MCP9808
5.2
SENSOR FEATURE DESCRIPTION
VDD
5.2.1
SHUTDOWN MODE
MCP9808
Shutdown mode disables all power consuming
activities (including temperature sampling operations)
while leaving the serial interface active. This mode is
selected by setting bit 8 of CONFIG to ‘1’. In this mode,
the device consumes ISHDN. It remains in this mode
until bit 8 is cleared to ‘0’ to enable Continuous
Conversion mode or until power is recycled.
RPU
Alert Output
The Shutdown bit (bit 8) cannot be set to ‘1’ while the
CONFIG<7:6> bits (Lock bits) are set to ‘1’. However, it
can be cleared to ‘0’ or returned to Continuous
Conversion mode while locked.
FIGURE 5-9:
Configuration.
Active-Low Alert Output
The status of the Alert output can be read using
CONFIG<4> (Alert Output Status bit). This bit can not
be set to ‘1’ in Shutdown mode.
In Shutdown mode, all registers can be read or written.
However, the serial bus activity increases the shutdown
current. In addition, if the device is in shutdown while
the Alert pin is asserted, the device will retain the active
state during shutdown. This increases the shutdown
current due to the additional Alert output current.
Bits 7 and 6 of the CONFIG register can be used to lock
the TUPPER, TLOWER and TCRIT registers. These bits
prevent false triggers at the Alert output due to an
accidental rewrite to these registers.
The Alert output can also be used as a critical temper-
ature output using bit 2 of CONFIG (Alert Output Select
bit). When this feature is selected, the Alert output
becomes a comparator output. In this mode, the
interrupt output configuration (Alert Output Mode bit,
CONFIG<0>) is ignored.
5.2.2
TEMPERATURE HYSTERESIS
(THYST
)
A hysteresis of 0°C, +1.5°C, +3°C or +6°C can be
selected for the TUPPER, TLOWER and TCRIT temperate
boundaries, using bits 10 and 9 of CONFIG. The
hysteresis applies for decreasing temperature only (hot
to cold) or as temperature drifts below the specified
limit.
5.2.3.1
Comparator Mode
Comparator mode is selected using bit 0 of CONFIG. In
this mode, the Alert output is asserted as active-high or
active-low, using bit 1 of CONFIG. Figure 5-10 shows
the conditions that toggle the Alert output.
The Hysteresis bits can not be changed if either of the
Lock bits (CONFIG<7:6) are set to ‘1’.
The TUPPER, TLOWER and TCRIT boundary conditions
are described graphically in Figure 5-10.
If the device enters Shutdown mode with asserted Alert
output, the output remains asserted during Shutdown
mode. The device must be operating in Continuous
5.2.3
ALERT OUTPUT CONFIGURATION
Conversion mode for tCONV. The TA vs. TUPPER
,
The Alert output can be enabled by using bit 3 of the
CONFIG register (Alert Output Control bit) and can be
configured as either a comparator output or as an
Interrupt Output mode using bit 0 of CONFIG (Alert
Output Mode bit). The polarity can also be specified as
active-high or active-low using bit 1 of CONFIG (Alert
Polarity bit). This is an open-drain output and requires
a pull-up resistor.
TLOWER and TCRIT boundary conditions need to be
satisfied in order for the Alert output to deassert.
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.
When the ambient temperature increases above the
critical temperature limit, the Alert output is forced to a
comparator output (regardless of CONFIG<0>). When
the temperature drifts below the critical temperature
limit minus hysteresis, the Alert output automatically
returns to the state specified by CONFIG<0> bit.
DS25095A-page 30
© 2011 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
