Microcontroller Solutions TechZone Magazine, April 2011 - Digikey

Examples of deeply embedded, Internet-connected devices include
information displays for the home that can integrate heating and
cooling control with lighting, music, and information displays (e.g.
a web browser); security still cameras that provide an extra level of
coverage for monitored services; appliances that allow time-of-start
control from utilities for smart-energy control, to reduce consumers’
electricity bills; heavy equipment with sensors that can provide
remote indication when in need of pending service; and toys or
multimedia handsets that could be configured for subscription
services, allowing updates during off periods.
The 802.11 protocol provides two basic methods of connectivity. The
primary method is called infrastructure, or basic service set, and is
implemented as a structured network with at least one central point
that routes traffic among the devices and onto other networks. In this
method, the central point is commonly called an access point, and all
communications occur through the access point. The second method
allows “unrelated” devices to connect temporarily. This mode is called
ad hoc, or independent basic service set. Ad hoc communication occurs
from device to device, but allows many devices to share the common
network. A primary advantage of 802.11 over other wireless protocols
is its intrinsic ability to connect devices to the Internet. However, this
functionality is provided primarily through the infrastructure mode of
operation. The Microchip TCP/IP stack and Microchip 802.11 radio
combination supports both infrastructure and ad hoc modes of operation.
Figure 2: Basic methods of connectivity.
One example of a use for a deeply embedded Wi-Fi solution is for
vehicle diagnostics. Considerable effort is spent designing the hardware
and software for user interfaces in these types of applications. However,
PDAs such as the iPod Touch or cellular phones such as the iPhone
are perfectly designed for user interfaces with a rich and intuitive
feature set. Most include Wi-Fi connectivity, which allows the use
of infrastructure or ad hoc modes in the Microchip Wi-Fi module. As
shown in Figure 3, the PDA or phone can connect to the Microchip Wi-Fi
module, and the PIC24F 16-bit microcontroller runs the Microchip TCP/
IP stack for ad hoc mode. The PIC24F microcontroller connects to the
vehicle using any of the standard On-Board Diagnostic (OBD) interfaces,
such as J1850, J1939, ISO 15765-4, etc. Vehicle status, such as RPM,
MPH, and battery voltage, can be displayed in a variety of forms such
as raw values, graphs, or gauges. Once disconnected, the PIC24F can
connect to a central server using infrastructure mode to upload data, or
download new firmware or parameters.
Figure 3: Application example.
Latency is not a critical factor for data transfers in most deeply
embedded devices. This means the data transfer has so little
latency that a user does not discern a delay from an action to the
corresponding reaction. Latency is often critical for handheld remotecontrol
devices, because users get frustrated and consider it a poor
user experience if the time for a local reaction to a button push on the
device is discernable (more than 250 ms).
The 802.11 specification has a mode called “power-save poll”
within the infrastructure service. It allows a device to go to sleep
while maintaining a connection to the access point. A device can
wake immediately and begin communication. Latency can be a little
more than 100 ms for an external client to begin transferring data
to a sleeping device. This is an ideal mode for remote controls using
infrastructure mode, as it provides the ability to power down without
a user-perceived reduction in latency. Battery life is only days with
this mode if the unit is frequently used, so this usage model should
be considered with a charging station. This scenario makes the usage
model for such a device similar to that of a cellular phone.
“Power-save poll” mode is also an efficient way to reduce power
consumption in devices where latency is not an important issue, such
as those that transfer data more than four times a minute. In such
cases, the act of reconnecting to the network consumes more energy
than keeping the device connected via power-save poll. For example,
the Microchip Wi-Fi radio provides the “power-save poll” mode
autonomously to the microcontroller. An added advantage of this is that
the entire client system can shut down during the sleep periods. An
interrupt from the radio will provide the wake indication, in case there
is data waiting on the AP. Additionally, for remote-control applications,
a wake-on-button-press or wake-on-motion can be provided via the
microcontroller, to maintain the illusion of always being on for the user.
For deeply embedded devices where latency is not critical, the best
methodology for increasing battery life is to turn off the radio. The off
state with leakage is the hibernate mode on the Microchip Wi-Fi radio.
Once outstanding traffic is resolved via the TCP/IP stack, a single
GPIO pin is used to deselect the device. This operation automatically
disconnects power from the Wi-Fi radio. This mechanism allows the
microcontroller and Wi-Fi radio combination to utilize about 120 nA in
power-down mode. Such a combination is ideal for deeply embedded
devices, because their low active power consumption stretches the
system’s battery life.
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