RFID Sensor Dog Door

This project describes how to use a GreenPAK™ SLG46531V as the main controller for an RFID based pet door.

We used an SM130 RFID module shown in Figure 1 to interface with the GreenPAK, and used the opportunity to apply serial communication principles.

PINs 16 and 17 play a major role in this project, as these two pins let us know when the SM130 is looking for a tag, and when it has found a new one. PIN 17, also known as OUTPUT ERROR, goes to HIGH when communication with a tag is not successful. PIN 16, labelled OUTPUT OK, goes to HIGH when the communication is successful.

This means that OUTPUT ERROR can signal when the SM130 is being ordered to "Seek a Tag", and OUTPUT OK will signal with a HIGH when a tag has been found. These two signals are going to be the base of the process for controlling the electronic lock. But before we jump into that, we have to send the SM130 a "Seek Tag" from the GreenPAK.

The SM130 has the UART protocol enabled by default, so we will be using that for this project. In Figure 2 we can see the UART frames as shown in the SM130 datasheet.

Below we described steps needed to understand how the solution has been programmed to design the smart pillow. However, if you just want to get the result of programming, download GreenPAK software to view the already completed GreenPAK Design File. Plug the GreenPAK Development Kit to your computer and hit the program to design the solution.

System function and circuit design

The SM130´s OUTPUT ERROR turns HIGH after the "Seek a Tag" command is received. OUTPUT OK turns HIGH when a tag is detected by the antenna, but this HIGH only lasts for a split second, so we have to plan accordingly.

The OUTPUT OK signal will be received by the GreenPAK as a trigger on PIN 17. Then, we’ll use a delay to keep the signal HIGH for 5 seconds, during which time the lock will remain open.

The lock used for this application is at 12 V DC with a current of 0.6 A, but it can be used with 8 V as well. We have two choices of circuit to drive the lock, both of which will be explored in this project.

To speed up development, you can buy the SparkFun DEV10406 Spark fun RFID Evaluation Shield. You can simply connect the UART Rx (Pin D8) to any of the GPIO pins on the SLG46531V.

Implementing UART Communication

In order to output our UART frames, we set up the GreenPAK to output all of the bits stored in its Asychronous State Machine’s RAM. We used the same technique discussed in AN-1137: Serial Output Tips and Techniques; please consult that app note for more information on how to assemble a serial output ASM.

At the hardware level, when the system is idling we need the data line to be HIGH, so we surrounded the frames with HIGH bits.

The ASM is configured as shown in Figure 4. The Start and Stop bits are labeled with a blue ‘S’ and a red ‘S’.

• 0xFF: Header Byte starts in State 0 and finishes in State 1
• 0x00: Reserved Byte starts in State 1 and finishes in State 2
• 0x01: Length Byte starts in State 2 and finishes in State 3
• 0x82: Command Byte starts in State 4 and finishes in State 5
• 0x83: Checksum starts in State 5 and finishes in State 6

This configuration will give us a waveform that, when analyzed by the receiver end, will output the command “Seek a Tag.” Figure 5 shows the waveform analyzed by Saleae Logic Analyzer.

The introduction of the Start and Stop bits make it a little more difficult to intuitively implement the code, so it's important to have a way to look at the waveform.

Next, we will be using the horizontal ASM serial output as shown in Figure 6.

As you can see in Figure 6, most of the resources of the GreenPAK are being used by the horizontal ASM serial output design, so we’ll have to make the lock operation as simple as possible.

Before we do that, we need to figure out what clock we are going to use. The SM130 has a default baud rate of 19200 bps. We could generate that CLK speed inside the GreenPAK, but since we are low on resources it makes more sense to generate the CLK outside the GreenPAK.

Another important point is that the UART idle state is HIGH, so we need to send a continuous HIGH state after the SM130 as already started looking for a tag.

Developing LOCK Control Using 2 LUTs

After the ASM is finished we only have two LUTs left to work with, both of which are 3bit LUTs.

