Datasheet

 2010-2013 Microchip Technology Inc. DS20002266E-page 9
MCP79410/MCP79411/MCP79412
4.0 RTCC FUNCTIONALITY
The MCP7941X family is a highly integrated RTCC.
On- board time and date counters are driven from a
low-power oscillator to maintain the time and date. An
integrated V
CC switch enables the device to maintain
the time and date, and also the contents of the SRAM
during a VCC power failure if an external supply is
connected to the V
BAT pin and configured.
4.1 Crystal oscillator
The crystal oscillator built into the MCP7941X has been
designed to operate with a standard 32.768 kHz tuning
fork crystal.
The MCP7941X family of devices require both an
external crystal and matching external load capacitors.
Capacitors are not included on-chip.
Suitable crystals have a load capacitance (C
L
) of 6-9
pF. We do not recommend using crystals with a load
capacitance (C
L
) of 12.5 pF.
Figure 9.1 shows the required external oscillator com-
ponents. C
X1
and C
X2
pin capacitance and stray capac-
itance form the crystal load. This is calculated using the
equation below:
EQUATION 4-1: CL CALCULATION
FIGURE 4-1: OSCILLATOR
SCHEMATIC
For a list of tested and recommended crystals, please
refer to AN1519, “Recommended Crystals for Micro-
chip Stand-Alone Real-Time Clock Calendar Devices”.
This document provides a reference for suitable
crystals and recommended load capacitors.
For information on suggested board layout, please
refer to AN1365, “Recommended Usage of Microchip
Serial RTCC Devices”.
It is recommended that the final application should be
tested with the chosen crystal and capacitor combina-
tions across all operating and environmental condi-
tions. Please also consult the crystal specification to
observe correct handling and reflow conditions during
assembly.
4.2 RTCC Memory Map
The RTCC registers are contained in addresses 0x00-
0x1f. 64 bytes of user-accessible SRAM are located in
the address range 0x20-0x5f. The SRAM memory is a
separate block from the RTCC Control and Configu-
ration registers. All SRAM locations are battery-
backed-up during a V
CC power fail. Unused locations
are not accessible, MCP7941X will noACK after the
address byte if the address is out of range, as shown
in the shaded region of the memory map in Figure 4-2.
• Addresses 0x00-0x06 are the RTCC Time and
Date registers.
• Addresses 0x07-0x09 are the device Configura-
tion, Calibration and ID Unlock registers.
• Addresses 0x0A-0x10 are the Alarm 0 registers.
These are used to set up the Alarm 0, the Inter-
rupt polarity and the Alarm 0 Compare.
• Addresses 0x11-0x17 are the same as 0x0Bh-
0x11h but are used for Alarm 1.
• Addresses 0x18-0x1F are used for the time-
stamp feature.
The detailed memory map is shown in Table 4-1.
No error checking is provided when loading Time and
Date registers.
C
L
C
x2
C
x1

C
x2
C
x1
+
-------------------------C
stray
+=
X1
X2
RTCC
C
X1
C
X2
Note: These are Read/Write registers. Care
must be taken when writing to these regis-
ters with the oscillator running. Incorrect
data can appear in the Time and Date reg-
isters if a write is attempted during the
time frame where these internal registers
are being incremented. The user can min-
imize the likelihood of data corruption by
ensuring that any writes to the Time and
Date registers occur before the contents
of the second register reach a value of
0x59.