3054
MULTIPLEXED
TWO-WIRE
HALL-EFFECT SENSOR ICs
ofthe address cycle. This response may be
used as an indication that the sensor is "alive
and well" on the bus and is called the
diag-
nostic
response. If the sensor detects an
ambient magnetic field, it continues with I
S
during the low portion of the address cycle.
This response from the sensor is called the
signal
response. When the next positive
(address) transition is detected, the sensor
becomes disabled, and its contribution to the
bus signal current returns to I
Q
.
Bus Current
Figure 1 shows the addressing protocol.
The top trace represents the bus voltage
transitions as controlled by the bus driver
(see Applications Notes for an optimal bus
driver schematic). The second trace repre-
sents the bus current contribution of Sensor
02. The
diagnostic
response from the sensor
indicates that it detected its address on the
bus. However, no
signal
current is shown,
which indicates that sufficient magnetic field
is not detected at the chip surface and that
pin 3 is open circuited. The third trace
represents the current drain of Sensor 03
when a magnetic field is detected. Note both
the
diagnostic
and
signal
currents from the
sensor. The last trace represents the overall
bus current drain. When no sensors are
addressed, the net bus current is the sum of
quiescent currents of all sensors on the bus
(for 'n' sensors, the bus current drain is
n • I
Q
).
Bus Issues
After a reset, while at the address LOW
voltage (VL), and before the first address
pulse, bus current calibration may be per-
formed. This feature allows for fail-safe
detection of signal current and eliminates
detection problems caused by low signal
current (I
S
), the operation of sensors at
various ambient temperatures, lot-to-lot
variation of quiescent current, and the
addition or replacement of sensors to the bus
while in the field. At present, a maximum of
30 active sensors can coexist on the same
bus, each with a different address. Address
TYPICAL DEVICE QUIESCENT CURRENT
2.0
IN
Q
mA
QUIESCENT CURRENT, I
1.5
1.0
T
A
= +25°C
0.5
0
0
3
6
9
12
15
SUPPLY VOLTAGE, V
IN
BUS
VOLTS
Dwg. GH-045
FIGURE 2
SENSOR CONNECTIONS
POSITIVE BUS SUPPLY
X
X
1
2
3
1
2
3
NC
SWITCH
BUS RETURN
Dwg. EH-004
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