Application of the hottest digital signature techn

2022-08-02
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The application of digital signature technology in handheld devices

Abstract: in view of the high cost paper signature in the current logistics industry, this paper analyzes the advantages of digital signature. Taking LCD control chip SED1335 and touch screen control chip ADS7846 as examples, this paper proposes a method to realize digital signature acquisition on handheld devices, and gives the implementation process of hardware and software

key words: digital signature LCD touch screen SED1335 ads7846

electronic signature technology is a technology that collects and verifies personal signatures through electronic devices and binds information together to achieve the same effect as paper signature, so as to realize paperless office. In the goods distribution market, the preferred technology to reduce paper as delivery evidence is signature collection technology. When problems occur in business exchange, digital signature will be a strong evidence. Therefore, it is hoped that a small, simple and easy-to-operate handheld signature information collector can replace the commonly used paper signature

1 hardware circuit and its working principle

in many application fields, the touch screen is the basic input device, and the display screen is the output device. To complete the acquisition of images such as signatures, it is necessary to input information on the touch screen and display the input information on the display screen. In this paper, SED1335 LCD controller is used to read and write data to LCD screen, and ADS7846 is used to collect data to four wire resistance touch screen. The following describes the hardware implementation process

1.1 LCD controller

sed1335 is a LCD controller produced by Seiko Epson company in Japan. It has the strongest function among similar products. Its structure is shown in Figure 1. The hardware structure of SED1335 can be divided into interface part, internal control part and driving part of LCD module LCM. The interface part consists of instruction input buffer, data input buffer, data output buffer and flag register. The selection of these buffer channels is jointly controlled by pin A0 and read and write operation signals. The core of the control unit SED1335 is composed of an oscillator, a functional logic circuit, a display RAM management circuit, a character library and its management circuit, and a timing generator that generates driving timing. The operating frequency of the oscillator can be selected in the range of 1 ~ 10MHz. SED1335 can quickly interpret the instruction code sent by MPU at a very high operating frequency, put the parameters into the corresponding registers, and trigger the operation of the corresponding logic function circuit. The control part can manage 64KB display RAM, display the embedded character generator and the extended character generator cgram or excgrom. The driving part has the synthetic display capability of each display area, the organization function of data transmission and the timing pulse signal required for the LCD module to account for only 5% of the total sales

1.2 ADS7846 touch screen controller

ads7846 is a new generation of 4-wire resistive touch screen controller introduced by burr Brown company, which is compatible with ADS7843. Through mechanical touch, the position signal of the touch point can be quickly obtained. It is a typical a/d converter with a continuous approximation register. It has an internal +2.5v reference voltage. The serial interface of the microprocessor can measure temperature and touch pressure. It has a programmable 8-bit or 12 bit resolution (the maximum accuracy can be resolved 4096 × 4096 points) and automatically enter the low power consumption mode. 750 power consumption at 2.7V and 125kHz conversion rate μ W; In off mode, the power consumption is only 0.5 μ W。 Tssop-16 and ssop-16 packages are provided. Pin functions are shown in Table 1

pin name function description +vcc logic positive power supply x+, y+ connected to touch screen positive electrode x-, y- connected to touch screen negative electrode GND grounded Vbat power supply monitoring input terminal penirq interrupt output terminal VREF reference voltage input output terminal

dout serial data output terminal DCLK external clock input terminal DIN serial data input terminal busy signal output terminal (low level valid) Cs chip selection

in order to complete a/d conversion, when a touch event occurs on the touch screen, Penirq is low level. The controller detects that penirq is at low level and sends control word to ADS7846 through serial port. When the chip selection CS is at the low level, after DIN inputs the 8-bit mode control word at the rising edge of DCLK, dout gets the output corresponding to the control word at the falling edge of DCLK. Figure 2 shows the timing chart of ADS7846 when it is converted to 16 clock cycles each time

