Real-Time Clock
20-4
Élan™SC520 Microcontroller User’s Manual
Figure 20-3
Circuit with Backup Battery
Software can read the RTC_VRT bit in the RTC Status D (RTCSTAD) register (RTC index
0Dh) at system boot time to determine whether or not the RTC time, date, and user RAM
are still valid since the last boot. This status bit is set based on the assertion of an internal
RTC-only reset signal. In systems with external backup batteries (as shown in Figure 20-3),
the RTC is reset when the main power supply is turned off and the backup battery is either
low or is not installed.
In these systems, the VCC_RTC pin is a dedicated power supply pin for the 32.768-kHz
oscillator and the RTC.
■
When the primary system power supply is turned on, the main power plane (V
CC
) drives
the VCC_RTC pin through an external diode.
■
When the primary power supply is turned off or nonfunctional, VCC_RTC is driven by
the backup battery through a second external diode.
■
The on-chip voltage monitor circuit monitors the voltage level of the backup battery
through the BBATSEN pin each time the PWRGOOD signal is asserted.
■
If the backup battery is sampled below 2.0 V, the RTC logic is reset. The read-only
RTC_VRT bit is cleared and latched in this state until the bit is read. After this bit is
initially read, it always reads back a value of 1 for all subsequent reads prior to an RTC
reset.
■
When the main system power supply is off and the backup battery is initially installed,
the external RC circuit consisting of R1 and C2 causes a slow rising edge on the
BBATSEN input, and the RTC is reset.
20.3.1.2
System without an External Backup Battery
For the system that is not using an external RTC backup battery, Figure 20-4 shows how
the circuit should be designed. It uses the same RC that is needed by the battery system,
but it is now connected to VCC_RTC.
VCC_ANLG is selected as the power plane for VCC_RTC because it is a well-filtered power
plane that is well below the VCC_RTC maximum of 3.3 V.
BATT
VCC_RTC
VCC_RTC
BBATSEN
C1
D1
D2
R1
C2
(3.3 V max)
10
Ω
Élan™SC520 Microcontroller
Содержание Elan SC520
Страница 1: ...lan SC520 Microcontroller User s Manual Order 22004A...
Страница 4: ...iv lan SC520 Microcontroller User s Manual...
Страница 28: ...Introduction xxviii lan SC520 Microcontroller User s Manual...
Страница 42: ...Architectural Overview 1 14 lan SC520 Microcontroller User s Manual...
Страница 78: ...System Initialization 3 22 lan SC520 Microcontroller User s Manual...
Страница 108: ...Clock Generation and Control 5 10 lan SC520 Microcontroller User s Manual...
Страница 118: ...Reset Generation 6 10 lan SC520 Microcontroller User s Manual...
Страница 148: ...System Arbitration 8 24 lan SC520 Microcontroller User s Manual...
Страница 214: ...SDRAM Controller 10 36 lan SC520 Microcontroller User s Manual...
Страница 230: ...Write Buffer and Read Buffer 11 16 lan SC520 Microcontroller User s Manual...
Страница 288: ...GP Bus DMA Controller 14 22 lan SC520 Microcontroller User s Manual...
Страница 316: ...Programmable Interval Timer 16 8 lan SC520 Microcontroller User s Manual...
Страница 328: ...Software Timer 18 4 lan SC520 Microcontroller User s Manual...
Страница 346: ...Real Time Clock 20 12 lan SC520 Microcontroller User s Manual...
Страница 360: ...UART Serial Ports 21 14 lan SC520 Microcontroller User s Manual...
Страница 414: ...AMDebug Technology 26 8 lan SC520 Microcontroller User s Manual...