Atmel AVR2016 Manual de usuario descargar pdf (Pagina 3)

The MCU also controls the MOSFET switches used for

battery charging via the solar panel, and for switching the

DTN node’s power supply. Current sensing is implemented

with INA138 integrated circuits. The voltages are measured

using potential dividers which are duty-cycled to preserve

energy. The remaining battery capacity is determined based

on the current and voltage data over time. If it falls below a

threshold value the DTN node is switched oﬀ, to preserve an

energy budget for emergency communications. It is switched

on again if the PMM’s low power radio receives a message

with an emergency ﬂag; and also if the battery is charged

above the threshold value. A DS1629 real time clock (RTC)

provides persistent time, so that the MCU can go to sleep

mode. As an additional feature the DS1629 contains a tem-

perature sensor, which is helpful to ensure that the battery

is not charged if its temperature speciﬁcations are exceeded.

All components are mounted to a single printed circuit board.

The photo in ﬁgure 3 shows the PMM’s topside.

Although the PMM was primarily designed for solar-powered

DTN nodes, it is notable that other unconventional power

sources like wind wheels or muscle powered generators also

can be used. It even may make sense to use the PMM with

a conventional power supply and without a battery, in or-

der to beneﬁt from the energy-saving discovery and wakeup

features.

Solar-

panel

MCU

RTC,

Thermometer

ADC

TWI U(S)ART

IEEE

802.15.4-

Radio

Battery

ADC

Embedded

Linux-System

U(S)ART

I, U I, U USB

I²C

switched

virtual link

integrated

in PMM

Figure 2: Architecture of the PMM

3.1.1 Radio Control Protocol

This section gives an overview of the currently implemented

PMM control messages on the 802.15.4 channel. It is very

likely that additional messages will be implemented, since

the PMM allows for a wide range of applications, which may

beneﬁt from future protocol extensions.

WHOISTHERE REQUEST and RESPONSE

A mo-

bile node sends this request as a broadcast message in

order to discover other stations. As a parameter, its

DTN name is transmitted. An answering node sends

its name, geographic position and remaining battery

capacity.

STATUS REQUEST and RESPONSE

Queries a known

node to send detailed information on voltages, currents

etc..

Figure 3: Top side of the PMM

Figure 4: 802.15.4 XBee radio plugged into the

radio-socket of the PMM

WAKEUP REQUEST and RESPONSE

This message

schedules a wakeup event at a remote PMM. It con-

tains the name of the DTN-node, time for the scheduled

wakeup, wakeup duration and a priority. The remote

PMM uses the priority and its remaining battery ca-

pacity to decide if the request is acknowledged.

GETNEXTEVENT REQUEST and RESPONSE

mobile node uses this command to ﬁnd out when (and

if) the DTN-system is powered up the next time.

BEACON

A node may announce its presence by periodi-

cally transmitting a short beacon with its name. This

message provides an alternative to the WHOISTHERE

query.

3.1.2 PMM Control Daemon

The PMM and the control daemon running on the em-

bedded Linux system communicate via an USB interface.

Applications can connect via TCP sockets to the daemon,

and request information on the current energy state. More-

over, registered applications are notiﬁed before the node is

powered oﬀ. The daemon also handles energy monitoring

and PMM conﬁguration. This includes uploading scheduled

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