20
Real-time clock
An additional device of the minimodule is the RTC clock operating with the DS1307 circuit connected to the
I2C bus. Along with the RTC circuit, there is a socket for lithium batteries mounted on the module, providing a
guarantee of many years of uninterrupted operation of the clock. The battery voltage is fed outside the
module, allowing supplying power to other elements from one battery or taking electric supply from the
outside. The I2C bus occupies two minimodule port terminals: PD0 and PD1. If the RTC clock is mounted,
these terminals can be used only as an I2C bus communicating with other peripherals, they cannot, however,
act as I/O ports.
32,768 KHz
3V CR2032
GND
X1
1
X2
2
Vbat
3
GND
4
SDA
5
SCL
6
SQW
7
VCC
8
DS1307
GND
+5V
Vbat
PD0
PD1
4k7 4k7
+5V +5V
Figure 10
Connection of the RTC circuit inside the module.
A detailed description of the DS1307 circuit is given on the Maxim Company page:
www.maxim-ic.com
.
Supply of power
The module requires a reg 5 V supply voltage. The + 3.3 V voltage, indispensable for the operation of
some circuits, is produced inside the module. It is also led out externally to be used by other system elements.
RESET circuit
The MMnet102 has a built-in voltage monitoring circuit constructed around the DS1811 integrated circuit. The
circuit generates a RESET signal in case when the supply voltage value is lower than 4.6 V. This takes place
when the supply voltage is switched on or off, when the VCC voltage changes its value from 0 to 5 V.
The guard circuit detects also momentary VCC voltage drops. A short duration drop of VCC below 4.6 V
causes the generation of a resetting signal of 100 ms duration. This signal is applied directly to the resetting
input of the microcontroller and through a simple inverter to the LAN91C111 circuit. The RESET signal is led
out to a module connector and it can be used as the zeroing output resetting external circuits and as the input
for resetting the module, e.g. by means of the RESET button. In such a case the RESET button can short the
RESET line directly to ground. An implementation of the reset circuit is presented in the diagram below.
