The first dummy column, at the beginning of the pixel array, is added to the sensor to act
as a specific easy-to-detect pattern, and represents the start of the frame tag.
The pixel array is always read in the following order: the first byte, following the 4 bytes
of the dummy column, which contains the value of the pixels physically located on the
upper left corner of the array, when looking at the die with bond pads to the right. Then
another 4 bytes are read that contain the value of the pixels located in the same column
from top to bottom. The next column on the right is output, and so on, until the last line
on the right, close to the bond pads, is output.
Even values are first sent during the data serialization for SPI transfer. Therefore, the
synchronization sequence on the chip’s MISO output is F0F00200.
Figure 16. Reading of Frame
SCK
MISO
F
0
F
0
0
2
0
0
P2
P1
P4
P3
P6
P5
P8
P7
P10 P9
Dummy Column
Second Pixel
Column
First Pixel
Column
Notes: 1. For the first array or frame reading, 40 dummy clock cycles must be sent before the first data arrives. This is necessary for
the initialization of the chip pipeline. Consequently, the first synchronization sequences appear after 40 clock cycles. For the
following array readings, data arrives at each clock cycle. One should implement a synchronization routine in the protocol to
look for the F0F00200 pattern.
2. The Most Significant Bit (MSB) is sent first.
Reading of Entire Image The FingerChip delivers fingerprint slices or frames with a height of 0.4 mm and a width
of 11.6 mm (this equals 8 × 232 pixels). Pixels are sampled/read sequentially and are
synchronous with SCK. Raw slices are captured by the acquisition system and over-
lapped with the corresponding X or Y finger displacement computed by Atmel
reconstruction software. This reconstruction software supports a sweeping speed from 2
to 20 cm/s.
The table below shows finger speeds according to the different clock frequencies. The
reconstruction results are obtained after acquisition of all slices.
Table 17. Finger Speeds Versus Clock Frequencies
Absolute
Fsck
(MHz)
Data Rate
(Mbit/s)
Slice Rate
(Slices/s)
MaximumFinger
Speed (cm/s)
Comments
Too slow
1
2
4
6
1
2
4
6
134
268
536
804
3
6
Too slow
12
18
Minimum
Normal speed
28
AT77C105A [Preliminary]
5419A–BIOM–01/05
ATMEL [ ATMEL ]
