Structural characteristics of sensor chip MBF200

With the development of network technology and communication technology, the traditional identity authentication technology is becoming more and more incapable of information security. Due to the uniqueness and stability of human physical characteristics, people began to turn their attention to biometric technologies, such as fingerprints, iris, DNA, palm recognition, etc. Among many feature recognitions, fingerprint collection devices are favored by people due to their small size, low power consumption, good stability, and low probability of misjudgment. The fingerprint sensor is the core device in the fingerprint collection device. Here, we choose MBF200 from Fujitsu. It is an advanced solid-state fingerprint sensor chip with high performance, low power consumption, low cost, and convenient interface. This makes it easier to implement data transmission in embedded systems. S3C2410 is an ARM920T core microprocessor introduced by Samsung. It is mainly targeted at handheld devices and high cost-effective, low-power applications. It integrates rich components on the chip, which is conducive to its expansion.

Introduction to the structure and characteristics of MBF200

MBF200 is a high performance, low power capacitive fingerprint sensor designed for embedded systems. The chip has a sensing area of 1.28 cm × 1.50 Cm, a 256 × 300 sensor array and a resolution of 500 dpi, an integrated 8-bit A / D converter, an operating voltage of 3.3 to 5 V, and automatic fingerprint detection. Provides three kinds of bus interfaces: 8-bit microprocessor bus interface (MCU), serial peripheral device interface (SPI), and integrated USB full-speed interface. In view of the simple interface, easy implementation and less I / O, this article uses the SPI interface method.

Among them, a 256 × 300 sensor array is used to generate an induced voltage; a function register is used to control the chip; a control circuit is used to control the sensor and an external interface circuit, and is responsible for reading and writing data; the address index register and the data register are respectively It is used to select the address of the function register and read and write data; the sample and hold and A / D conversion circuit is used to sample the voltage generated by the sensor array. In addition, a multi-frequency oscillation circuit is used to provide a clock signal for the chip.

MBF200 has a total of 19 registers. The read / write operation to the chip is actually an operation on the register, so the function of the chip can be learned by understanding the function of the register. Because this article uses the serial peripheral device interface (SPI) slave operation mode, only some important registers used here are explained.

(1) CTRLA (control register A)

Write this register to start image conversion. Read this register to get A / D conversion result. The three bits GETSUB, GETIMG and GETROW select a corresponding image access mode, and initialize the conversion sequence of the A / D converter. The ANISEL bit determines the source of the A / D conversion. Set the GETSUB bit to 1 to start capturing the rectangular graphics area determined by RAH, RAL, CAL, REH, REL, and CEL. Set the GETIMG bit to 1 to start capturing the entire image regardless of the row and column registers. Set the GETROW bit to 1 to start capturing the row unit data determined by RAH, RAL. Writing a "1" to any one of GETSUB, GETIMG, and GETROW will abandon the image acquisition of the current corresponding mode and restart the image acquisition of the new mode, and the three bits can only have at most 1 bit at a time Is set to 1. If more than one of these 3 bits is set to 1, the conversion of the image will not start. When GETIMG is set to 1, MBF200 will complete the following series of actions:

① The row address is set to O;

② The column address is set to O;

③ Automatically start line acquisition of images;

④ The A / D conversion of the first pixel starts automatically.

When "O" is written to CTRLA, it will not cause MBF200 to give up the current image except to clear ANSEL. Read the status of CTRLA to get the result of A / D conversion.

(2) CTRLB (control register B)

In SPI mode, AFDEN is used in conjunction with ENABLE to wake up the microprocessor and start the A / D conversion enable by generating an interrupt, which can save power. The AUTOINCEN bit in the control register CTRLB is used to set whether to start automatically increasing the value of the row and column address registers. When this bit is set to 1, if the A / D conversion value of a certain pixel is read, the column address is automatically increased, and the A / D conversion of the next pixel is automatically started. Once the gray value of the last pixel of a row is read, the row address is automatically increased, and the A / D conversion of the first pixel of the row is automatically started.

