The digital scanning probe microscope (scanning probe microscope, SPM) is a research nanometer important tool, it uses the probe and the sample different interaction surveys the surface or the contact surface the physical property which and the chemical property displays in a nanometer criterion, its being published to the superficial science, the physics, microelectronics, the electronic material study, the advanced material and nanomaterials and so on research area technology vital significances, at the same time, the digital signal processing technology already developed quite maturely, the DSP technology also already widely applied in the correspondence, the survey, multimedia, domains and so on expense electronic products, because DSP and SPM unifies in is the inevitable direction which together the SPM instrument develops, itCan cause the SPM performance tends the consummation, for this reason, this article introduced how to use TMS320C5416 to realize this tentative plan!
SPM system plan and flaw
The existing SPM system mainly based on the PCI form, in this plan, the computer maintains the correspondence through the PCI card and the SPM control panel, entire electronic control system’s flow chart as shown in Figure 1.
This system by the scanner, the electronic control panel and the control processing software three parts is composed, the scanner is the operative paragraph, it realizes through the unit motor and the piezoelectricity vitrified pipe’s three dimensional expansion scans the probe to the sample surface scanning; The control processing software is the central control part, may establish the scanning parameter, to scan the process to implement the real-time adjustment and the monitoring through the control software (again line scanning control) as well as carries on the analysis to the scanned picture
Processing (off-line data analysis), the electronic control panel is connects the control software and scanner’s middle part, is shouldering in the instruction which when in the scanning process’s control software will issue solid transformation for to scanner’s concrete operations duty, therefore, electronic control system’s precise degree and to the instruction reaction rate immediate influence image formation’s effect, will be designing the good electronic control system to entire STM very important.
Although based on the PCI form’s system in the general control, the transmission speed and the image formation effect can achieve the essential requirements, but takes the precision instrument, its effect far insufficient, the major object is also as follows:
(1) the PC machine switching power supply is too big to the high accuracy A/D, D/A chip disturbance.
(2) the PCI card each time can only to a group signal addressing, system’s timeliness be low.
(3), because needs the big computing resource expenses and must move a complex gamma correction algorithm, this control panel needs a handling ability strong processor.
(4) the memory and the handshake way is not very ideal.
System design thought
In order to solve the above shortcoming, the author has given one kind based on the DSP new digital SPM system’s design proposal, new plan’s system diagram as shown in Figure 2.
What this plan and Figure 1 obvious difference is, Figure 2 the plan increased a DSP chip on the SPM control panel, the SPM control panel and the computer exchange of information will make corresponding processing first through DSP, then delivers to opposite party again, DSP and computer’s correspondence uses the full-duplex RS-232 serial port mailing address.
DSP control panel’s structure and function
Passes through to the SPM instrument’s control flow, the succession request, the scanning way, the feedback model and timeliness carries on the overall analysis, and to several kind of DSP chip’s performance’s comparison, this design decided that uses TI Corporation’s 54X the series DSP chip, this system’s DSP operation processing speed, the processing precision, the power loss can satisfy the SPM application system’s feedback request.
TMS320VC5416 is TI Corporation’s 16 fixed-point DSP, its clock rate is 160MHz, can realize the high speed operation (160MIPS) and the large capacity memory, on the piece has 128×16 position SRAM and 16K×16 position ROM. The TMS320VC5416 chip essence and the I/O mouth uses 1.5V and the 3.3V power supply separately, therefore may reduce the power loss effectively.
Theoretically, on this DSP piece the SDRAM capacity should be able to satisfy the data storage request, therefore, to reduce system’s complexity, no longer carries on outside the piece the SRAM expansion, this has also limited the system later consummation and the promotion, to propose that the program run speed, the design uses Flash the Bootloader way, namely drops the first procedure to the piece outside in Flash, after DSP electricity, the system will be automatic Flash in the procedure reads to the piece on RAM moves, therefore this design also exterior expands a 256×16 position in DSP Flash. Time design what selects is AMD Corporation’s AM29F800B model Flash, the capacity is 8Mbit, may operate in 128×16bit and 512K×16bit the data storage form, this design uses 512K×16bit, its hardware connection as shown in Figure 3.
DSP and computer’s serial port correspondence uses a piece of asynchronous transceiver and more than a piece of agreement transceiver, the asynchronous transceiver selects TL16C52B, this component’s transmission receive has 64 byte FIFO and the Modem connection signal respectively, and minute A, the B two groups receiving and dispatching, the highest transmission speed may reach the 1.5Mbps baudrate. Uses the 3.3V power source power supply, moreover the connection is simple, may with the DSP direct connection, each channel’s 18 registers be possible to use in controlling the serial asynchronous communication the working and the feedback condition, after uses a0-A2 addressing. The multi-agreement transceiver uses the MAX3160 chip, its asynchronous serial port level may dispose the RS323/RS485/RS422 many kinds of connection level standard, this system chooses RS232, and uses the four-wire system (RXD, TXD, RTS, CTS). Its hardware connection as shown in Figure 4.
