Homework 4
RFID Shopping Cart Lane
Group 12
Team Members:
Saqib Malik
Beau Benjamin
Guy Barcelona
Rajat Bhardwaj
Homework 4: Design Constraint Analysis and Component Selection Rationale
Group No. 12
Team Member Completing This Homework: Saqib Malik
Introduction
The RFID Shopping Cart Lane is an intelligent method to check out at a department store. The checkout system consists of a RFID reader to read the items from the shopping car, an LCD interface to display the final total of the items, a keypad interface to enter the credit card number to pay the amount, and receipt printer that would print out the list of items in the shopping cart. The microcontroller will be part of the RFID reader and will control the other interfaces. The RFID reader will read the RFID tags on the items and send the information to the microcontroller. An antenna will be connected to the RFID reader so it is able to read all the items in the range of the shopping cart. The microcontroller will be reading and storing information from the reader and then calculate the total using the information received from the tags. The microcontroller will drive the LCD to display the total and the keypad to get customer information. Once the transaction is complete, it will send all the item data to the receipt printer for a receipt. There is not much user interaction with this system, except entering payment information using the keypad.
The design constraints are considered for the following parts: the microcontroller, RFID kit, LCD display, keypad, power supply constraints, packaging constraints, and cost constraints.
Real World Design Constraints
Computational Requirements:
The microcontroller is the central component of the design. The system will be reading from 1 to around 100 items at a time. Therefore, the speed of the system is heavily dependent on the amount of items in the cart.The communication speed between the reader and the microcontroller is 115200 baud. At this rate, the reader can read 60 tags per second.. In order for the microcontroller to interface correctly the microcontroller has to be able to run at about 15 MHz. Most microcontrollers are usually greater then that so that’s not really a large problem. With this in mind, the memory of the microcontroller as well as the peripherals becomes much more of a priority than the speed of the microcontroller's clock.
Interface Requirements:
There are four components that the microcontroller will interface with. The RFID reader IC, LCD screen, keypad and the receipt printer. The two main constraints for the interface are:
- I/O pins
- Serial Communication
I/O Pins:
One of the most important constraints is I/O pins. The microcontroller has to be able to connect to all the components listed above. An estimate of I/O pints needed is in the table below.
Component / I/O PinsRFID Reader IC / 4
LCD Display / 5
Receipt Printer / 5
Keypad / 10
Total / 23
Table 1 – Estimated Number of I/O Pins
The total number of I/O pins is not going to be a problem since most microcontrollers have 30+ I/O pins and are also expandable. We will make sure we have a few extra I/O pins open for any additions or changes made to the components.
Serial Communication:
Communication with RFID reader IC will be done using a serial interface. The microcontroller has to have a serial driver which is able to communicate at 115200 baud. Also the printer uses an asynchronous serial interface for communication purposes so a serial communication method is necessary for this project.
Power Supply Constraints:
Each component used in our design has its own specifications for power supply requirements. These requirements have been summarized in Table 2 below:
Component / VDD (V) / Current (mA)Microcontroller / 2.7-5.5 / 100
LCD Display / 5 / 1.6
Receipt Printer / 24 / 100
Keypad / 12 / 5
RFID Reader IC / 3.3-5 / 200
Table 2 – Power Requirements By Component
Packaging Constraints:
For packaging, we will have something that looks like a security gate at the airports used for metal detectors, just smaller in size. The size has to be enough for a shopping cart to be pushed through. It will be 3.5ft x 2.5ft depending on the materials and shopping carts we use. We will place the circuitry with reader on top of the platform, with the LCD placed on the side so the user can easily read the information. The keypad will be right under the LCD for easy interaction. The antenna for the reader will be placed on the left side of the platform that the cart goes through to maximize the reading range for the reader. The side panels will be large so it will also give us room to build a larger antenna for a better range. We will try to use the least amount of metal or any kind of shielding that will cause any coupling to the antenna. We don’t want to deal with any of those problems, since the antenna measurements are going to be really precise. There fore for this project we will try to use plastic or wood enclosure for now.
Cost Constraints:
Cost is an area that provides constraints which are not completely clear. One of our main goals for this project is to keep our cost an minimum as possible. The most expensive part in this project was the RFID reader kit, which costs around $1000+. We were mainly looking at Texas Instrument readers, but due to the cost we contacted them and were able to get the schematics and the Reader IC as samples. There fore the biggest cost constraint was taken care off, except that it added more work to our project, where we have to build the reader and the antenna. The antenna costs are not exact yet, because we are searching for testing equipment on campus, but it should not exceed $50.00. For the microcontroller, another expensive part, we took advantage of the Renesas contest where we get the free microcontroller development kit. Next major part was the printer, which we got a great deal on. We got the $200 Verifone printer for $50 with all documentation needed. We also got the LCD for free; because one of the group members had it from a previous project in which they didn’t end up using it. We need to buy a 10 button keypad for transaction which costs around $20.The cost of each of the parts chosen is displayed in Table 3 below:
Component / Cost (US $)Renesas SKP16C62P Microcontroller / 0.00
LCD Display / 0.00
10 Button Keypad / 20.00
Printer / 50.00
RFID Reader IC / 0.00
Antenna / 50.00
Total: / 120.00
Table 3 – Cost By Component
Some other costs that are not included in this table will be included in the final price. These costs will be of the mounting and screws used to package the product adequately.
