FM33256/FM3316 SPI Companion w/ FRAM
capacitance (switch open circuit) allowed on the CNT
pin is 100pF.
seconds and minutes match select bits causes an
exact match of these values. Thus, an alarm will
occur once per hour. Setting seconds, minutes, and
hours causes a match once per day. Lastly, selecting
all match-values causes an exact time and date match.
Selecting other bit combinations will not produce
meaningful results, however the alarm circuit will
follow the functions described.
Serial Number
A memory location to write a 64-bit serial number is
provided. It is a writeable nonvolatile memory block
that can be locked by the user once the serial number
is set. The 8 bytes of data and the lock bit are all
accessed via unique op-codes for the RTC and
Processor Companion registers. Therefore the serial
number area is separate and distinct from the memory
array. The serial number registers can be written an
unlimited number of times, so these locations are
general purpose memory. However once the lock bit
is set, the values cannot be altered and the lock
cannot be removed. Once locked the serial number
registers can still be read by the system.
There are two ways a user can detect an alarm event,
by reading the AF flag or monitoring the ACS pin.
The interrupt pin on the host processor may be used
to detect an alarm event. The AF flag in register 00h
(bit 6) will indicate that a time/date match has
occurred. The AF flag will be set to ‘1’ when a match
occurs. The AEN bit must be set to enable the AF
flag on alarm matches. The flag and ACS pin will
remain in this state until the AF bit is cleared by
writing it to a ‘0’. Clearing the AEN bit will prevent
further matches from setting AF but will not
automatically clear the AF flag.
The serial number is located in registers 10h to 17h.
The lock bit is SNL, register 18h bit 7. Setting the
SNL bit to a 1 disables writes to the serial number
registers, and the SNL bit cannot be cleared.
The RTC alarm is integrated into the special function
registers and shares its output pin with the 512Hz
calibration and square wave outputs. When the RTC
calibration mode is invoked by setting the CAL bit
(register 00h, bit 2), the ACS output pin will be
driven with a 512 Hz square wave and the alarm will
continue to operate. Since most users only invoke the
calibration mode during production this should have
no impact on the otherwise normal operation of the
alarm.
Alarm
The alarm function compares user-programmed
values to the corresponding time/date values and
operates under VDD or VBAK power. When a match
occurs, an alarm event occurs. The alarm drives an
internal flag AF (register 00h, bit 6) and may drive
the ACS pin, if desired, by setting the AL/SW bit
(register 18h, bit 6) in the Companion Control
register. The alarm condition is cleared by writing a
‘0’ to the AF bit.
The ACS output may also be used to drive the system
with a frequency other than 512 Hz. The AL/SW bit
(register 18h, bit 6) must be ‘0’. A user-selectable
frequency is provided by F0 and F1 (register 18h, bits
4 and 5). The other frequencies are 1, 4096, and
32768 Hz. If a continuous frequency output is
enabled with CAL mode, the alarm function will not
be available.
There are five alarm match fields. They are Month,
Date, Hours, Minutes, and Seconds. Each of these
fields also has a Match bit that is used to determine if
the field is used in the alarm match logic. Setting the
Match bit to ‘0’ indicates that the corresponding field
will be used in the match process.
Depending on the Match bits, the alarm can occur as
specifically as one particular second on one day of
the month, or as frequently as once per second
continuously. The MSB of each Alarm register is a
Match bit. Examples of the Match bit settings are
shown in Table 3. Selecting none of the match bits
(all ‘1’s) indicates that no match is required. The
alarm occurs every second. Setting the match select
bit for seconds to ‘0’ causes the logic to match the
seconds alarm value to the current time of day. Since
a match will occur for only one value per minute, the
alarm occurs once per minute. Likewise setting the
Following is a summary table that shows the
relationship between register control settings and the
state of the ACS pin.
Table 2.
State of Register Bit
CAL AEN AL/SW
Function of
ACS pin
/Alarm
Sq Wave out
512 Hz out
Hi-Z
0
0
1
0
1
X
X
0
1
0
X
1
Rev. 1.0
Dec. 2006
Page 7 of 28
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