Moreover, this system might select piece of CSC Corporation’s CPLD chip (model is CY37032) realizes digital logic operation between each connection (for instance Flash control logic, serial port control logic, on SPM control panel A/D and D/A control logic as well as read-write signal and so on). System’s control logic clear order, moreover uses the VHDL language write program not to be complex, the system debugging uses TI Corporation’s CCS2.2 development environment, this platform and may carry out the code generation tool including the code edition and the debugging, can support the entire flow which designs and develops.
The system electrical power system mainly has ±12V, ±150V, ±15V, ±5V, 3.3V, 1.5V several parts, ±12V to the analogous circuit power supply, ±150V is the piezoelectric ceramics scanning high pressure transports puts electric circuit’s power source, ±15V to step-by-steps the electromechanical power supply, other is the digital circuit power supply, because the entire electric circuit power source type are many, the massive chips simultaneously open and the closure can create in the power source and the grounding voltage and the electric current fluctuate greatly, affect the chip the normal work, therefore, besides between the grounding and the power source the shunt capacitance, increases Pi the filter, also to the analogous circuit and the digital circuit, the high-speed circuit and the low speed electric circuit carry on the district layout, by sharpens system’s antijamming ability as far as possible.
Based on DSP SPM system architecture and function
SPM system before movement, through control software’s on scanning control kneading board, the user may adjust the time-base generator electric circuit’s operational parameter, like the swath range, X displacement, the Y displacement, the sweep speed and so on, then through the reaction control kneading board, the user may real-time adjust Z to the feedback electronics active status, like the proportional gain, the integral increase, feedback loop and bias. Finally establishes the advance, the backlash or the stop through the unit motor control panel, and establishes the step to give the motor driver corresponding drive pulse. Chart
5 show is the SPM control panel’s structure diagrams.
On what the control panel uses is a piece of A/D switch, the model is MAX120, it can accept the peaceful magnitude electric current signal conversion VZin digital signal. MAX120 is the 12bitA/D switch, uses the 5V power supply, the slewing rate may reach 1.6μs, the sampling rate reaches 500KHz, it has five kind of transformation patterns, the entire control model, the independent control pattern, the slow memory pattern, the ROM pattern and transform the pattern continuously, the mode control pin and other pin combinatory logic may choose one of five kind of patterns, what this design choice transforms the pattern continuously, its FIFO read cycle may reach 15ns, therefore may reduce the interrupt latency time, can suit in the system speed request, four piece of D/A switches select piece of 12bit AD565 and three piece of 14bit AD7840, AD565 use in transforming Z by to obtain electric current signal VZout to the control information,Transports again after the high pressure puts actuates the motor to have the displacement. Three piece of AD7840 respectively transforms Vb, Vx and the Vy three direction control informations, AD7840 uses ±5V the double power source power supply, the switching time is 21ns, its internal input latch and DAC latch, but the effective guarantee transformation data does not lose, but AD565 uses ±12V the double power source power supply, the switching time is 30ns.
TMS320VC5416 internal has 16 timers, timer’s output can start 12bitA/D to transform, and may the sampling Z signal (adjustment scanning probe with sample surface distance), the timer interruption service routine which may shield install the XY scanning algorithm and scanner’s gamma correction algorithm, the XY scanning algorithm uses for carries on X to with Y to the scanning displacement to calculate, the gamma correction algorithm makes the corresponding adjustment according to the flying spot on tunneling current’s size, then starts 3 group D/A to transform, to the feedback X, Y, the Z three groups signals carries on the enlargement, affects in the SPM forehead. Because its tunneling current signal only then accepts the peaceful magnitude, not easily by direct measurement, therefore should its enlargement for the corresponding voltage signal, carry on again corresponding processing, to scans between the probe and sample bias voltage Vb, may when the scanned picture causes its size to be constant, because requests the noise is very small. Therefore may use the standard voltage power supply, guarantees the low noise, but when carries on the scanning, Vb no longer is a constant value, but is must the change relations which the control software establishes to change, Vset is the standard voltage which establishes through the control software, it may control Z to the voltage value scope, each time gathers Z carries on the comparison to the voltage value and Vset, its output transports after the proportion puts and the integrator may decide that after D/A switch’s voltage whether to affect in the SPM forehead.
In carries on the debugging and the improvement after the SPM electronic control system and the control software, the author obtained Jin who as shown in Figure 6 Mo the surface image.
Concluding remark
Because DSP is the high speed processor, therefore this design compared to wants complex based on the DSP design proposal many, simultaneously designs when must realize the signal complete question specially the importance, therefore middle the design must control to the impedance, the reflection and the signal terminal carries on the match, and to DSP, A/D, the D/A component carries on the physical isolation, simultaneously must consider that questions and so on crosstalk, power source decoupling, avoid the signal integrity as far as possible to the design performance influence.
The experiment proved that realizes the SPM feedback system design using DSP with to compare based on the PCI system, has the connection to be simple, the stability is good and the precision higher merit, the author will further design from now on also the new DSP algorithm, and strengthens the image characteristic, simultaneously must enhance system’s extension on the hardware, reduces feedback system’s noise, the enhancement operating system’s stability.
