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FINE–TUNING AN EMBEDDED IDEA

Part 3: Armed and Ready
by Fred Eady

Start ý Retro Rabbit ý Preflight Checklist ý Firmware Developmentý Left Turn, Clyde• Successý Sources and PDF

PREFLIGHT CHECKLIST

Letýs check off what the upgrade requirements were versus the RCM2300ýs capabilities. The first concern, size, was satisfied by the RCM2300ýs compact form factor. The relay controller module needed the ability to keep track of relay closures and durations. In addition, the relay controller module had to have a means of identifying itself to the master controller. If the relay controller has to be moved to another master controller network on the plant floor, the relay controller ID will need to be changed and remembered.

Because the specifications called for unlimited reads and writes to nonvolatile storage, I considered using RAMTRONýs ferro-based nonvolatile serial memories for this, but as good as that sounds, it adds complexity and parts to the solution. The RCM2300 has all of the necessary circuitry onboard to support battery-backed SRAM operation. I had to add a battery to the solution to support the real-time clock anyway. So, with the battery being a necessary item, the RAMTRON part was one less part I needed to add.

By simply attaching a 950-mAh lithium cell between the RCM2300 VBAT pin and ground, unlimited read/write nonvolatile storage is effected and the real-time clock is implemented. The final requirement of serial communications was satisfied by the RCM2300ýs abundance of serial ports. In addition to the programming port, the RCM2300 comes standard with a serial port configuration capable of two 2-wire interfaces or one 5-wire interface. The upgrade uses the services of one of these serial ports in 2-wire mode for asynchronous communications between the RCM2300 complex and an external terminal or personal computer. If the need arises later, the remaining 2-wire port can be equipped with a standard RS-485 converter, and with the help of a single I/O for transmit/receive switching, the RCM2300 embedded complex can become a fully functional RS-485 network node.

Basically, the hardware consists of a simple printed circuit board, a lithium backup battery, the RCM2300, and an RS-232 converter IC. If youýre wondering why I included the printed circuit board as part of the hardware, youýll get your answer by studying Photo 3. What you see is a 28-pin dual inline arrangement that is designed to be plugged into the socket of the microcontroller the RCM2300 complex replaced. This arrangement allows the RCM2300 embedded computer to access the resources of the original relay controller via the pinout of the microcontroller it replaced.

Photo 3ýThis is the bottom side of the RCM2300-based upgrade module. Note the dual inline header pins. These pins allow the enhanced RCM2300-based embedded complex to assume the pinout of the microcontroller it is replacing.

 

For instance, if pin 1 was an output pin on the original microcontroller, All I had to do was assign and hardwire one of the RCM2300 I/O pins to the IC socketýs pin 1 and code it for output in the code. The original equipment microcontroller had a single UART that was pinned for use with an external RS-232 converter. To mimic this functionality, I simply designed the RCM2300 motherboard to allow one of the RCM2300 serial ports to interface with the original UART pins. To go one better, I added the RS-232 converter IC to the RCM2300 motherboard in the form of the Sipex SP233. I chose the SP233 because it does not require the charge pump capacitors to operate in true RS-232 mode.

Moving to the topside of the RCM2300 motherboard (see Photo 4), youýll find the female headers that support the RCM2300 core module and the 950-mAh lithium backup battery. This view gives you an idea of what can take place under the RCM2300 module. As you can see, just in case I need them, I added some SMT resistor pads to accommodate pullup resistors on the old microcontroller input pins, which are mapped to I/O pins on the RCM2300 module. Photo 5 is a view of the complete RCM2300-based embedded upgrade module with the RCM2300 mounted on the motherboard.

Photo 4ýThis is a look under the RCM2300 module. As a precaution, I added pads for SMD pullup resistors on the input pins of the original microcontroller. I also made provisions for the installation of a couple through-hole MOSFETS to drive high current outputs. Itýs best to have more pads than you need than to need them and not have them.

Photo 5ýI can assemble this upgrade module in about 15 min.

Thereýs one more piece of hardware I want to mention before I get into the development of the final firmware. Although this hardware doesnýt have any direct bearing on the operation of the RCM2300-based upgrade module, it does have a big impact on the design of the upgrade moduleýs printed circuit board. The piece of hardware Iým talking about is the RCM2300 Development Kit. I used the RCM2300 Development Kit to baseline the upgrade moduleýs design.

As you can see from Photo 6, I added the optional RS-232 converter IC to my RCM2300 Development Kit to allow testing of my firmware with a known good serial configuration. To help in firmware development, I wired the upgrade moduleýs serial interface just like it is wired on the RCM2300 Development Kit. My RCM2300 Development Kit came with Dynamic C SE, a RCM2300 core module, prototype board, programming cable, power supply, a "Getting Started" manual, and a Rabbit 2000 Easy Reference poster.

Photo 6ýI added this Sipex 232 RS-232 converter IC to initially test the upgrade moduleýs serial port design. The RCM2300 Development Kit has provisions for this addition. All you have to do is add the parts.

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