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71M6541D/F/G and 71M6542F/G Data Sheet

Rev 5

2.5.5

71M654x Temperature Sensor

The 71M654x includes an on-chip temperature sensor for determining the temperature of its bandgap

reference. The primary use of the temperature data is to determine the magnitude of compensation

required to offset the thermal drift in the system for the compensation of current, voltage and energy

measurement and the RTC. See 4.7 Metrology Temperature Compensation on page 97. Also see 2.5.4.4

RTC Temperature Compensation on page 53.

Unlike earlier generation Maxim SoCs, the 71M654x does not use the ADC to read the temperature

sensor. Instead, it uses a technique that is operational in SLP and LCD mode, as well as BRN and MSN

modes. This means that the temperature sensor can be used to compensate for the frequency variation

of the crystal, even in SLP mode while the MPU is halted. See 2.5.4.4 RTC Temperature Compensation

on page 53.

In MSN and BRN modes, the temperature sensor is awakened on command from the MPU by setting the

TEMP_START (I/O RAM 0x28B4[6]) control bit. The MPU must wait for the TEMP_START bit to clear before

reading STEMP[10:0] and before setting the TEMP_START bit once again. In SLP and LCD modes, it is

awakened at a regular rate set by TEMP_PER[2:0] (I/O RAM 0x28A0[2:0]).

The result of the temperature measurement can be read from the two I/O RAM locations STEMP[10:3]

(I/O RAM 0x2881) and STEMP[2:0] (I/O RAM 0x2882[7:5]). Note that both of these I/O RAM locations must

be read and properly combined to form the STEMP[10:0] 11-bit value (see STEMP in Table 46). The

resulting 11-bit value is in 2’s complement form and ranges from -1024 to +1023 (decimal). The equations

below are used to calculate the sensed temperature from the 11-bit STEMP[10:0] reading.

The equations below are used to calculate the sensed temperature. The first equation applies when the

71M654x is in MSN mode and TEMP_PWR = 1. The second equation applies when the 71M654x is in

BRN mode, and in this case, the TEMP_PWR and TEMP_BSEL bits must both be set to the same value, so

that the battery that supplies the temperature sensor is also the battery that is measured and reported in

BSENSE. Thus, the second equation requires reading STEMP and BSENSE. In the second equation,

BSENSE (the sensed battery voltage) is used to obtain a more accurate temperature reading when the IC

is in BRN mode.

For the 71M654x in MSN Mode (with TEMP_PWR = 1):

325

)

(

⋅

STEMP

Temp

For the 71M654x in BRN Mode, (with TEMP_PWR=TEMP_BSEL):

609

00218

325

)

(

⋅

−

⋅

BSENSE

STEMP

Temp

Table 46 shows the I/O RAM registers used for temperature and battery measurement.

If TEMP_PWR selects VBAT_RTC when the battery is nearly discharged, the temperature

measurement may not finish. In this case, firmware may complete the measurement by selecting

V3P3D (TEMP_PWR = 1).

Table 46: I/O RAM Registers for Temperature and Battery Measurement

Name

Location

Rst

Dir

Description

TBYTE_BUSY

28A0[3]

Indicates that hardware is still writing the 0x28A0

byte. Additional writes to this byte are locked out

while it is one. Write duration could be as long as 6 ms.

TEMP_PER[2:0]

28A0[2:0]

–

R/W

Sets the period between temperature measurements.

Automatic measurements can be enabled in any

mode (MSN, BRN, LCD, or SLP).

TEMP_PER

Time

Manual updates (see TEMP_START)

1-6

2 ^ (3+TEMP_PER) (seconds)

Continuous

71M6542F/G Datasheet(PDF) 56 Page - Maxim Integrated Products

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