TCN75A
5.3.4.1
Shutdown Mode
5.3.4.3
ALERT Output Configuration
The Shutdown mode disables all power-consuming
activities (including temperature sampling operations)
while leaving the serial interface active. The device
consumes 2 μA (max.) in this mode. It remains in this
mode until the Configuration register is updated to
enable continuous conversion or until power is
recycled.
The ALERT output can be configured as either a
comparator output or as Interrupt Output mode using
bit 1 of CONFIG. The polarity can also be specified as
an active-high or active-low using bit 2 of CONFIG.
The following sections describe each output mode,
while Figure 5-6 gives a graphical description.
5.3.4.4
Comparator Mode
In Shutdown mode, the CONFIG, TA, TSET and THYST
registers can be read or written to; however, the serial
bus activity will increase the shutdown current.
In Comparator mode, the ALERT output is asserted
when TA is greater than TSET. The pin remains active
until TA is lower than THYST. 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.
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
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 TA below THYST, for the
ALERT output to be deasserted.
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 TA in One-shot mode
consumes 200 μA (typ.) for 30 ms and 0.1 μA (typ.)
during shutdown.
5.3.4.5
Interrupt Mode
In Interrupt mode, the ALERT output is asserted when TA
is greater than TSET. However, the output is deasserted
when the user performs a read from any register. This
mode is designed for interrupt-driven, microcontroller-
based systems. The microcontroller receiving the inter-
rupt 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 TA
drifts below THYST, 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.
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 tCONV, 30ms for 9-bit data. Once the
conversion is completed, TA is updated and bit 7 of
CONFIG becomes cleared <0> by the TCN75A.
TABLE 5-6:
SHUTDOWN AND ONE-SHOT
MODE DESCRIPTION
One-Shot Shutdown
Operational Mode
(Bit 7)
(Bit 0)
TSET
Continuous Conversion
Shutdown
0
0
1
0
1
0
TA
THYST
Continuous Conversion
(One-shot is ignored)
One-shot (Note 1)
1
1
ALERT
Comparator mode
Active-low
Note 1: The shutdown command <01> needs to
be programmed before sending a one-
shot command <11>.
ALERT
Interrupt mode
Active-low
Register
Read
*
* See Section 5.3.4.5 “Interrupt Mode”
FIGURE 5-6:
Alert Output.
© 2006 Microchip Technology Inc.
DS21935C-page 21
MICROCHIP [ MICROCHIP ]
