11

5364A–BIOM–09/05

AT77C102B

Figure 3-6.

Functional Description

3.2

Sensor

Each pixel is a sensor in itself. The sensor detects a temperature difference between the begin-

ning of an acquisition and the reading of the information: this is the integration time. The

integration time begins with a reset of the pixel to a predefined initial state. Note that the integra-

tion time reset has nothing to do with the reset of the digital section.

Then, at a rate depending on the sensitivity of the pyroelectric layer, on the temperature varia-

tion between the reset and the end of the integration time, and for the duration of the integration

time, electrical charges are generated at the pixel level.

3.3

Analog-to-digital Converter/ Reconstructing an 8-bit Fingerprint Image

An analog-to-digital converter (ADC) is used to convert the analog signal coming from the pixel

into digital data that can be used by a processor.

As the data rate for the parallel port and the USB is in the range of 1 MB per second, and at least

a rate of 500 frames per second is needed to reconstruct the image with a fair sweeping speed

of the finger, two 4-bit ADCs have been used to output two pixels at a time on one byte.

3.4

Start Sequence

A reset is not necessary between each frame acquisition.

The start sequence must consist in:

1.

Setting the RST pin to high.

2.

Setting the RST pin to low.

3.

Sending 4 clock pulses (due to pipe-line).

4.

Sending clock pulses to skip the first frame.

Note that after a reset it is recommended to skip the first 200 slices to stabilize the acquisition.

Figure 3-7.

Start Sequence

8

latches

Chip

temperature

sensor

Column selection

Line sel

Odd

Even

8 lines of 280 columns of pixels

4-bit

ADC

ADC

8

1 dummy column

4

4

Amp

De0-3

Do0-3

1

2

3

4

4-bit

1

4

3

1

2

1

1124

Clock PCLK

Reset RST

4 + 1124 clock pulses to skip the first frame