PC87307VUL National Semiconductor, PC87307VUL Datasheet - Page 52

• Manufacturer Part Number: PC87307VUL
• Description: PC87307/PC97307 Plug and Play Compatible and PC97 Compliant SuperI/O
• Manufacturer: National Semiconductor

www.national.com
System Bus Lockout
As the RTC switches to battery power all, input signals are
locked out so that the internal registers can not be modified
externally.
Power Up Detection
When system power is restored after a power failure, the
power failure lock condition continues for a delay of 62
msec (minimum) to 125 msec (maximum) after the RTC
switches from battery power to system power.
The power failure lock condition is switched off immediately
in the following situations:
Oscillator Activity
The RTC internal oscillator circuit is active whenever power
is supplied to the RTC with the following exceptions:
These conditions disables the oscillator.
When the oscillator becomes inactive, the APC is disabled.
4.1.4
The RTC logic device has a single Interrupt Request line,
IRQ, which handles three interrupt conditions. The Periodic,
Alarm, and Update-Ended interrupts are generated (IRQ is
driven low) if the respective enable bits in Control Register
B are set when an interrupt event occurs.
Reading RTC Control Register C (CRC) clears all interrupt
flags. Thus, it is recommended that when multiple interrupts
are enabled, the interrupt service routine should first read
and store the CRC register, then deal with all pending inter-
rupts by referring to this stored status.
If an interrupt is not serviced before a second occurrence of
the same interrupt condition, the second interrupt event is
lost. Figure 4-5 illustrates interrupt and status timing in the
part.
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•
•
•
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If the Divider Chain Control bits (DV2-0, bits 6-4 in Con-
trol Register A) specify any mode other than 010, 100 or
011, all input signals are enabled immediately upon de-
tection of system voltage above that of the battery volt-
age.
When battery voltage is below 1 volt and MR is 1, all in-
put signals are enabled immediately upon detection of
system voltage above that of battery voltage. This also
initializes registers at indexes 00h through 0Dh.
If the VRT bit (bit 7 in Control Register D) is 0, all input
signals are enabled immediately upon detection of sys-
tem voltage above that of battery voltage.
trol bits (DV2-0), i.e., bits 6-4, of Control Register A,
and the RTC is supplied by V
The RTC is supplied by V
trol Register D is 0.
Software wrote 000 or 001 to the Divider Chain Con-
Interrupt Handling
Real-Time Clock (RTC) and Advanced Power Control (APC) (Logical Device 2)
BAT
BAT,
and the VRT bit of Con-
or
52
4.2 THE RTC REGISTERS
The RTC registers can be accessed at any time during non-
battery backed operation. These registers cannot be written
to before reading the VRT bit (bit 7 of the “RTC Control Reg-
ister D (CRD), Index 0Dh” on page 55), thus preventing
bank selection and other functions. The user must read the
VRT bit as part of the startup activity.
These registers are listed in Table 4-1 and described in de-
tail in the sections that follow.
4.2.1
The CRA register controls periodic interrupt rate selection
and bank selection.
Bits 3-0 - Periodic Interrupt Rate Select (RS3-0)
Bits 6-4 - Divider Chain Control (DV2-0)
Bit 7 - Update in Progress (UIP)
7
0
These read/write bits select one of fifteen output taps
from the clock divider chain to control the rate of the pe-
riodic interrupt. See Table 4-2 and Figure 4-5.
Master reset does not affect these bits.
These read/write bits control the configuration of the di-
vider chain for timing generation and memory bank se-
lection, as shown in Table 4-3.
Master reset does not affect these bits.
This read only bit is not affected by reset.
0 - An update will not occur within the next 244 sec.
1 - Timing registers are updated within 244 sec.
6
0
UIP
Bit 7 (the SET bit) of Control Register B (CRB) is 1.
RTC Control Register A (CRA), Index 0Ah
5
DV2
1
FIGURE 4-4. CRA Register Bitmap
DV1
4
0
DV0
3
0
RS3
2
0
RS2
1
0
RS1
0
0
Power-Up
Reset
Required
RS0
RTC Control
Register A
Index 0Ah
(CRA)