First, we have to find a way to extend the duration of the OUTPUT OK signal from a few fractions of a second to at least 5 seconds. Luckily, the GreenPAK has no problem dealing with this situation.

We’ll reconfigure 3bit LUT8 as a delay block. Using the falling edge of the OUTPUT OK signal, we’ll trigger a falling edge delay of 5 seconds. This means we are going to delay the change from HIGH to LOW for 5 seconds.

We’ll use OSC0/64, predivided by 8, to create a delay of 5 seconds, successfully using one LUT to drive the lock, as shown in Figure 7.

Finally, to drive the ASM output HIGH, we will use the OUTPUT ERROR signal, which can only be HIGH when OUTPUT OK is LOW.

Here we find a problem: it is possible for the OUTPUT ERROR to be HIGH when the door is open, leading to some undesirable side effects. To prevent this, we will use the Delayed_State signal to drive the last 3bit LUT.

The states are as follows:

• Delayed_State is HIGH, so the bus is not sending the UART message (it doesn't matter what the other signals are).
• Delayed_State its LOW, but OUTPUT_ERROR is HIGH, so the bus is not sending the UART message.
• Delayed_State is LOW and OUTPUT_ERROR is LOW, so the bus transmits the UART message.

In the LUT table we assign Delayed_State to IN2, OUTPUT_ERROR to IN1, and the ASM output to IN0. This turns the LUT into an OR gate, as the only time that a LOW state is sent through the bus is when the three signals are in a LOW state.

Driving the Lock

The lock used for this project is a 12v DC Cabinet Drawer Electric Solenoid Lock, and is pictured in Figure 9. When powered on, the shaft retracts and stays retracted as long as power is still supplied.

As stated before, this project covers two ways to drive the system:

• Using the L298n HBridge module
• Using transistors

It’s true that driving the lock with the L298n its easier and more convenient, but if we want to make a custom PCB for this project, it would be a good idea to dive a little into driving the lock using transistors.

The lock is rated at 12 V DC and 0.6 A, which means the lock has a resistance of approximately 20 Ohms. The lock also works at 8 V with a current of 0.4 A. Usually transistors are rated with a maximum of 0.5 A, so knowing that we can drive the lock with less is a good thing.

If we supply the motor with the 12 V it requires, we’ll need a resistor to limit the current. That resistor will go in series with the motor. A resistor value of 10 Ohms is enough, but we need to know how much power it’s going to be dissipating. Since the resistor is going to be dissipating more than 1 W, we’ll choose a value that withstands at least 2 W.

In Figure 10 we can see the transistors switching ON and OFF the lock.

In Figure 10, Q1 is an NPN BJT. When its base is LOW, Q1 behaves like an open switch, so R2 and R3 are in series and the voltage in Q2’s base is similar to the voltage in its emitter, meaning Q2 also behaves like an open circuit.

When Q1’s base is HIGH, it transmits some of the voltage between R2 and R3, making Q2’s base voltage significantly smaller than the voltage in its emitter, allowing Q2 to turn ON the lock.

Results

Figure 11 shows the completed system. An Arduino Uno is used as an external clock, but the whole system is driven by the GreenPAK. As the command is already sent, the LED that signals searching turns on.

Figure 12 shows a tag in the antenna prompting the GreenPAK to wait for the programmed time, after which the message is sent again.

This GreenPAK is using nearly all of its resources, so implementing the 19200 Baudrate becomes difficult. Further customizing the serial output could free enough resources to implement an internal clock.

Conclusion

This project was created to lay out the basics of how to create a design for the GreenPAK SLG46531V to work as a master IC in some applications. It described a low security system that is certainly more than enough to drive a doggy door, but that can be perfected by adding (for example) another GreenPAK to analyze the response of the SM130, which could be used to authenticate the tag used. Furthermore, the system could use another method to drive the lock, or we could even select another kind of lock such as an electromagnetic one.