![](bg4.png)
would get through to the receiving station, but not
all, at a distance too short for link failure.
The device worked properly when an arbitrary
delay was inserted between transfers of each byte
from memory to transceiver. This delay was then
calibrated to its approximate minimum value that
would still allow for proper functioning.
The resulting transmission time was 5,5 seconds.
It is more than double the theoretical, ideal value. It
corresponds to the transmission occurring 15,5% of
the time.
3.3 Discussion
The acquisition device was not able to function with
transmission time lower that 5,5 seconds, that is, it
did not function properly without deliberate delays
that increased the transmission time to 5,5 seconds.
The microcontroller code keeps the I2C interface
from getting ahead of the UART interface, that is,
the next byte is read from the memory only after the
last byte is forwarded to the transceiver.
The UART interface is very much slower than
the rf link, and the former shouldn’t overwhelm the
latter. However, the transceiver was configured not
to wait for any number of bytes to form an rf packet,
that is, as soon as a byte is received by the UART, it
is put in an rf packet with significant overhead. (This
configuration is necessary to transmit one-byte
control messages used for reliability.) Therefore, if
assembling the packet takes too long, perhaps the
transceiver’s transmission buffer overflows.
At the receiving station’s side, the rf link, being
faster then the UART interface, could overwhelm it
This shouldn’t happen since all the data coming
through the rf link has passed by the UART interface
at the acquisition device. However, non-uniform
delays due to rf packet assembling and
disassembling could cause the receiving buffer to
overflow.
4 CONCLUSIONS
This work presented a wireless microcontrolled
solution for an ECG home monitoring system
application. It was based in the concept of battery
power saving through intermittent data transmission.
It was observed that in practice the transmission
time is much higher than the ideal theoretical
minimum. Further experimentation is necessary to
discover whether it can be reduced.
The use of intermittent data transmission has
opened another strategy to reduce battery
consumption by setting the power-down modes of
the microcontroller and transceiver, but that wasn’t
achieved with this prototype. For example, if the
storage phase power consumption can be ten percent
of the transmission phase power consumption, then
the total consumption as a percentage of the
transmit-all-the-time consumption will be 24%.
Therefore, it is useful to use intermittent
transmission instead of transmitting all the time.
ACKNOWLEDGEMENTS
This work is supported by FAPES – Fundação de
Apoio à Ciência e Tecnologia do Espírito Santo
(30899583/2005), and partially by CNPq.
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