In each living thing recognition technology, the fingerprint recognition technology is maturest, accurate and easiest to use. But fingerprint gathering takes in a fingerprint recognition system’s important link also more and more to be valued people’s, the high grade fingerprint gathering technology has become an important research subject. Fingerprint image’s gathering is the automatic fingerprint recognition system (AFIS-Automation Fingerprint Identification System) the important component, gathers fingerprint image quality quality, immediate influence to the following fingerprint imagery processing process. The high grade fingerprint image may simplify the fingerprint imagery processing greatly the algorithm, enhances the recognition rate, reduces resists knowing rate.
Along with the new semiconductor fingerprint gathering sensing component and the DSP, CPLD technology’s development, the automatic fingerprint recognition technology is turning toward the miniaturization and the embedded direction develops. This article introduces is based on the DSP fingerprint gathering system.
This system’s principle
In this system’s fingerprint sensor uses the FPS200 solid state fingerprint chip, its image formation principle as shown in Figure 1. May see by Figure 1, a picture element lights has a metal electrode, the finger skin is another electrode, between both has formed electric capacity CP. On the finger skin’s keel (ridge) and the valley (valley) will produce different CP. Because CP is very small, is unable the direct measurement, therefore uses the following methods: By fixed time to the CP charge (SW1 closes first, and SA2 opens), then by fixed time to the CP electric discharge (SW1 opens, and the SW2 pass), emits the electrical energy will dump to Cc. In each charge electric discharge cycle, because the charging voltage is the same, the duration of charging is the same, therefore the different CP value will cause CP to save the different electrical energy. These electrical energies when the CP electric discharge will dump to Cc in causes the Cc voltage the markup. After therefore the CP value different will cause a charge electric discharge cycle to end, the Cc voltage markup value different (is proportional), finally will cause the Cc voltage to advance the charge electric discharge cycle number of times which needs to the reference voltage differently (to be in reverse proportion), like this might survey CP through the charge electric discharge cycle number of times.
Figure 1 FPS200 fingerprint image formation principle
The sensor array including 256 row ×300 line of sensor electrodes, each row has two sampling - maintains the electric circuit relates with it. Gathers when a fingerprint image gains a line of data, but this process altogether has two stages. The first stage, in the sensor electrode the line which chooses is charged in advance the VDD level, an internal signal permits the first sampling - maintains the electric circuit set preserves a pre-charge line of level. The second stage, the good sensor electrode by certain electric current electric discharge, each unit’s discharge ratio becomes the proportion with “the discharging current”. After period of time (i.e. “discharge time”), an internal signal permits the second sampling - - maintains the electric circuit set preservation final electrode’s level. After the pre-charge and between the electric discharge electrode level’s difference lies in the sensor electric capacity. After a line of gain had ended, might carry on A/D to line each unit to transform. The chip sensitivity may controls through the adjustment discharge time and the discharging current; The current supply reference value decided by the external connection between ISET and the place resistance that it (DCR) controls by the discharging current register; The discharge time (DTR) controls by the discharge time register.
When in the fingerprint bulge part puts on the sensing electric capacity picture element electrode, the electric capacity can increase, increases the electric capacity through the examination to be able to carry on fingerprint gathering. In sensor’s picture element size for 45μm2, is separated for 50μm, the picture element array’s resolution slightly is higher than 500dpi, based on one kind of standard sole polycrystalline silicon three metal 0.5μm CMOS craft.
Is in the fingerprint under the bulge picture element (electric capacity high) to discharge slowly, but is in the fingerprint under the depression picture element (electric capacity low) to discharge quickly. This kind of different discharge ratio may maintain through the sampling (S/H) the electric circuit examines and transforms a 8 digital quantity output, this examination method has the high sensitivity to the fingerprint bulge and the indentation, and may form the very good primitive fingerprint image. The fingerprint image carries on turn by a line of gathering, each metal electrode as electric capacity’s one extremely, with it contact’s finger is electric capacity’s another extremely. Has a deactivated level in the component surface, takes dielectric sheets between the electric capacity two electrode’s. Puts on the sensor when the finger, in the fingerprint bulge and will indent will have the different capacitance value in the array, and will constitute a view picture image which will use in authenticating.
System hardware design
This system’s work mainly constitutes by the below part: Fingerprint image gathering part, procedure and data storage part, overall situation logical control part as well as data communication part.
Fingerprint image gathering part: The system using the software inquiry way judges whether to carry on the fingerprint gathering. When carries on fingerprint gathering, the fingerprint sensing chip and the analog picture will transform the digital image according to the hypothesis parameter gathering fingerprint, then under the DSP control the data storage in the exterior data space, waited for that carries on next step processing.
