Rainbow Electronics DS2422 - Manual (Page 22)

DS2422
22 of 48
SERIAL DATA INTERFACE TUNING
The serial interface consists of several signals that are intended to control external circuitry, such as an analog-to-
digital converter (see Figure 9A). There is one signal, called CNVST, which can be used to load data into a shift
register or to trigger a data conversion. The delay t
SP
from the activation of the serial interface (PUMP_ONZ) to
CNVST is user-programmable through the Delay Register. When used with a charge pump such as the MAX619,
the variable delay t
SP
is used to give the charge pump adequate time to stabilize before a conversion starts. If no
charge pump is used, the delay may be set to 00h to begin the conversion sooner.
Delay Register
ADDR b7 b6 b5 b4 b3 b2 b1 b0
0400h delay value
During a mission, there is only read access to this register.
The Delay Register holds the preset value of a counter that determines the duration of t
SP
. The number format is
unsigned integer with values ranging from 0 to FFh (0 to 255 decimal). This is equivalent to a range from 0 to
127.5ms. The power-on value of this register is 08h.
TEMPERATURE CONVERTER TRIM
The DS2422 leaves the factory fully tested, but not trimmed for temperature accuracy. The actual trim values
consist of two sets, Temperature Counter Reset and Temperature Conversion Length, which need to be
determined individually for each device during a 2-point calibration step. These trim values need to be written to the
respective registers in the Trim Register Page before the device meets the accuracy specification shown in the
graphs at the end of this document.
Temperature Counter Reset Register
ADDR b7 b6 b5 b4 b3 b2 b1 b0
0404h Temperature Counter Reset Low Byte
0405h 0 0 0 Temperature Counter Reset High Byte
There is always full read/write access to this register. Bits 5-7 of the High Byte are always 0 and cannot be written
to 1. The power-on default is 6Bh (0404h) and 11h (0405h).
The Temperature Counter Reset value provides a purely vertical shift along the Temperature Transfer Curve in
order to reset the zero point. The algorithm to determine the correct Temperature Counter Reset value is included
in the application note that describes the 2-point calibration trim.
Temperature Conversion Length Register
ADDR b7 b6 b5 b4 b3 b2 b1 b0
0406h Temperature Conversion Length Low Byte
0407h 0 0 0 Temp Conversion Length High Byte
There is always full read/write access to this register. Bits 5-7 of the High Byte are always 0 and cannot be written
to 1. The power-on default is A6h (0406h) and 12h (0407h).
The Temperature Conversion Length value provides a vertical and horizontal shift of the Temperature Transfer
Curve. The algorithm to determine the correct Temperature Counter Reset value is included in the application note
that describes the 2-point calibration trim.
DATALOG MEMORY USAGE
Once setup for a mission, the DS2422 logs the temperature measurements and/or external data at equidistant time
points entry after entry in its datalog memory. The datalog memory is able to store 8192 entries in 8-bit format or
4096 entries in 16-bit format (Figure 10A). If temperature as well as external data is logged, both in the same
format, the memory is split into two equal sections that can store 4096 8-bit entries or 2048 16-bit entries (Figure
10B). If the device is set up to log data in different formats, e. g., temperature in 8-bit and external data in 16-bit
format, the memory is split into blocks of different size, accommodating 2560 entries for either data source (Figure
10C). In this case, the upper 256 bytes are not used. In 16-bit format, the higher 8 bits of an entry are stored at the