correctly and RTC can detect the voltage of VBAT?
4.) If I try to operate RTC with battery only (e.g., coin-cell battery), is power input not VBAT but VCC?
There is no VBAT_POR specification because that is not supported. The RTC does not support operation with only VBAT supply. At initial power-up, the VCC must be powered above VCCST startup voltage before the VBAT circuit becomes active. Please refer to section 5.13 "Power Control and Switching" for more details. The following statement explains that the RTC will not come out of POR unless VCC is supplied: If VBAT rises above VBATSW in the POR state, the AM18X5 remains in the POR state. This allows the AM18X5 to be built into a module with a battery included, and minimal current will be drawn from the battery until VCC is applied to the module the first time. If the RTC and MCU will be powered from the same coin cell, then they should power RTC vrom VCC. If coin cell will only be powering the RTC, then they can connect the copin cell to VBAT, but they will need to provide power to VCC for initial startup, such as when MCU is configuring RTC operation.
|AM08X5 Datasheet page71|
6.10.2 0x21 - BREF Control