Datasheet

• Watching the serial line for any activity during the boot sequence reveals that the
serial line is not involved in the authentication process. This leaves the Serial
Pheripheral Interface (SPI) as a promising candidate.
• The Adesto Technologies datasheet for the AT45DB011 DataFlash chip [20] is an in-
teresting read, as it provides several hints of what mechanisms can be employed
in the authentication process — without adding additional hardware components
to the circuitry.
• Hooking up a logic analyzer such as the Saleae Logic16 [15] will quickly reveal the
communication taking place on the SPI lines during the console boot sequence.
• Hooking up a BusPirate [18] and having a chat with the AT45DB011 DataFlash
chip found in the original data loggers aid in extracting interesting data from the
DataFlash chips.
3.3.1 An introduction to the Serial Pheripheral Interface (SPI)
The Serial Pheripheral Interface is used throughout the Davis consoles for communi-
cation between different subsystems. The SPI lines are also utilized to authenticate an
attached data logger during the console boot sequence. Another use is to actually write
data to the memory chip in the data logger.
A basic SPI system consists of a master unit and one or more slave units. At least
three wires are required for a basic SPI system consisting of only one master and one
slave. For systems consisting of one master unit and multiple slave units, four wires
are required. The SPI signal names are as follows:
MOSI Master Out, Slave In — data carrying line.
MISO Master In, Slave Out — data carrying line.
CLK Clock pulse — provided by the master unit.
SS Slave Select (also known as CS — Chip Select) — used to ”address” one par-
ticular slave unit.
Readers unfamiliar with the SPI can find numerous informative articles online. A
good starting point is the Wikipedia article [23] on the Serial Pheripheral Interface. Note
that SPI signals are routed ”straight through” — MOSI from the master should be
routed to MOSI on the slave, MISO from the slave should be routed directly to MISO
on the master, etc.
In the case of Davis consoles, the ATmega128L MCU takes the role as master, whereas
the AT45DB011 DataFlash chip steps into the slave role. That said, the fact that Davis re-
sorts to fairly standard communication routines for chip-to-chip communication inside
their consoles only aids the process of determining the exact nature of the data logger
authentication process.
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