Specifications

PC/II+p Board Technical Reference Manual 26

5.4 Voltage Monitor

A MicroMonitor is included on the board, the Dallas Semiconductor DS1706S, to provide both a watchdog

function and a dual-voltage monitor function. This section discusses the voltage monitor function; refer to

section 7.29

for a description of the watchdog function.

+3.3V Monitor

The +3.3V power supply is monitored by the DS1706S. When the +3.3V power supply falls below the

minimum Vcc tolerance (2.85v to 3.00v), the RST# output of the part is pulled low for a minimum of 130ms.

The RST# output is tied to the PC/II+p POWERGOOD signal that signals a reset to the board. The DS1706S

part's power supply voltage detection tolerance (2.85v to 3.00v) provides a minimum of 10% variation in the

+3.3V power supply, and a maximum of 15% variation in the +3.3V power supply. Megatel generates +3.3V

from the upstream supply (+5V) regulated to within 5% of +3.3V, which meets the requirements of

components on the board.

+5V Monitor

The upstream +5V power supply is monitored as well by the DS1706S. When the IN pin falls below the

minimum IN trip point (Vtp) tolerance (1.20v to 1.30v), the NMI# output of the part is pulled low. Because The

NMI# output of the part is tied to the RBRST# input of the part, the part pulls the RST# output of the part low

for a minimum of 130ms. The RST# output is tied to the PC/II+p POWERGOOD signal that signals a reset to

the board. A 1% precision voltage divider allows sense of the +5V supply using the IN pin of the part, and is

set to allow the +5V supply to drop to a minimum of 4.53V before tripping. The trip range, allowing for

tolerances, is 4.05V to 4.53V. Megatel recommends that the board should be operated with +5.0V on

the board, and in no case should the +5V supply be allowed to drop below 5% of its nominal value.

Other voltage planes on the board are regulated locally from the +5V supply rail.

Table 6 Electrical – Voltage Monitor Characteristics

Parameter Symbol Min Typical Max Units

DS1706 VCCM VCCM 3.15 3.3 3.45 V

VCCM Trip Point DS1706 (+3.3v

monitor)

VCCTP 2.85 2.93 3.00 V

IN - VCC5 Input Trip Point (+5v

monitor)

VTP 4.05 4.29 4.53 V

Reset Active Time TRST 130 205 285 ms

VCC Detect to RST and RST# TRFP 130 204 285 ms

PBRST# Stable Low to RST

and RST#

TDLY 250 ns

VIN Detect to NMI# TIPD 5 8

µs

Over Recommended Operating Conditions