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AN1176.0
June 20, 2005
by clipping a test lead to the JP1 shunt, or if an external V
CC
supply is used it should be connected to the center pin of
JP1 (shunt is removed). A ground connection is available at
J3.
BACKUP SUPPLY
There are two sources of backup supply available. A 0.22µF
supercapacitor (C10) is on the board. It will provide enough
backup power for the typical RTC chip to last many days at
room temperature when V
CC
is set to 5V. There is also a
CR2032 battery included in a socket to allow removal/
replacement without soldering. This battery will last up to ten
years in normal backup operation. The backup supply is
selectable via jumper JP2 as shown in Figure 3. One row of
JP2 is connected to the device V
BACK
, and the other row to
the power sources. There are three positions possible on
JP2, Supercap, Battery, and Ground. If no backup is used
the Ground position should be used. The jumper may be
removed altogether and an alternative source for V
BACK
can
be attached to the board via the exposed header.
MICROCONTROLLER
The microcontroller used is the PIC16C63A. It has a
dedicated serial interface which connects to the FDTI USB
device. There is a dedicated I
2
C serial interface connected
to the Intersil RTC device, EEPROM, and the temperature
sensor. The power for the microcontroller comes from the
USB 5V input, so the microcontroller will always be powered
from 5V. Note that the RB1 to RB4 I/O pins for the
microcontroller are available at the daughtercard connector,
J3. This is to enable identification of the RTC daughtercard
and thus enable functions for a particular device family. The
device is socketed to allow removal or replacement for
development activity and software upgrades with a newly-
programmed device.
DAUGHTERCARD CONNECTOR (J3)
This connector provides power and interface signals to the
daughtercard containing the Intersil RTC device as follows:
Pin 1: V
CC
Pin 2: RB4
Pin 3: RB3
Pin 4: RB2
Pin 5: RB1
Pin 6: RB0 or Event Input
Pin 7: SDA
Pin 8: SCL
Pin 9: V
BAT
Pin 10: GND
This configuration allows these signals to control the Intersil
device on an external board by connecting the serial
interface and ground, available at the connector. Also
available are the RB0 to RB4 I/O signals from the
microcontroller (see Microcontroller section).
SUPERCAP
BACKUP
BATTERY
SOCKET
V
CC
SELECT
DAUGHTERBOARD
CONNECTOR
SHUNT
FOR V
BACK
SELECT
FIGURE 3. RTC-USB MOTHERBOARD LAYOUT
Application Note 1176
