that is greater than + 3.0 volts. The
PIC16LF876A is the execution unit
that supports the RPM3’s radio
packet modem subsystem which is
clocked by a 9.216 MHz crystal.
The radio packet modem directly
interfaces with the host
microcontroller. The smaller
PIC16F628A has the same 10 MHz
clock frequency characteristics
when operated at + 3.0 volts.
However, the PIC16F628A can
operate with clocks up to 20 MHz
when powered with a + 5.0 volt
power source. Theoretically, the
PICs in our five volt RPM3 could
operate at + 3.0 volts. The caveat is
that we really don’t know if the RF
section is voltage sensitive. The PIC16F628A clocks
mnemonics that form the Fast Radio Packet Controller
which interfaces directly to the radio packet modem on
one side and the actual RF transceiver on the other.
The controller is serviced by an 8.192 MHz crystal.
Figure 1 sums up the RPM3 subsystems. The bottom
line is that whatever data we pump into the
transmitting + 5 volt powered RPM3 has a very good
chance of being delivered to any other desired RPM3
that is in receiving range.
PHOTO 2. As you can see, our RPM3 is a five volt version
that is tuned to the North American 900 MHz frequency.
RPM3 Modes and Pins
end device. Once the logical connection is established,
the contacted RPM3 enters a connected state which
allows data to be transferred.
Serial data is transferred between the embedded
host and RPM3 via the RPM3’s TXD and RXD pins.
Data flow control can be implemented by monitoring
the RPM3’s CTS pin. The RPM3 will take its CTS line
logically high when its serial receive buffer reaches
66% of its capacity. When the serial receive buffer
empties to 33% of capacity, the RPM3 will drop the
CTS to a logically low level. Our host microcontroller
has enough extra I/O pins available to allow us to
dedicate one to the RPM3’s CTS output.
Figure 2 provides us with the
lay of the land as far as the RPM3
is concerned. The active-low
WAKE/DTR pin will force the
RPM3 into shutdown state when
it is driven logically high. The
RPM3 is not accessible while in
shutdown state. We will assign a
host microcontroller I/O pin to
the WAKE/DTR input as the
shutdown state is actually low
power sleep mode.
The RPM3 will automatically
enter the standby state when
powered up. Standby state is
maintained until a connection
request is received. A host can
initiate a connection request by
simply transferring data to the
RPM3 via the serial interface.
Thus, a logical connection is
established between the host and
FIGURE 1. This graphic depiction of the RPM3 pretty much says it all.
The BiM3A transceiver easily handles the RF traffic as the
FIGURE 2. This is vital information.
SERVO 02.2011 51