RE46C800
DS20005172B-page 10 2013 Microchip Technology Inc.
3.6 LED Driver
The LED drive circuit provides power to an LED, which
can be used as a visual indicator by the system. The
LED drive circuit can also be used as part of a battery
check function in battery-powered applications. When
LEDEN is asserted high the LED will load the V
BST
output and the microcontroller can monitor the battery
operation under load. In low-voltage systems the boost
regulator will be placed into high-boost operation when
LEDEN is asserted high. The load current is set by the
resistor in series with the LED.
3.7 Interconnect Operation
The IO circuitry provides the means for the CO detector
to be connected to other CO detectors or smoke
alarms. Tab le 3 -2 below provides the truth table for the
interconnect circuit operation. IO1 is a bidirectional pin
that connects to other CO detectors or smoke alarms.
IO2 is a bidirectional pin that connects to the
microcontroller. IODIR connects to the microcontroller
and determines when IO1 and IO2 act as an input or
output. When IO1 is used as an output asserting a logic
high, the IO1 output acts as current source that is
biased from V
BST
. In low-voltage applications where
the boost regulator is enabled, the boost regulator will
operate in High-Boost mode. When IO1 is used as an
output asserting a logic low, the IO1 output acts as
current sink. IO2 logic levels are referenced to V
REG
.
TABLE 3-2: INTERCONNECT LOGIC
TRUTH TABLE
IODIR
IO2 IO1
Input Output Input Output
10 —— 0
11 —— 1
0 — 00—
0 — 11—
