Datasheet

TCN75A
DS21935D-page 24  2010 Microchip Technology Inc.
5.3.4.1 Shutdown Mode
The Shutdown mode disables all power-consuming
activities (including temperature sampling operations)
while leaving the serial interface active. The device
consumes 2 µA (maximum) in this mode. It remains in
this mode until the Configuration register is updated to
enable continuous conversion or until power is
recycled.
In Shutdown mode, the CONFIG, T
A
, T
SET
and T
HYST
registers can be read or written to; however, the serial
bus activity will increase the shutdown current.
5.3.4.2 One-Shot Mode
The TCN75A can also be used in a One-shot mode that
can be selected using bit 7 of the CONFIG register. The
One-shot mode performs a single temperature
measurement and returns to Shutdown mode. This
mode is especially useful for low-power applications
where temperature is measured upon command from a
controller. For example, a 9-bit T
A
in One-shot mode
consumes 200 µA (typical) for 30 ms and 0.1 µA
(typical) during shutdown.
To access this feature, the device needs to initially be
in Shutdown mode. This is done by sending a byte to
the CONFIG register with bit 0 set <1> and bit 7 cleared
<0>. Once the device is in Shutdown mode, the
CONFIG register needs to be written to again, with bit
0 and bit 7 set <1>. This begins the single conversion
cycle of t
CONV
, 30ms for 9-bit data. Once the
conversion is completed, T
A
is updated and bit 7 of the
CONFIG register becomes cleared <0> by the
TCN75A.
TABLE 5-2: SHUTDOWN AND ONE-SHOT
MODE DESCRIPTION
5.3.4.3 ALERT Output Configuration
The ALERT output can be configured as either a
comparator output or as Interrupt Output mode using
bit 1 of the CONFIG register. The polarity can also be
specified as an active-high or active-low using bit 2 of
the CONFIG register. The following sections describe
each output mode, while Figure 5-7 gives a graphical
description.
5.3.4.4 Comparator Mode
In Comparator mode, the ALERT output is asserted
when T
A
is greater than T
SET
. The pin remains active
until T
A
is lower than T
HYST
. The Comparator mode is
useful for thermostat-type applications, such as turning
on a cooling fan or triggering a system shutdown when
the temperature exceeds a safe operating range.
In Comparator mode, if the device enters the Shutdown
mode with asserted ALERT output, the output remains
active during shutdown. The device must be operating
in continuous conversion, with T
A
below T
HYST
, for the
ALERT output to be deasserted.
5.3.4.5 Interrupt Mode
In Interrupt mode, the ALERT output is asserted when
T
A
is greater than T
SET
. However, the output is deas-
serted when the user performs a read from any
register. This mode is designed for interrupt-driven,
microcontroller-based systems. The microcontroller
receiving the interrupt will have to acknowledge the
interrupt by reading any register from the TCN75A.
This will clear the interrupt and the ALERT pin will
become deasserted. When T
A
drifts below T
HYST
, the
TCN75A outputs another interrupt and the controller
needs to read a register to deassert the ALERT output.
Shutting down the device will also reset, or deassert,
the ALERT output.
Operational Mode
One-Shot
(Bit 7)
Shutdown
(Bit 0)
Continuous Conversion 00
Shutdown 01
Continuous Conversion
(One-shot is ignored)
10
One-shot (Note 1) 11
Note 1: The shutdown command <01> needs to
be programmed before sending a
one-shot command <11>.
T
SET
T
HYST
ALERT
ALERT
Comparator mode
Interrupt mode
Active-low
Active-low
T
A
Register
Read
* See Section 5.3.4.5 “Interrupt Mode”
*