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參數(shù)資料
| 型號： | LV8571A |
| 文件頁數(shù)： | 7/24頁 |
| 文件大?。?/td> | 353K |
| 代理商： | LV8571A |
Functional Description
(Continued)
INITIAL POWER-ON of BOTH V
BB
and V
CC
V
BB
and V
CC
may be applied in any sequence. In order for
the power fail circuitry to function correctly, whenever power
is off, the V
CC
pin must see a path to ground through a
maximum of 1 M
X
. The user should be aware that the con-
trol registers will contain random data. The first task to be
carried out in an initialization routine is to start the oscillator
by writing to the crystal select bits in the Real Time Mode
Register. If the LV8571A is configured for single supply
mode, an extra 50
m
A may be consumed until the crystal
select bits are programmed. The user should also ensure
that the TCP is not in test mode (see register descriptions).
REAL TIME CLOCK FUNCTIONAL DESCRIPTION
As shown in Figure 2, the clock has 10 bytes of counters,
which count from 1/100 of a second to years. Each counter
counts in BCD and is synchronously clocked. The count se-
quence of the individual byte counters within the clock is
shown later in Table VII. Note that the day of week, day of
month, day of year, and month counters all roll over to 1.
The hours counter in 12 hour mode rolls over to 1 and the
AM/PM bit toggles when the hours rolls over to 12
(AM
e
0, PM
e
1). The AM/PM bit is bit D7 in the hours
counter.
All other counters roll over to 0. Also note that the day of
year counter is 12 bits long and occupies two addresses.
Upon initial application of power the counters will contain
random information.
READING THE CLOCK: VALIDATED READ
Since clocking of the counter occurs asynchronously to
reading of the counter, it is possible to read the counter
while it is being incremented (rollover). This may result in an
incorrect time reading. Thus to ensure a correct reading of
the entire contents of the clock (or that part of interest), it
must be read without a clock rollover occurring. In general
this can be done by checking a rollover bit. On this chip the
periodic interrupt status bits can serve this function. The
following program steps can be used to accomplish this.
1. Initialize program for reading clock.
2. Dummy read of periodic status bit to clear it.
3. Read counter bytes and store.
4. Read rollover bit, and test it.
5. If rollover occured go to 3.
6. If no rollover, done.
To detect the rollover, individual periodic status bits can be
polled. The periodic bit chosen should be equal to the high-
est frequency counter register to be read. That is if only
SECONDS through HOURS counters are read, then the
SECONDS periodic bit should be used.
READING THE CLOCK: INTERRUPT DRIVEN
Enabling the periodic interrupt mask bits cause interrupts
just as the clock rolls over. Enabling the desired update rate
and providing an interrupt service routine that executes in
less than 10 ms enables clock reading without checking for
a rollover.
READING THE CLOCK: LATCHED READ
Another method to read the clock that does not require
checking the rollover bit is to write a one into the Time
Save Enable bit (D7) of the Interrupt Routing Register, and
then to write a zero. Writing a one into this bit will enable the
clock contents to be duplicated in the Time Save RAM.
Changing the bit from a one to a zero will freeze and store
the contents of the clock in Time Save RAM. The time then
can be read without concern for clock rollover, since inter-
nal logic takes care of synchronization of the clock. Be-
cause only the bits used by the clock counters will be
latched, the Time Save RAM should be cleared prior to use
to ensure that random data stored in the unused bits do not
confuse the host microprocessor. This bit can also provide
time save at power failure, see the Additional Supply Man-
agement Functions section. With the Time Save Enable bit
at a logical 0, the Time Save RAM may be used as RAM if
the latched read function is not necessary.
INITIALIZING AND WRITING TO THE
CALENDAR-CLOCK
Upon initial application of power to the TCP or when making
time corrections, the time must be written into the clock. To
correctly write the time to the counters, the clock would
normally be stopped by writing the Start/Stop bit in the Real
Time Mode Register to a zero. This stops the clock from
counting and disables the carry circuitry. When initializing
the clock’s Real Time Mode Register, it is recommended
that first the various mode bits be written while maintaining
the Start/Stop bit reset, and then writing to the register a
second time with the Start/Stop bit set.
The above method is useful when the entire clock is being
corrected. If one location is being updated the clock need
not be stopped since this will reset the prescaler, and time
will be lost. An ideal example of this is correcting the hours
for daylight savings time. To write to the clock ‘‘on the fly’’
the best method is to wait for the 1/100 of a second period-
ic interrupt. Then wait an additional 16
m
s, and then write
the data to the clock.
PRESCALER/OSCILLATOR FUNCTIONAL
DESCRIPTION
Feeding the counter chain is a programmable prescaler
which divides the crystal oscillator frequency to 32 kHz and
further to 100 Hz for the counter chain (see Figure 3 ). The
crystal frequency that can be selected are: 32 kHz, 32.768
kHz, 4.9152 MHz, and 4.194304 MHz.
Once 32 kHz is generated it feeds both timers and the
clock. The clock and timer prescalers can be independently
enabled by controlling the timer or clock Start/Stop bits.
TL/F/11416–7
FIGURE 3. Programmable Clock Prescaler Block
7
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| LV8572AMX | Real-Time Clock |
| LV8572AVX | Real-Time Clock |
| LV8800C | Optoelectronic |
| LVA12D | Analog IC |
| LVA12S | Analog IC |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LV8571AN | 制造商:Rochester Electronics LLC 功能描述:- Bulk 制造商:Texas Instruments 功能描述: |
| LV8572A | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:LV8572A Low Voltage Real Time Clock (RTC) |
| LV8572AM | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:LV8572A Low Voltage Real Time Clock (RTC) |
| LV8572AMX | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Real-Time Clock |
| LV8572AN | 制造商:Rochester Electronics LLC 功能描述:- Bulk |
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