Rationale For Component Selection
There were five major component selections that needed to be made. A significant amount of time and research went into the selection of each of the components. These components are:
Microcontroller
LCD Display
Keypad
RFID Reader
Receipt Printer
Microcontroller:
Numerous factors played a role in which microcontroller to choose for our design. The computational constraints were not too big of a problem since most of the microcontrollers are faster then 15 MHz. Also we needed a serial communication port for the reader IC and the printer. An Ethernet interface was not necessary so we left the rabit family out. Mainly we were looking at the Free Scale 9SHC12 family since it was fast enough, had a serial port and plenty of I/O pins. But once we found out about the Renesas competition, where we can get the SKP16C62P development kit for free, we looked more in to it. It is 24 MHz, which is plenty fast for our purposes, has 87 I/O pins which is also way more then we need for our project, it has plenty of memory, 5 Serial I/O channels and a SPI interface and most of all it was free. Since it fit all our requirements and more for future upgrades, we went ahead and ordered the Renesas SKP16C62P [1].
LCD Display:
When choosing our LCD display, we decided up front that we did not need any kind of graphics module. A simple text module would be sufficient to display transaction information. We were looking at the Crystal Fontz 632 16x2 and 634 20x4 Serial LCD Modules. Since we were looking for something real simple, we decided to go with the 632 16x2 LCD, also because we got that for free so again it was a really easy decision for us [2].
Keypad:
In deciding which keypad to use we figured the first step was to figure out how many keys would be needed for our functionality. The keypad usage will be only there for transaction purposes. There fore we decided we needed a 10 button keypad, so its easy for the user to enter credit card info. We looked at the Grayhill 84 series keypads, the 3x4 and the 4x4. They have a built in debounce so no extra circuitry would be needed. The 12-button keypad has too many buttons for us, so we decided on the 84A1-B102 [3] since it’s a 3x4, it’s more then enough for what we need. Also since we have enough I/O pins, all 10 keys can be direct inputs and no decoding will be needed.
RFID Kit:
This was the biggest cost constraint for our project, and was making us think twice about this idea. But after TI gave us the reader IC, schematics for the reader and the antenna design, we decided to build our own reader and antenna. First we were looking at the TI S2000 low frequency reader, but the range wasn’t enough for that so we then looked at the TI S2000 and TI S6000 high frequency readers at 13.56 MHz. But none of those readers were with in our price range so we will go with our own reader using the S6700 Transceiver IC [5] and our own antenna. We can make a fairly large antenna to increase the range of the reader. The choice wasn’t too hard for us again, so we went with the IC and will build our own reader.
Receipt Printer:
For the printer, we were just looking for a plain receipt printer with a serial connection to the microcontroller. We were looking between the Samsung SRP-270A and the Verifone 250 Printer [4]. Both have a serial port and functionality was plenty for our project, since speed or resolution was not much of an issue. The cheapest place to get these was EBay, and we decided to get which ever one was cheaper. We ended up getting the Verifone for $50.00 which was a great deal since it retails for $200 and it came with all documentation needed.
List of Major Components
All of the major components used in the refrigerator and their information are shown in Table 4 below. These parts either have been ordered or will be ordered very soon.
Part Name / Vendor / Part Number / Quantity / Unit Cost / Total CostMicrocontroller / Rabbit Semiconductor / RCM2200 / 1 / $0.00 / $0.00
LCD Display / Crystal Fontz / CFAH20004A-NYA-JP / 1 / $0.00 / $0.00
Keypad / Digi-Key / MGR1523-ND / 1 / $20.00 / $20.00
Receipt Printer / Verifone / 10692 / 1 / $50.00 / $50.00
Antenna / Custom / NA / 1 / $50.00 / $50.00
RFID Kit / Texas Instruments / RI-R6C-001A / 1 / $0.00 / $.00
Table 4 – Major Components Used in the “Cold as Ice” Fridge
Reference
1-Renesas Microcontroller
2- 632 16x2 Crystal Fontz Serial LCD
3- Gray hill 84A1-B102 Keypad
4- Verifone Printer 250
5- RFID Reader IC
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