MAX809 View Datasheet(PDF) - Philips Electronics
Part Name
Description
Manufacturer
MAX809 Datasheet PDF : 12 Pages
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Philips Semiconductors
3-pin microprocessor resets
Product data
MAX809/MAX810
APPLICATION INFORMATION
Detailed description
A microprocessor reset ensures that the microprocessor starts in a
known and safe state. The reset asserts a reset signal to prevent
code execution errors during power up, power down, or brownout
conditions.
A reset must assert an output within a predictable range of the
supply voltage. The common threshold voltage range is between 5%
and 10% of the nominal supply voltage. MAX809/MAX810 have
4.63 V, 3.08 V and 2.63 V options for 5 V, 3.3 V and 3.0 V supplies
respectively. They have high accuracy (within ±3%) that ensures
that the reset thresholds occur within the safe operating range.
Whenever the supply voltage falls below the reset threshold, the
reset signal is asserted. It remains asserted for at least 140 ms after
the supply voltage rises above the threshold. At this point the reset
is released. This delay time helps ensure valid reset signals despite
erratic changes in supply voltage. The MAX809/MAX810 have a
push-pull output stage and do not require a pull-up resistor.
Negative-going VCC transients
The MAX809/MAX810 are relatively immune to short negative-going
VCC transients and glitches. Figure 5 shows the maximum pulse
width a negative-going transient can have without causing a reset
signal. As the magnitude of the transient increases below the reset
threshold, the maximum allowable pulse width decreases. Typically,
for the 4.0 V, 4.38 V, and 4.63 V versions of the MAX809/MAX810, a
VCC transient that goes 100 mV below the reset threshold and lasts
20 µs or less will not cause a reset signal. To provide additional
transient rejection, connect a 10 nF bypass capacitor as close as
possible to the VCC pin.
450
400
350
300
250
200
MAX809/MAX810J/L/M
150
100
MAX809/
50 MAX810R/S/T/Z
0
1
10
100
1000
RESET COMPARATOR OVERDRIVE, Vth – VCC (mV)
SL01728
Figure 5. Maximum transient duration without causing a
reset pulse versus reset comparator overdrive.
Ensuring a valid reset output down to VCC = 0 V
When VCC falls below 1 V, the MAX809 RESET no longer sinks
current (i.e., it becomes open circuit). A high impedance CMOS
logic input connected to RESET can drift to undetermined voltages.
In most applications in which the microprocessor circuitry is
inoperative below 1 V, this will not represent a problem. However, in
applications in which RESET must be valid down to 0 V, use a
relatively large resistor from RESET to ground as shown in Figure 6.
100 kΩ is small enough to provide a path for any leakage currents to
flow to ground (holding RESET LOW); while it is large enough not to
load RESET. Conversely, a 100 kΩ pull-up resistor is recommended
for MAX810 if RESET is required to remain valid for VCC < 1 V.
VCC
VCC
VCC
RESET
MAX809
RESET
100 kΩ
MICROPROCESSOR
GND
GND
SL01726
Figure 6. RESET valid to VCC = 0 V circuit.
Interfacing to microprocessors with bi-directional
reset pins
Microprocessors with bi-directional reset I/Os, such as the Motorola
68HC11 series, can be connected to the MAX809 RESET output. To
ensure a correct output on the MAX809, even when the
microprocessor reset I/O is in the opposite state, connect a 4.7 kΩ
resistor between the reset pins as shown in Figure 7. This allows the
microprocessor to issue commands to the system regardless of the
state of the RESET. The bi-directional microprocessor reset
functions both as a driven reset input and as an active reset driver.
VCC
VCC
RESET
4.7 kΩ
MAX809
GND
VCC
RESET
MOTOROLA
68HCxx
GND
SL01727
Figure 7. Interfacing to microprocessor with bi-directional
reset I/O
2003 Aug 08
7
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