Datasheet

Table 3. WOE Thresholds
WOE
3–1
(hex) V
WOE
(mV)
0h n/a
1h 3.840
2h 1.920
3h 1.280
4h 0.960
5h 0.768
6h 0.640
7h* 0.549
* Default value after POR.
Temperature
The bq2018 has an internal temperature sensor which is
used to set the value in the temperature register
(TMP/CLR) and set the self-discharge count rate value.
The register reports the temperature in 8 steps of 10°C
from <0°C to >60°C as Table 4 specifies. The bq2018 tem-
perature sensor has typical accuracy of
±
2°Cat 25°C.
See the TMP/CLR register description for more details.
Clear Register
The host system is responsible for register maintenance.
To facilitate this maintenance, the bq2018 has a Clear
Register (TMP/CLR) designed to reset the specific coun-
ter or register pair to zero. The host system clears a reg
-
ister by writing the corresponding register bit to 1. When
the bq2018 completes the reset, the corresponding bit in
the TMP/CLR register is automatically reset to 0, which
saves the host an extra write/read cycle. Clearing the
DTC register clears the STD bit and sets the DTC count
rate to the default value of 1 count per 0.8789s. Clearing
the CTC register clears the STC bit and sets the CTC
count rate to the default value of 1 count per 0.8789s.
Calibration Mode
The system can enable bq2018 V
OS
calibration by setting
the calibration bit in the MODE/WOE register (Bit 6) to
1. The bq2018 then enters calibration mode when the
HDQ line is low for greater than 10 seconds and when
the signal between SR1 and SR2 is below V
WOE
. Cau-
tion: Take care to ensure that no low-level exter-
nal signal is present between SR1 and SR2 because
this affects the calibration value that the bq2018
calculates.
If HDQ remains low for one hour and |V
SR
|<V
WOE
for
the entire time, the measured V
OS
is latched into the
OFR register, and the calibration bit is reset to zero, indi
-
cating to the system that the calibration cycle is com
-
plete. Once calibration is complete, the bq2018 enters a
6
bq2018
TD201801.eps
Break
Written by Host to bq2018
LSB
0 1 2 3 4 5 6 7
MSB
110
01110
CMDR = 73h
73h = 0 1 1 1 0 0 1 1
MSB
LSB
Received by Host from bq2018
LSB
0
1 2 3 4 5 6
7
Data (OFR) = 65h
MSB
1
010011
0
65h = 0 1 1 0 0 1 0 1
MSB LSB
Figure 4. Typical Communication with the bq2018
Temp Value (hex) SDR Count Rate
<0° 0h
×
1/8
0–10° 1h
×
1/4
10–20° 2h
×
1/2
20–30° 3h 1 count/hr.
30–40° 4h
×
2
40–50° 5h
×
4
50–60° 6h
×
8
>60° 7h
×
16
Table 4. Temperature Steps