(3) ICR interrupt control register

This register mainly controls the two interrupt sources of the fingerprint sensor. Interrupt request O corresponds to the finger detection interrupt. Interrupt request 1 corresponds to the external interrupt of pin EXTINT.

Hardware interface design

2.1 SPI interface characteristics of MBF200

In the SPI slave mode, the data transmission formats supported by the sensor are SPImode (O, 0) and SPI mode (1,1). MBF200 timing requirements are as follows:

① During data transmission, SCS remains low;

② The data on the MOSI line is sampled on the rising edge of SCK;

③ SCK can be high level or low level in idle state;

④ The data on the MISO line changes at the falling edge of SCK;

⑤ The highest position is removed first;

⑥ The instruction format is 8-bit instruction code plus 8-bit address code and 8-bit data.

2.2 S3C2410 SPI Interface Features

The S3C2410 has two serial peripheral device interfaces (SPI). Each SPI interface has two 8-bit shift registers for sending and receiving. In SPI communication, data is transmitted (serial shifted out) and received (serial shifted in) at the same time. The transmission rate of 8-bit serial data is determined by the relevant control register.

SPI interface characteristics: compatible with SPI interface protocol v2.11; 8-bit shift register for transmission; 8-bit shift register for reception; 8-bit prescaler logic; query, interrupt and DMA transfer modes.

The S3C2410's SPI is configured as the master device, and the interface design for MBF200 is completed. The hardware connection is shown in Figure 3. SPIMOSI0 is used as the output of the master device, SPIMISOO is used as the input of the master device, and SPICLK0 is used as the serial clock for SPI communication. And S3C2410 supports 4 different transmission formats, which can ensure the consistency of the timing of the master and slave devices.

S3C2410 SPI interface operation: Through SPI interface, S3C2410 can send and receive 8-bit data with MBF, 200 at the same time. The serial clock line is synchronized with the two data lines for shifting and data sampling. The data transmission rate is controlled by the SPPREn register, and the baud rate is adjusted by modifying the prescaler value. When the GPGO output is low, write byte data to the SPDATn register, and SPI transmit and receive operations are initiated simultaneously.

Software design

The software design mainly implements the S3C2410 to control the MBF200 type fingerprint sensor for operation. It is written in the ARM integrated development environment ADS1.2 using the highly portable C language. After the target board is powered on, the entire acquisition system is initialized, including the initialization of the ARM system and the MBF200, and parameter configuration. After the initialization is completed, it starts to detect whether there is a finger on the fingerprint sensor. If there is an interruption, the fingerprint image is collected and saved in the interrupt service routine.

3.1 S3C2410 system initialization

The initial task of any system startup is the initialization of the hardware. Prepare some basic hardware environments for the execution of subsequent applications, including watchdog and interrupt settings, CPU speed and clock frequency configuration, and related memory settings. , And the settings of registers related to SPI communication. Part of the code is as follows:

3.2 MBF200 initialization

MBF200 has a total of 19 registers. Except for SRA, CIDH and CIDL, they must be initialized before fingerprint collection.

3.3 Fingerprint acquisition program design

The fingerprint acquisition starts by detecting the interruption of the finger, and writes 0x02 to the CTRLA register to start the fingerprint image data acquisition of the entire area; after the line capture time and A / D conversion time, the CTRLA register is read, which is the 8-digit number of the current pixel Value. Each time the value of the current pixel is read, the A / D conversion of the next pixel is automatically performed, and the data of a fingerprint image can be obtained by reading the value of CTRLA to the specified space.

Because the structure and acquisition principle of MBF200 and fingerprint sensor FPS200 are similar, according to the algorithm of line capture time and A / D conversion time of FPS200, the line capture time TROW and A / D conversion time TA / D of MBF200 can be estimated. The calculation method is as follows:

Conclusion

With the increasing attention to information security, the automatic fingerprint identification system will have a wider application space. In this paper, the front-end of the fingerprint identification system, the fingerprint acquisition module, combines the characteristics of MBF200 itself, and proposes an SPI communication method with S3C2410. This method is simple in structure, convenient to use, and easy to embed.

Тег: MBF200

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