MCP9805
DS21977C-page 14 2005-2013 Microchip Technology Inc.
4.1.4 AMBIENT TEMPERATURE
REGISTER (T
A
)
The MCP9805 uses a band gap temperature sensor
circuit to output analog voltage proportional to absolute
temperature. An internal ADC is used to convert the
analog voltage to a digital word. The converter resolu-
tion is set to 0.25 °C/LSb + sign (11-bit data). The digital
word is loaded to a 16-bit read-only Ambient Tempera-
ture register
(T
A
) that contains 11-bit temperature data
in two’s complement format.
The T
A
register bits (bits 12 thru 0) are double-buffered.
Therefore, the user can access the register while, in the
background, the MCP9805 performs an analog-to-dig-
ital conversion of the band gap temperature sensor.
The temperature data from the ADC is loaded in
parallel to T
A
at t
CONV
refresh rate.
The T
A
magnitude in decimal to ambient temperature
conversion is shown in Equation 4-1:
EQUATION 4-1: DECIMAL CODE TO
TEMPERATURE
CONVERSION
In addition, the T
A
register uses three bits (bits 15, 14
and 13) to reflect the Event pin state. This allows the
user to identify the cause of the Event output trigger
(see Section 4.2.3 “Event Output Configuration”);
bit 15 is set to ‘1’ if T
A
is greater than or equal to T
CRIT
,
bit 14 is set to ‘1’ if T
A
is greater than T
UPPER
and bit 13
is set to ‘1’ if T
A
is less than T
LOWER
.
The T
A
register bit assignment and boundary
conditions are described in Register 4-5.
T
A
Code 2
2–
=
Where:
T
A
= Ambient Temperature (°C)
Code = MCP9805 output magnitude in
decimal
REGISTER 4-5: AMBIENT TEMPERATURE REGISTER (T
A
) ADDRESS ‘0000 0101’b
bit 15 T
A
vs. T
CRIT
(1)
bit:
0 =T
A
T
CRIT
1 =T
A
T
CRIT
bit 14 T
A
vs. T
UPPER
(1)
bit:
0 =T
A
T
UPPER
1 =T
A
T
UPPER
bit 13 T
A
vs. T
LOWER
(1)
bit:
0 =T
A
T
LOWER
1 =T
A
T
LOWER
bit 12 SIGN bit:
0 =T
A
0°C
1 =T
A
0°C
Note 1: Not affected by the status of the Event output configuration (bits 5 to 0 of CONFIG) and T
HYST
= 0°C,
Register 4-3.
2: Bit 1 may remain set ‘1’ for some devices indicating 2
-3
°C/LSb or 0.125°C temperature resolution,
depending on the state of the device calibration code.
Upper-Half:
R-0 R-0 R-0 R-0 R-0 R-0 R-0 R-0
T
A
Vs. T
CRIT
T
A
Vs. T
UPPER
T
A
Vs. T
LOWER
SIGN 2
7
°C/LSb 2
6
°C/LSb 2
5
°C/LSb 2
4
°C/LSb
bit 15 bit 8
Lower-Half:
R-0 R-0 R-0 R-0 R-0 R-0 U-0 U-0
2
3
°C/LSb 2
2
°C/LSb 2
1
°C/LSb 2
0
°C/LSb 2
-1
°C/LSb 2
-2
°C/LSb —
(2)
—
bit 7 bit 0
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = bit is set ‘0’ = bit is cleared x = bit is unknown
