Application of PIC16C65 single chip microcomputer

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Application of PIC16C65 single chip microcomputer in scanning tunneling microscope

Abstract: This paper briefly expounds the basic composition and scanning imaging principle of scanning tunneling microscope, which is expected to have an annual compound growth rate of 7.2% from 2014 to 2020, and introduces the composition, interface circuit and some interface subroutines of a scanning tunneling microscope electronic system with PIC16C65 single chip microcomputer as microprocessor

key words: scanning tunneling microscope; PIC16C65; MAX195; AD1866

Application of the PIC16C65 Single Chip to Scanning Tun nel Microscope

YUAN Fang1, YANGSheng2

(stitute of Information Engineering,Shanghai Maritime University, Shanghai, 200135,China;

iversity of Science and Technology of China,Hefei,230026,China)

Abstract:The paper summarises the mainly parts of the scanni ng tunnel microscope and the principle of the scanning imagings, And int hydraulic universal experimental machine produces the interface circuit and the subprogram of the PIC16C65 with the other parts

keywords:scanning tunnel microscope; PIC16C65; MAX195; AD1 866


scanning probe microscope (STM) is the most important tool in the development of nanotechnology. Based on the tunnel penetration theory in quantum mechanics, his core is a needle tip with an atomic diameter that can scan on the surface of a metal sample and has a certain bias voltage with the sample. When the distance between the sample and the tip is very close, electrons can be transferred from the tip to the sample or from the sample to the tip through the tunneling effect, thus forming a tunneling current. The magnitude of the tunnel current has a negative exponential relationship with their spacing [1]:

I ∝ B ∝ exp (-ks) (1)

where: I is the tunnel current between the sample and the tip of the needle; B is the coefficient related to the bias between the sample and the needle tip; K is the coefficient related to the mass of free electrons and the effective average barrier height; S is the tunnel current between the sample and the needle tip

The basic structure of scanning tunneling microscope can be divided into three parts: head system, electronics system and computer system. The scanner in the head system is a key component. The quality of the scanner determines the control accuracy of the distance between the tip and the sample, which determines the quality of the STM image. At present, tubular PZT piezoelectric ceramic materials are generally used. The scanner can make nano scale precise movement in the X, y and Z directions. The XY scanning voltage generator generates scanning waveforms such as triangular waves and controls the scanner to scan the samples line by line. The tip of the needle is fixed on the scanner and moves with the scanner. Under the action of the bias voltage between the tip and the sample, when the tip and the sample are close enough, tunnel current will be generated. The sensitive current amplifier detects the tunnel current, converts it into voltage, and compares it with the current set point. The comparison result reflects the deviation between the distance between the tip and the sample and the set value. Generally, a proportional integral controller is introduced into the STM electronic system as a feedback circuit to adjust the movement of the scanner in the Z direction to keep the tunnel current constant. At this time, the distance between the needle tip and the sample in the Z axis direction reflects the fluctuation of the surface height of the sample. This is the constant current mode of STM [2]

at present, there are many domestic manufacturers of scanning tunneling microscope. The internal electronics part is basically composed of DSP chip as the core and external high-precision a/d or d/a and other peripheral devices. Due to the complexity of DSP chip structure and function, it is not convenient to use and learn. The following introduces an electronic system of scanning tunneling microscope with PIC16C65 mid-range MCU produced by microchip company as the core and 16 bit a/d converter MAX195 and d/a converter ad1866 devices attached to the periphery

2 interface between PIC16C65 single chip microcomputer and MAX195

pic series single chip microcomputer produced by microchip company has Harvard dual bus structure, adopts quasi reduced instruction set and instruction pipeline structure, and runs very fast, which can completely replace DSP to work [3]. MAX195 is a 16 bit serial output a/d converter produced by Maxim company. It has high speed, low power consumption and self calibration ability. MAX195 is mainly used for collecting tunnel current. The connection between MAX195 and PIC16C65 and the peripheral connection of MAX195 are shown in Figure 1

in Figure 1, PIC16C65 single chip microcomputer is connected with MAX195 and mc14555 through 7 pins of general i/o port D port. The a/d conversion process is controlled by the program of PIC16C65. This program adopts asynchronous reading mode, that is, read the data after the completion of 16 bit data conversion. At the beginning of the conversion, first set/conv as low level to start the conversion. After the conversion starts, the MCU constantly checks whether the level of/eo C pin becomes low. If it becomes low, it indicates that the conversion has ended. The MCU constructs 16 serial output pulses through rd5 port and sends them to MAX195. The 16 bit data obtained after conversion is sent to PIC MCU bit by bit through RD4 port for reading and processing<1. The subroutine of selecting torque loading level/p>

max195 for a/d conversion is as follows:

3 interface between PIC16C65 and ad1866

ad1866 is a dual channel 16 bit d/a converter with good performance produced by ad company. Each channel is composed of 1 reference voltage, 1 16 bit input register, 1 16 bit input latch, 1 16 bit DAC and 1 output amplifier. The overall level of ad186 is still at the level of foreign countries in the early 1990s. 6 the working process is also controlled by the program of PIC single chip microcomputer. Firstly, PIC constructs the output pulse through the ra2 and RB3 ports, and sends the 16 bit data requiring d/a conversion from high to low into the data register of ad1866, then constructs the falling edge pulse on the LL and LR pins, sends the 16 bit data in the data register into the DAC for d/a conversion, and updates the output of the DAC accordingly. Its interface circuit is shown in Figure 2

some of its interface subroutines are as follows:

its working process is as follows: first, energize the stepping motor, let the motor run continuously, gradually approach the sample downward, and the probe with small diameter is installed in front of the stepping motor to move with the motor. When the probe is close to the sample, the stepping motor changes its operation mode from continuous operation to inching operation. At the same time, bias the probe until the preset tunnel current can be detected, and then compare the distance in the z-axis direction of the piezoelectric ceramic with the preset value to obtain the surface height of the point. In this way, the surface shape of all samples can be scanned, and then the data can be sent to the computer. Through certain image processing, the surface profile of the sample can be obtained

4 conclusion

the electronic system of the scanning tunneling microscope with PIC16C65 as the core can clearly display the appearance of the metal sample surface through the interface with PC and the display and image processing program. This scanning tunneling microscope has the characteristics of high cost performance and clear image display. It has a wide application prospect in precision measurement, micro drive and material research


[1] Zhang Lide Development and Prospect of nano metrology [J] Modern scientific instruments, 1998, (21)

[2] Atherton P D. nanometer precision mechanisms [J] me measurement+control 1998, 31 (3)

[3] Yu Guangyun Pic Series MCU development and application technology [M] Beijing: Electronic Industry Press, 2000 (end)

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