1.3 touch display circuit

the hardware circuit of terminal touch display is shown in Figure 3. The circuit is mainly composed of microcontroller, LCD and LCD controller, touch screen and touch screen controller, memory and power supply circuit. The microprocessor adopts the W78E58 of Winbond company, which is compatible with the industrial standard MCS-51 series. It has 3 16 bit timers, 12 interrupt sources, 2 interrupt priorities, 2 enhanced serial ports, 32KB flash EPROM, programmable watchdog, and dual dptr pointers. Two pieces of 62256 are used as LCD display memory and signature data memory respectively. The LCD touch screen controller adopts the controller described in the previous two sections

the display used in the design is the LCD pda320240a produced by Beijing PENGYUAN company. Pda320240 LCD is a dot matrix display with high precision, which has the advantages of small volume, light weight, flexible display, etc. It has two display modes: text display and graphic display. In order to provide users with a simpler operation interface, the auxiliary function modules of many display screens have been integrated. For pda320240a, only 18 signal lines need to be operated (pin definitions are listed in Table 2) to complete the corresponding display function

pin name function description el1 and EL2 display screen backlight input line x+, y+ is connected to the positive electrode x-, Y- connected to the negative electrode of the touch screen CL1 display data latch pulse signal Cl2 display enable signal input penirq interrupt output terminal d0 ~ D3 display data input line vdd+5v power supply VSS grounding input line VLCD display drive power supply (+18) FLM frame scanning signal input line reset LCD reset disp display screen drive output (grounding)

as can be seen from Figure 3, four analog signals x+, x-, y+, y-, It is converted into quantized digital coordinates through ADS7846, and the coordinates are controlled and displayed on the LCD screen by SED1335 after conversion. After signing, the image information is read through xd0 ~ xd3 on SED1335. The signature information collection memory 2 is firmly integrated into ram62256 (1) around the high end. The serial port on W78E58 can transmit the signature data to the printer through RS-232-C, or wirelessly transmit it to other devices through Bluetooth interface

2 software design

the software implementation of digital signature is to convert the touch coordinates into display coordinates in real time, and display them directly on the screen. Then how can many experimental machine manufacturers choose the experimental machine produced by Jinan Shijin? Give it to the LCD controller for processing, and store the signature and other image information in the displayed format. The practice shows that this method is operable and fast

the process is shown in Figure 4. MCU is always in the state of waiting for touch after power on. Once INT0 is detected as low level, and INT0 is still low level after 10ms delay, touch occurs. Execute the touch control program touch() to obtain the 12 bit precision coordinate touch of the touch position_ XY, calculate the display coordinate sign on the screen_ XY。 Judge whether it is a signature area by coordinates. If yes, the touch point will be displayed on the screen. After the signature is completed, the touch control program will jump out through coordinate judgment, and all information in the signature box will be read out and saved

there are many ways to save data. One is to record the coordinates of the current point and store them in the storage every time an effective touch event occurs; Second, after the signature is completed, scan the signature box displayed on the LCD screen by line, read the data in the signature box, and then store it in the memory. Compare the above two methods. In the first method, each point corresponds to two signature coordinates. For 160 × 50 dot matrix signature box, with a storage capacity of up to 160 when all are blackened × fifty × 2=16000 bytes, so this method is more suitable for the collection of a small amount of data; The second method is relatively flexible. Regardless of the number of signatures, the maximum storage capacity for the same signature box is 160 × 50/8=1000 bytes, so we use this method

3 application example

at present, both China's postal service and article delivery companies store their article delivery confirmation information in the form of paper media, which is very inconvenient for information collection and use. For example, the operation of parcel delivery is that after the postal delivery personnel verify the recipient's certificate, the parcel recipient fills in the corresponding receiving information on the parcel list and signs for it. The signed parcel list is handed over to the post office for storage by the delivery personnel. Now we use the hand-held equipment developed by us for delivery confirmation to replace the original paper signature, in which the signature data is compressed and encoded. Small amount of data and strong confidentiality; The data is stored in the nonvolatile memory and can be recovered in case of power failure; The signature data can be sent to the upper computer through the Bluetooth module on the device, and the data can be restored to the state at the time of signature after being touched

4 conclusion

the digital signature handheld device composed of SED1335 and ADS7846 and controlled by 51 series single chip microcomputer has the advantages of low cost and clear signature effect. In the embedded field, with the continuous emergence of new devices, some microcontrollers (such as Epson Series MCU and Motorola's Longzhu chip) can directly drive the LCD display, which can greatly reduce the design of hardware circuit. (end)

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