Rainbow Electronics AT77C104B - Manual (Page 25)

25
AT77C104B
5347B–BIOM–08/04
Figure 14. Fast SPI Communication
Communication Speed The acquisition speed of the pixels is linked to the clock’s communication speed. The
faster the communication clock, the faster the authorized maximum finger sweeping
speed. The sensor supports fast communications up to 16 Mbps.
Reading of Frame A frame consists of 232 true columns and 1 dummy column of 8 pixels of 4 bits each. A
frame starts with a dummy column.
Figure 15. Example of a Frame
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.
Bit2 Bit3
MSB Bit1 Bit0 Bit3 Bit2 Bit1 Bit0 Bit2 Bit1 Bit0 Bit3 Bit2 Bit1 Bit0 Bit3
MSB
SCK (Pixel Clock)
MISO
Transmission Clock
Edge (Sensor)
Reception Clock
Edge (Host)
Pixel 2i Pixel 2i - 1 Pixel 2i + 2 Pixel 2i + 1
p1
p2
p3
p4
p5
p6
p7
p8
0
F
0
F
2
0
0
0
Pixel Frame
P9
p10
p11
P12
P13
P14
P15
p16
Synchro = F0F00200
Dummy
Column
232 x 8 Pixels Column