Philips Semiconductors

Product data

MAX809/MAX810

3-pin microprocessor resets

2003 Aug 08

7

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.

The MAX809/MAX810 are relatively immune to short negative-going

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

20

µs or less will not cause a reset signal. To provide additional

transient rejection, connect a 10 nF bypass capacitor as close as

SL01728

0

50

100

150

200

250

300

350

400

450

1

10

100

1000

MAX809/

MAX810R/S/T/Z

MAX809/MAX810J/L/M

Figure 5. Maximum transient duration without causing a

reset pulse versus reset comparator overdrive.

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 k

Ω 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 k

Ω pull-up resistor is recommended

GND

GND

RESET

RESET

SL01726

MAX809

MICROPROCESSOR

100 k

Ω

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 k

Ω

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.

GND

GND

SL01727

MAX809

MOTOROLA

68HCxx

4.7 k

Ω

RESET

RESET

Figure 7. Interfacing to microprocessor with bi-directional

reset I/O