SS8017TR SSC [Silicon Standard Corp.], SS8017TR Datasheet - Page 9

• Manufacturer Part Number: SS8017TR
• Description: Two Remote Temperature Sensors with SMBus Serial Interface and System Reset
• Manufacturer: SSC [Silicon Standard Corp.]

Rev.2.01 6/06/2003
Ensuring a Valid Reset Output Down to V
When V
put no longer sinks current-it becomes an open cir-
cuit. Therefore, high-impedance CMOS logic inputs
connected to RESET can drift to undetermined volt-
ages.
This presents no problem in most applications, since
most µP and other circuitry is inoperative with V
below 1V. However, in applications where RESET
must be valid down to 0V, adding a pull-down resistor
to RESET causes any stray leakage currents to flow
to ground, holding RESET low (Figure 3). R1's value
is not critical; 100k
Interfacing to µPs with Bi-directional Reset Pins
A µP with bi-directional reset pins (such as the Mo-
torola 68HC11 series) can connect to the SS8017
reset
pull it low, indeterminate logic levels may result. To
correct this, connect a 4.7k
SS8017 RESET output and the µP reset I/O (Figure
4). Buffer the SS8017 RESET output to other sys-
tem components.
Benefits of Highly Accurate Reset Threshold
Most µP supervisor Ics have reset threshold voltages
between 5% and 10% below the value of nominal
supply voltages. This ensures a reset will not occur
within 5% of the nominal supply, but will occur when
the supply is 10% below nominal.
When using ICs rated at only the n ominal supply
±5% this leaves a zone of uncertainty where the sup-
ply is between 5% and 10% low, and where the reset
may or may not be asserted.
Fig 3. RESET Valid to V
RESET and small enough to pull RESET to ground.
RESET output is asserted high and the µP wants to
SS8017
output.
CC
GND
GND
V
V
falls below 1V, the SS8017 RESET out-
RESET
RESET
CC
CC
If,
for
is large enough not to load
CC
example,
= Ground Circuit
R1
R1
100k
100k
resistor b etween the
www.SiliconStandard.com
the
CC
SS8017
= 0V
CC
The SS8017 uses highly accurate circuitry to en-
sure that reset is asserted close to the 5% limit,
and long before the supply has declined to 10%
below nominal.
SMBus Digital Interface
From a software perspective, the SS8017 appears as
a set of byte-wide registers that contain temperature
data, alarm threshold values, fan speed data, or con-
trol bits, A standard SMBus 2-wire serial interface is
used to read temperature data and write control bits
and alarm threshold data. Each A/D and fan control
channel within the device responds to the same
SMBus slave address for normal reads and writes.
The SS8017 employs four standard SMBus protocols:
Write Byte, Read Byte, Send Byte, and Receive
Byte (Figure 5). The shorter Receive Byte protocol
allows quicker transfers, provided that the correct
data register was previously selected by a Read Byte
instruction. Use caution with the shorter protocols in
multi-master systems, since a second master could
over-write the command byte without informing the
first master.
The temperature data format is 7bits plus sign in
twos-complement form for each channel, with each
data bit representing 1°C (Table3), transmitted MSB
first. Measurements are offset by +1/2°C to minimize
internal rounding errors; for example, +99.6°C is re-
ported as +100°C.
Fig 4. Interfacing to µPs with Bi-directional Reset
I/O
SS8017
GND
GND
V
V
CC
CC
RESET
RESET
4.7k
4.7k
RESET
RESET
BUFFER
BUFFER
GND
GND
µP
µP
V
V
CC
CC
BUFFERED RESET
BUFFERED RESET
TO OTHER SYSTEM
TO OTHER SYSTEM
COMPONENTS
COMPONENTS
SS8017
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