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Programmable Hardware Manual (PHM)
© Tibbo Technology Inc.
4.2.2.7
Real-time Counter
The real-time counter (RTC) of the EM1000 is a free-running 40-bit register that
increments at a rate of 128Hz.
As a source of backup power, the EM1000 can rely on a supercapacitor. Option "-S"
of the EM1000 (see
Specifications and Ordering Info
) has this onboard
supercapacitor. To enable charging, connect 3.3V power to the VCCB pin of the
EM1000, preferably through a current-limiting resistor (50 Ohm is a good value). A
fully discharged supercapacitor creates a nearly short-circuit current inrush when it
starts charging and this can damage the power supply of the host board.
The EM1000-...-S carries the supercapacitor on the bottom side of its PCB (see
). With this supercapacitor present, it is impossible to solder
the module into the host PCB directly and the module can only be installed on a
socket. If this is not acceptable you can use a "plain" EM1000 (non- "-S") and
connect an external supercapacitor to the SCAP pin of the EM1000. This option is
only available on the newer
EM1000-...- 01
device (
EM1000-...- 00
does not have
the SCAP input).
The supercapacitor has many advantages -- it charges almost instantly and has
virtually unlimited lifespan. The disadvantage is that the supercapacitor is only able
to sustain the RTC of the EM1000 for several days at most (about 6 days for the 4F
supercapacitor of the EM1000-...-S), which may appear to be insufficient.
Remember, however, that the EM1000 is usually connected to the network and can
always synchronize its clock* with an Internet time server or a master clock on the
main server of your system. Therefore, the role of the supercapacitor is to provide
the backup power during relatively short periods of power interruption, i.e. when the
device is unplugged and moved to another location, or when the device is powered
off over the weekend.
It is also possible to use a 3V lithium battery to power the RTC (in this case, do not
use the EM1000 with "-S"). Connect the battery to the VCCB pin through a small
Schottky diode. This diode is necessary to slightly reduce the voltage on the VCCB
pin. You can calculate the time the battery will be able to sustain the EM1000 from
the average backup current, which is ~13uA. Note that the VCCB pin cosumes a
much larger current (~1mA) when the Vcc is applied and the EM1000 is running.