Basic Connections

Once you have assembled your board, you need to know how to connect power, then set it up for programming. The MC68HC812A4 part comes from the factory completely blank. Therefore, you must program it before it can do anything.

To program this part, you will need a BDM12 interface pod, or an equivalent board from another manufacturer. My instructions are going to assume you are using a BDM12 interface pod.

Setup for Programming and Debugging

Here is a list of things to do in setting up the board for programming.

1. Connect a 5 volt power supply to the POWER pins
2. Connect a BDM12 to the BDM12 port. Be sure you understand where pin 1 is on the cable and on the board.
3. Using your BDM12 download software, issue a reset command, then use the load command to download your .S19 file to the EEPROM
4. If needed, you can run your debugger software to single step and otherwise debug your code.

These steps should get you setup to program your board.

Normal Operation

For normal operation, it is recommended that you do the following:

1. Connect +5 volts to POWER

On powerup, the chip will enter normal single chip operation mode, and execute whatever software you have previously loaded.

Optional changes to the board

I have configured this board to run just fine in stand alone mode. I have also added a few options via cut jumpers in the event that you wish to change the configuration on the board. The following describe what each of the cut jumpers on the board do. I have organized them according to functionality.

First a quick word about cut jumpers.

The cut jumpers are made up of three surface mount pads. To cut a jumper, you need to remove the pad that connects the outer two pads. This creates a break in the surface. You can now use the outer pads to solder on a jumper wire to connect the circuit to some other connection point. I recommend using AWG #30 Kynar wire wrap style wire for your jumpers on logic level devices. This wire is small enough to be soldered to these pads with plenty of room to spare.

Jumper Diagram

All of the cut jumpers on the 812A4 are on the backside of the board. Here is a diagram of where to find them on the back.

A/D Conversion Voltages

The A/D converter comes default using Vdd and Vss as the reference voltages. If you would like to connect your own reference voltages, you will need to cut jumpers JP1 and/or JP3 on the bottom of the board. JP1 is connected to the Vrh pin and to Vdd. If you cut JP1, you can replace Vrh with your own signal by either soldering a jumper to the appropriate pad on JP1, or connecting to pin 85 of the CPU. Likewise, JP3 is connected to pin 86 which is Vrl.

The A/D converter also comes default with VSSA and VDDA connected to Vss and Vdd respectively. If you wish to change this default behaviour, you will need to cut JP2 and JP4.

Serial EEPROM

The Serial EEPROM used on the 812A4 board is a Microchip 25LC640, which is an 8k x 8 serial EEPROM that is SPI compatible. I have wired this part directly to the SPI port on the board, and it uses the SS pin of the SPI port as the chip select. If you wish, you can omit the serial EEPROM if you don't plan to use it.

If you do install the EEPROM, there are several cut jumpers that you can use to change the configuration. The default wiring works great when this device is the only thing on the SPI bus.

If your design plans to use the SS pin, then you may need to reconfigure the board so that the serial EEPROM uses a different chip select. The default wiring has the IC4's chip select connected to the SS pin on the SPI port. To change this, you will need to cut jumper JP7, and rewire a different chip select via an I/O output pin.

Two other pins are connected to defaults through cut jumpers. JP6 defaults the HOLD\ pin to high, and JP5 disables the write protect mechanism. You need to understand how the serial EEPROM works before changing these defaults.

RS232/TTL Conversion

One of the larger optional sections is the RS232/TTL conversion section. This section uses a MAX3221CAE part, which is a low power converter. The nice thing about this converter is it uses extremely little power in its shutdown state (approximately 5 uA). Thus, if you wish, you can install the part and practically ignore it. Otherwise, you can leave off IC3, X1, C1, C2, C3, and C4. The board will function without them.

The MAX3221 part will stay in shutdown until it finds a valid signal on the RX line of the SERIAL0 connector. When this happens, the part will resume operation by starting the charge pump. If you wish, there is an additional connection that isn't implemented on this part called the INVALID/ pin. I have brought this connection out to an unmarked pad on the bottom side of the board. It is called the INVALID pad, and can be seen in the above diagram.

Empty SMT layouts

The 812A4 board has a fairly large prototyping area for using standard through hole parts. I also included a few empty SMT layouts in some spare rooom I found on the backside of the board. These parts are labeled EMPTY1-5, and are available for your use.