Issue
160 November 2003
RF
Made Simple
DEAD-EASY
APPLICATION
Well,
almost. This is an embedded project, and you all know
the first rule of embedded programming (repeat after
me): nothing is free.
I
wanted various microcontroller-supported easy-Radios
to report to a central module that reports to me with
an e-mail message. The idea was to have the CH2124-equipped
easy-Radio station poll the remote easy-Radio stations
for their data. Every easy-Radio station within an earshot
picks up a transmission, so I’m also considering having
each radio listen for a transmission, wait for a predetermined
amount of time, and then transmit its data. Whether
to poll or use a timed sequence for transmission depends
on the application.
In
any case, the central CH2124-equipped easy-Radio station
simply assembles the incoming data from the radios in
the network into an e-mail I can read and shoots it
down the line using the CH2124. The concept is simple;
however, the implementation is a click above dead easy.
In the CH2124 and the easy-Radio, we have two intelligent
devices. The easy-Radio and the CH2124 have serial ports,
which presents Problem 1. Although both of the smart
devices have serial interfaces, the easy-Radio serial
interface’s speed seems to be fixed at 19,200 bps. The
CH2124 wants to see a 2400-bps serial datastream at
its serial interface, which stands to reason, because
it’s an intelligent 2400-bps modem module.
Checking
the CH2124 datasheet, the CH2124 cannot be commanded
to do 19,200 bps at its serial interface. My goal was
to not have a microcontroller in the CH2124-equipped
easy-Radio station. There’s more than enough intelligence
between the easy-Radio and the CH2124 to make this RF-to-e-mail
thing happen. Now “dead easy” has turned into “dead
in the water.”
OK,
if I have to add a microcontroller to solve Problem
1, it’s going to be an itty-bitty one, and the code
is going to be itty-bitty, too. Because the CH2124 won’t
be retrieving e-mails in this application, all that’s
needed is to convert the easy-Radio’s 19,200 bps to
2400 bps, and then feed the lowered data rate data to
the CH2124’s 2400-bps serial interface.
The
production easy-Radio/CH2124 units didn’t need the RS-232
conversion circuitry present on both the CH2124 and
easy-Radio evaluation platforms. But remember that this
was supposed to be easy, so I verified the data rate
converter hardware and firmware using the known-good
easy-Radio and known-good CH2124 evaluation hardware
already on hand. After I had a working data rate converter
circuit, I was able to eliminate the RS-232 conversion
ICs and stick the itty-bitty microcontroller between
the easy-Radio and CH2124 serial interfaces.
REMODELING
THE BOARD
The
blue power indicator LED on the easy-Radio evaluation
boards stayed. Right now, I don’t know about the rest
of the easy-Radio’s evaluation board’s power train.
There it is, the potential Problem 2. Easy-Radio engineers
warned me that I didn’t have the latest spin of the
easy-Radio evaluation boards. They also said that the
voltage regulator currently on the evaluation boards
wasn’t up to snuff for the new 900-MHz hoppers.
I’m
not planning on adding much more of a load to the easy-Radio
evaluation board’s power supply, but I don’t want to
add enough load to cause a shutdown of the evaluation
board’s on-board 78L05 SMT voltage regulator. There’s
ample ground plane area on the bottom of the evaluation
board to clear some heatsink space for a small-package
78M05 voltage regulator that can handle up to 500 mA,
which is more than enough to handle the easy-Radio hopper
module and what little else I’ll add.
You
can power an easy-Radio evaluation board with either
a 9-VDC wall wart or 9-V battery. The evaluation board’s
on-board, 5-V SMT regulator pre-regulates the incoming
9 VDC provided by the wall wart. Pre-regulation is a
good thing, because as the average unregulated 9-VDC
wall wart actually supplies about 12 VDC under light
load conditions. The easy-Radio has an internal 3.6-V
regulator and a 5 V-tolerant I/O interface. It’s a safer
bet than applying 5 VDC, because it won’t tax the easy-Radio’s
internal regulator.
The
easy-Radio and the CH2124 are both configured as DCE,
which allows a straight-through cable to connect either
of them to a standard DTE PC serial port. Because I
wanted to stick with the CH2124 and easy-Radio evaluation
boards, one of them had to be converted to a DTE configuration.
I’m already chopping on the easy-Radio evaluation board,
and, in the traditional sense, the CH2124 evaluation
board should be the DCE device because it holds the
modem, which is considered a DCE device. So, one of
the Easy-Radio evaluation board’s serial receive and
transmit lines will be swapped to transform the board
into a DTE device.
If
I poll for data, I will need a microcontroller on the
CH2124 end of the RF link to initiate the polling sequence.
If I allow the easy-Radios to police themselves, I still
would require a microcontroller on the CH2124 side of
the RF link because I need data rate conversion between
the CH2124 and the easy-Radio module. So, it looks like
my no-microcontroller-at-the-modem-station idea is a
bogus concept. With that, I’ve got a couple of ideas
about which microcontroller I should use.
My
first inclination was to use an eight-pin PIC12F675
for the data rate converter. If I go with the little
eight-pin PIC, I’ll have to allow the easy-Radios to
operate on a timed basis, because I’ll most likely consume
all of the PIC12F675’s processing time converting data
rates and transferring data. To allow the CH2124-equipped
easy-Radio station to poll, I’ll need a microcontroller
with a USART and USART interrupt capability. That takes
the “easy” out of it, so I’ll go with the itty-bitty
PIC.
The
PIC12F675 doesn’t need an external crystal because it
contains an internal 4-MHz oscillator. All I need externally
to support the PIC12F675 is a 0.1-µF bypass capacitor.
The
simplicity of the PIC12F675-based data rate converter
allows you to mount the PIC12F675 in an in-line fashion
using one of the unused in-line radio receptacles on
the easy-Radio evaluation board. The evaluation board
is a simple double-sided board, which allows you to
cut and reroute existing traces to accommodate the data
rate converter’s power and I/O. All of the data rate
conversion is performed on the TTL side of the serial
link. The CH2124 and the easy-Radio are both designed
to use TTL levels on the serial link and eliminate the
need for RS-232 voltage levels to communicate serially
with a host processor.
Normally,
the addition of a microcontroller would allow for the
buffering of the data between the easy-Radio and the
CH2124. In this case, the little PIC has no USART or
USART interrupt structure, and I’ve tied up the PIC12F675’s
time to such an extent that there may not be enough
cycles to perform a buffering operation without missing
an incoming bit. The solution is to not attempt to buffer
the data with the PIC and allow the data rate-converted
data to flow freely from the easy-Radio to the CH2124
modem.
The
CH2124 can be instructed to store a message and transmit
the message using @T iModem commands. Because the channel
is always open between the easy-Radio and the CH2124,
the @T commands can be embedded in the incoming datastream
that originates at a remote easy-Radio station. A typical
message from a remote easy-Radio is shown in Photo 4.
|
(Click
here to enlarge)
|
Photo
4—The remote non-modem-equipped easy-Radio generated
the entire line beginning with @TM1. The end of
a message is signaled by a carriage return dot carriage
return. The @TDM1 was also generated by the easy-Radio
on the non-modem side of the RF link. The rest of
the messages, which were ignored, are from the CH2124.
You can see them because I have Serialtest Async
tracing both the CH2124 and the easy-Radio serial
connections. |
