Uploading the biometric detection sketch
With the sensor now trained to recognize your fingerprint, we'll upload a sketch that will
lock out the CryptoCape until you present your fingerprint. Specifically, it will prevent ac-
cess to the chips on the CryptoCape from the BBB. We'll accomplish this by jamming the
SCL line on the I2C bus.
One weakness of I2C is that one misaligned device can disrupt the entire bus. On the
CryptoCape, every IC is connected to the same I2C bus. We'll exploit this for this project.
The ATmega will hold the SCL low until a successful fingerprint is received. This essen-
tially jams the bus since the master, the BBB, can't generate the start condition.
The software running on the BBB will hang until the ATmega releases the lock. Typically,
this is a very undesirable effect. For this project, however, it illustrates how two micropro-
cessors can interact and even interfere with one another. In the following screenshot, you
can see the effect of this jamming of Salee Logic Analyzer software. Once the SCL is re-
leased, the BBB is finally able to send a start condition.