Rainbow Electronics AT77C101B FingerChip™ - Manual (Page 16)

16
AT77C101B
2150A–BIOM–02/02
Thermometer The dummy bytes DB3 and DB4 contain some internal and temperature information.
The even nibble nnnn in DB3 can be used to measure an increase (or decrease) of the
chip temperature, using the difference between two measures of the same physical
device. The following table gives values in Kelvin.
For code 0 and 15, the absolute value is a minimum (saturation).
When the image contrast becomes low because of a low temperature difference
between the finger and the sensor, it is recommended to use the temperature stabiliza-
tion circuitry to increase the temperature of two codes (i.e. from 8 to 10), to get at least
an increase >1.4 Kelvin of the sensor. This enables to recover enough contrast to get a
proper fingerprint for recognition purpose.
nnnn
Decimal
nnnn
Binary
Temperature differential with code 8
in Kelvin
15 1111 11.2
14 1110 8.4
13 1101 7
12 1100 5.6
11 1011 4.2
10 1010 2.8
9 1001 1.4
8 1000 0
7 0111 -1.4
6 0110 -2.8
5 0101 -4.2
4 0100 -5.6
3 0011 -7
2 0010 -8.4
1 0001 -11.2
0 0000 < -16.8