April 2004, Issue 165

BasicCard 101 (Part 2):
Use in a Liquid Nitrogen Monitor

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Start • Interface to the AVR MCU • Card Firmware • Give it a Try • Sources and PDF

Last month, I introduced you to ZeitControl’s BasicCard, which is a smartcard that is programmed in Basic. In Part 2, I’d like to further explore the programming of these cards and discuss a project.

I built a device that allows you to use BasicCards like debit cards to control the dispensing of liquid from a liquid nitrogen generator/storage tank. The idea is to issue each liquid nitrogen user here at Dalhousie University with a BasicCard. Each card is personalized by entering the user’s name, account number, and a zero balance. Card personalization is done on a PC using a Windows-based application. This user-friendly application interfaces to the BasicCard using the CyberMouse reader that comes with the development kit.

To access some liquid nitrogen (LN²), insert your BasicCard into the custom controller connected to the ² generator. The controller checks to ensure that the card is properly personalized for this use, and displays your name, account number, and the number of liters of ² previously consumed during the current billing period. You then enter the amount of ² desired on the controller’s keypad. After updating the BasicCard EEPROM variable, which stores the accumulated ² usage, the controller activates a relay that opens a valve and lets the liquid nitrogen flow for a long enough period to dispense the necessary amount of ².

Periodically, the BasicCards are collected and another PC application is run that zeroes out the accumulated ² total. It transfers that figure to a table that is used for the actual billing process.

In the past, an honor system was used in which users entered their ² usage on a sign-out sheet posted next to the generator. We lost some ² billings here at the university because of dishonesty and low-ball estimates of the actual amounts taken. Also, there was a significant amount of clerical time spent tallying all of the entries on the daily sign-up sheets.  

CIRCUIT DETAILS

Because the prototype would be the only unit I would need, I wanted to use a commercial development board and just add the few custom parts needed for the design. I chose the Futurlec 8535 development PCB because it’s inexpensive, contains headers for commonly used peripherals, and has a large enough prototype area to mount the few extra components I needed to complete the design.

Figure 1 is a diagram of the circuit. Some of the circuitry on the Futerlec board that is not required for this project is not shown in the diagram. For example, the in-circuit serial programming port for the 8535’s flash program memory is not shown. There is one small change that I had to make to the Futurlec board itself. The board comes fitted with an 8-MHz MCU clock crystal. I changed that to 3.579 MHz so that I could use a common clock for both the MCU and BasicCard, which has a 5-MHz maximum clock rate and communicates at 9600 bps when clocked at 3.579 MHz.

(Click here to enlarge)

Figure 1—The liquid nitrogen dispensing monitor was built on a Futurlec 8535 development board. Apart from the card socket, keypad/LCD, and relay driver, most of the circuitry already exists on the development board.

I mounted an Amphenol C702 10M0008 2834 smartcard socket in the prototype area of the PCB. This socket contains an NC switch that opens up when a card is inserted, making it easy for the MCU to know when to establish communications with the card. Although I connected this line to port D3 of the ’8535 (INT1), I don’t use this pin as an interrupt input, instead I poll the pin to see when a card is either inserted or removed. There are a few different phases in the program, and the removal of a card must be handled differently in each case, so using an interrupt to signal card-in-socket status was not ideal for this project.

The BasicCard interfaces to the ’8535 using only two port lines. The *RESET input to the BasicCard is connected to port D2, and the I/O line connects to port D4. As I mentioned earlier, the BasicCard gets its clock signal directly from the ’8535’s XTAL1 pin, which is its oscillator buffer output. Although you have to be careful tapping off a clock signal from an MCU’s oscillator output, the BasicCard presents, in this case, such a small load that the oscillator is unaffected by its presence.

The user interface is the standard keypad/LCD. I had a Grayhill series 88 keypad on hand, which contains the numbers in a telephone-like arrangement, as well as several extra keys labeled Enter, Clear, etc. that provide all the necessary input functions. Eight lines are needed to interface this Matrix keypad, and I used port A for that purpose. Doing so meant I could not make use of the eight-channel ADC contained in the ’8535, but I didn’t need an ADC for this project. The Futurlec board has a 10-pin header connected to this port, which made it easy to connect the keypad using a ribbon cable.

I didn’t need a large LCD, because there isn’t a lot of information to display. Therefore, I used a 2 × 16 display, and connected it to the ’8535 in 4-bit mode using six lines of port C. The Futurlec board also provides a 14-pin header for the LCD, with all the signal and power wiring taken care of, as well as POT1 for contrast adjustment.

Apart from wiring a few lines to the smartcard socket, all the circuitry I needed to add was the driver and relay to activate the dispensing valve. This was a solenoid valve that required 120 VAC to operate, so I decided to use a relay to keep any inductive kickback from its coil away from the MCU circuitry. Any relay with a 12-V coil and contacts able to switch 1-A AC will do for K1. If the coil needs more than a few hundred milliamps to operate, you’ll need a larger transistor than a 2N3904 for Q1. Diode D1 is placed across the relay coil to protect Q1 from the spike that occurs when the relay turns off.

The 5-V power supply regulator is contained on the Futurlec PCB, but the power transformer, bridge, and filter capacitor are mounted externally. Photo 1 shows the development board with the extra components needed for this project.

(Click here to enlarge)

Photo 1—Check out the monitor before it’s mounted in its cabinet. Note the BasicCard sticking out of the card socket on the right-hand side.

The development board comes with a MAX232 for RS-232 communications purposes. Although I did not use this feature, it would be easy to wire the controller to a remote PC, for example, if you want to log all the transactions in real time to a computer file.

Atmel’s AT90S8535 is a versatile MCU. In some respects it is overkill for this application. With the program written in compiled Basic, much of its 8-KB flash program memory is unused. It contains 512 bytes of EEPROM, which is unused apart from a few bytes used to store some valve time calibration parameters. The eight-channel ADC, UART, and SPI functions weren’t needed either. However, the assembled Futurlec development board, including the ’8535 MCU and programmer cable, cost less than $30, so I thought it made a lot of sense to do things this way.