Figure 2 system hardware functional block diagram
Procedure and data storage part: The temporary data space which this part by SRAM and DSP the internal DARAM constitution, SRAM uses in depositing the fingerprint image and providing when program run needs.
Overall situation logical control part: This part completes by CPLD, realizes the following three aspect function:①Carries on the time sharing addressing to the DSP data space;②Produces in the system each chip to select patches or strips of land as worth saving for seed the signal;③Produces in the system each chip read-write signal.
Data communication part: This part has designed the serial port correspondence as well as USB corresponds two kind of patterns, may work alone. What the serial port correspondence uses is TI Corporation’s asynchronous serial receiving and dispatching chip TL16C550, coordinates piece of MAX232 then to realize the computer and the target system data correspondence; The USB correspondence part used Nanjing to seep permanent electronic company’s USB correspondence chip CH375.
1 TMS320VC5402 chip
TMS320VC5402 has the optimized CPU structure, the interior has 1 40 arithmetic logic unit (including a 40 barrel type shift register and two independent 40 accumulators), a 17×17 multiplier and 40 special-purpose accumulators, 16K×16bit RAM space and 4K×16bit ROM space; The altogether 20 address wires, the addressable 64K×16bit data area and 1M×16bit the procedure area, has 64K the I/O space; The processing speed is 100MIPS, the speed is high, the power loss is low.
2 memory electric circuit’s design
Usually a DSP system besides the DSP chip, but also needs the exterior memory. The exterior memory has two kinds generally, namely the stored routine and fixed data’s EPROM and may read-write fast RAM. This system uses the memory is CY7C1041, is section of 256K×16bit static RAM. Considered from the connection way that exterior memory divides into the serial port memory and the mouth memory 2 kinds. In the DSP system, because requests to exchange the data high speed, generally uses and the mouth memory.
Takes the exterior program memory’s piece route selection by DSP procedure space selection signal PS, read-write line R/W takes the exterior memory’s read-write pilot wire, memory gating signal MSTRB takes the exterior program memory to enable the line, address wire A0~A17 to take the address wire addressing 256K×16 procedure storage space. In DSP with in the exterior data-carrier storage’s connection, read-write line R/W takes the exterior memory’s read-write pilot wire, takes the exterior data-carrier storage by data space selection signal DS to select patches or strips of land as worth saving for seed the signal, the memory chooses line of flux MSTRB to take the exterior data-carrier storage to enable the line, because C54XX the series DSP address wire only then A0~A15 can take the exterior data-carrier storage and the I/0 mouth addressing, therefore wants the addressing space to surpass 64KB, must expand the address wire.
3 DSP and FPS200 connection designs
This system TMS320VC5402 and the fingerprint sensor FPS200 connection realizes uses the microprocessor connection pattern, its connection form is simple. What needs to explain, in this chip, the address selection and the data reads in is divides two steps completes, sets first through A0 0 writes the address reference register, then sets again to A0 1 comes the read-write correspondence address the data register.
The fingerprint sensor chooses its function register through the table of contents address table. In the chip has 8 bit data lines (D[7:0]) and an address selection line (A0). This address wire uses for to choose the table of contents register and the data register. If A0 is low, then selects the table of contents address; If A0 is high, selects the data register through the table of contents address, but in the table of contents register the data maintains the original value, until is rewritten or the chip replacement.
The chip has four control input pin: RD, WR, CS0, CS1. If causes CS0 is low, and CS1 is high, then selects the chip, the data is locked the existence writes (WR) the rise edge.
System software design
The software realizes including two major parts; first, fingerprint data acquisition control section; second, fingerprint data communication transmission part.
The fingerprint data acquisition control section is uses the C language to realize, but the fingerprint data transmission uses the C/C language to realize, afterward transfers TMS320VC5402 the C compiler to translate it the assembly language, then delivers the TMS320VC5402 assembler to carry on the assembly again.
Figure 3 under CCS environment procedure development flow
This system’s fingerprint image gathering work is controls the FPS200 fingerprint sensor by DSP to carry on, based on under DSP integrated development environment CCS2.0 software design flow chart as shown in Figure 3.
This system’s master routine flow chart as shown in Figure 4.
Figure 4 master routine flow chart
Conclusion
This system the DSP technology introduction fingerprint gathering domain, these components compared to the traditional monolithic integrated circuit, the discrete component not only in the reliability, the processing speed, the stability greatly enhances, moreover causes the design the board card volume to reduce greatly, the power loss reduces greatly, entered the civil domain for the fingerprint technology to create the advantage.
