4.0 DEVICE OPERATION
4.1 General Description
For many of today’s microcontroller applications, care
must be taken to prevent low-power conditions that can
cause many different system problems. The most
common causes are brown-out conditions, where the
system supply drops below the operating level
momentarily. The second most common cause is when
a slowly decaying power supply causes the microcon-
troller to begin executing instructions without sufficient
voltage to sustain volatile memory (RAM), thus
producing indeterminate results.
The TC127XA family (TC1270A, TC1270AN, and
TC1271A) are cost-effective voltage supervisor
devices designed to keep a microcontroller in Reset
until the system voltage has reached and stabilized at
the proper level for reliable system operation. These
devices also operate as protection from brown-out
conditions when the system supply voltage drops
below a safe operating level.
A Manual Reset input (MR pin) is provided. This allows
a push button switch to be directly connected to the
TC127XA device, and is suitable for use as a push
button Reset. This allows the system to easily be reset
from the external control of the push button switch. No
external components are required.
The Reset pin (RST or RST) will be forced active, if any
of the following occur:
• During device power up
• VDD goes below the device threshold voltage
• The Manual Reset input (MR) goes low
Figure 4-1 shows a high level block diagram of the
devices. The device can be described with three
functional blocks. These are:
• Voltage Detect circuit
• Manual Reset with Glitch Filter circuit
• Reset Generator circuit
The Reset Generator circuit controls the reset delay
time of the reset output signal.
There are three Reset Delay time options. Depending
on the option, the reset signal (RST/RST pin) will be
held active for a minimum of 1.09 ms, 17.5 ms, or
The TC1271A has an active-high RST output while the
TC1270A and TC1270AN have an active-low RST
The TC1270A and TC1271A have a push-pull output
driver, while the TC1270AN has an open-drain output.
Figure 4-2 shows a typical circuit for a push-pull device
and Figure 4-3 shows a typical circuit for an open-drain
TC127XA High Level Block
The TC1270A and TC1271A devices are available in a
4-Pin SOT-143 package to maintain footprint compati-
bility with the TC1270, TC1271, TCM811, and TCM812
devices, and the SOT-23-5 package. The TC1270AN is
only available in the SOT-23-5 package.
Low supply current makes these devices suitable for
battery powered applications.
Device specific block diagrams are shown in Figure 4-4
through Figure 4-6.
© 2007 Microchip Technology Inc.