Rainbow Electronics DS1615 - Manual (Page 16)

DS1615
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A second security feature lies in the fact that once the sample rate has been selected by writing to the
Sample Rate register, it cannot be changed to another value without resetting the recorded temperature
data. This prevents gathering many data samples at a fast sample rate and then lowering the sample rate
to give the appearance that the data was recorded over a longer period of time. The Sample Rate register
can only be written to a new value if the MEM CLR bit is set to one.
A third security feature lies in the two integrated sample counters - the Current Samples Counter and the
Total Samples Counter. These two counters can be used to guarantee that the DS1615 data has not been
cleared at any time during a given period of time. The Current Samples Counter counts the number of
samples that have occurred since the most recent data acquisition operation was started (i.e., the number
of samples since the Sample Rate register was written to a non-zero value). The Total Samples Counter
counts the total number of samples that have been recorded in the life of the device (assuming the lithium
energy source has not been removed during that time). If the end user knows the value in the Total
Samples Counter before the data acquisition operation is started, he can guarantee that the DS1615 has
not been cleared. If the Current Samples count equals the difference between the ending value and
beginning value of the Total Samples Counter, then the DS1615 data has not been cleared during that
time frame.
As a fourth security measure, changing any value in the RTC and Control registers (with the exception of
the Status register) will stop datalogging and clear the Mission-in-Progress (MIP) bit.
SERIAL INTERFACE
The DS1615 provides two different serial communications options; asynchronous and synchronous. Both
communications options will transmit the data LSb first, MSb last.
The mode of communication is selected via the COMSEL pin. When this pin is pulled high, the DS1615
operates in synchronous mode. In this mode, communication with the DS1615 is facilitated by the
SCLK, I/O, and
RST pins. When COMSEL is pulled low or floated, asynchronous communications is
selected and communication with the device occurs over the TX and RX pins. The operation of each
mode is discussed in further detail below.
Asynchronous Communication
In asynchronous mode, the DS1615 operates as a slave peripheral device which is read and written over a
half duplex asynchronous data interface at the fixed rate of 9,600 bits per second. Data is received and
transmitted in 8-bit bytes using a standard asynchronous serial communications format as shown in
Figure 3. This format is easily generated by the UART in most systems. The DS1615 data format
implements 10 bit words including one start bit, eight data bits, and one stop bit. Data is received by the
DS1615 on the RX pin and transmitted by the TX pin.
COMMUNICATION WORD FORMAT Figure 3