Table 1. Temperature Data Format (Local and Remote Temperature High Bytes)

Table 2. Decimal Fraction Temperature Data Format (Local and Remote Temperature Low Bytes)

8.3.1.1 Standard Binary to Decimal Temperature Data Calculation Example

High-byte conversion (for example, 0111 0011):

Convert the right-justified binary high byte to hexadecimal.

From hexadecimal, multiply the first number by 16⁰ = 1 and the second number by 16¹ = 16.

The sum equals the decimal equivalent.

0111 0011b → 73h → (3 × 16⁰) + (7 × 16¹) = 115.

Low-byte conversion (for example, 0111 0000):

To convert the left-justified binary low-byte to decimal, use bits 7 through 4 and ignore bits 3 through 0 because they do not affect the value of the number.

0111b → (0 × 1 / 2)¹ + (1 × 1 / 2)² + (1 × 1 / 2)³ + (1 × 1 / 2)⁴ = 0.4375.

8.3.1.2 Standard Decimal to Binary Temperature Data Calculation Example

For positive temperatures (for example, 20°C):

(20°C) / (1°C per count) = 20 → 14h → 0001 0100.

Convert the number to binary code with 8-bit, right-justified format, and MSB = 0 to denote a positive sign.

20°C is stored as 0001 0100 → 14h.

For negative temperatures (for example, –20°C):

(|–20|) / (1°C per count) = 20 → 14h → 0001 0100.

Generate the twos complement of a negative number by complementing the absolute value binary number and adding 1.

–20°C is stored as 1110 1100 → ECh.

8.5.1.1 Bus Overview

The TMP461 device is SMBus-interface-compatible. In SMBus protocol, the device that initiates the transfer is called a master, and the devices controlled by the master are slaves. The bus must be controlled by a master device that generates the serial clock (SCL), controls the bus access, and generates the start and stop conditions.

To address a specific device, a start condition is initiated. A start condition is indicated by pulling the data line (SDA) from a high-to-low logic level when SCL is high. All slaves on the bus shift in the slave address byte, with the last bit indicating whether a read or write operation is intended. During the ninth clock pulse, the slave being addressed responds to the master by generating an acknowledge bit and pulling SDA low.

Data transfer is then initiated and sent over eight clock pulses followed by an acknowledge bit. During data transfer, SDA must remain stable when SCL is high. A change in SDA when SCL is high is interpreted as a control signal.

After all data are transferred, the master generates a stop condition. A stop condition is indicated by pulling SDA from low to high when SCL is high.

8.5.1.2 Bus Definitions

The TMP461 device is two-wire- and SMBus-compatible. Figure 15 and Figure 16 illustrate the timing for various operations on the TMP461 device. The bus definitions are as follows:

1. Slave address 1001100 is shown.

Figure 15. Two-Wire Timing Diagram for Write Word Format

1. Slave address 1001100 is shown.

2. The master must leave SDA high to terminate a single-byte read operation.

Figure 16. Two-Wire Timing Diagram for Single-Byte Read Format

8.5.1.3 Serial Bus Address

To communicate with the TMP461 device, the master must first address slave devices using a slave address byte. The slave address byte consists of seven address bits and a direction bit indicating the intent of executing a read or write operation. The TMP461 allows up to nine devices to be connected to the SMBus, depending on the A0, A1 pin connections as described in Table 3. The A0 and A1 address pins must be isolated from noisy or high-frequency signals traces in order to avoid false address settings when these pins are set to a float state.

8.5.1.4 Read and Write Operations

Accessing a particular register on the TMP461 device is accomplished by writing the appropriate value to the pointer register. The value for the pointer register is the first byte transferred after the slave address byte with the R/W bit low. Every write operation to the TMP461 device requires a value for the pointer register (see Figure 15).

When reading from the TMP461 device, the last value stored in the pointer register by a write operation is used to determine which register is read by a read operation. To change which register is read for a read operation, a new value must be written to the pointer register. This transaction is accomplished by issuing a slave address byte with the R/W bit low, followed by the pointer register byte; no additional data are required. The master can then generate a start condition and send the slave address byte with the R/W bit high to initiate the read command; see Figure 16 for details of this sequence.

If repeated reads from the same register are desired, continually sending the pointer register bytes is not necessary because the TMP461 retains the pointer register value until it is changed by the next write operation. The register bytes are sent MSB first, followed by the LSB.

Terminate read operations by issuing a not-acknowledge command at the end of the last byte to be read. For a single-byte operation, the master must leave the SDA line high during the acknowledge time of the first byte that is read from the slave.

8.5.1.5 Timeout Function

The TMP461 device resets the serial interface if either SCL or SDA are held low for 25 ms (typical) between a start and stop condition. If the TMP461 device is holding the bus low, the device releases the bus and waits for a start condition. To avoid activating the timeout function, maintaining a communication speed of at least 1 kHz for the SCL operating frequency is necessary.

8.5.1.6 High-Speed Mode

For the two-wire bus to operate at frequencies above 1 MHz, the master device must issue a high-speed mode (HS-mode) master code (0000 1xxx) as the first byte after a start condition to switch the bus to high-speed operation. The TMP461 device does not acknowledge this byte, but switches the input filters on SDA and SCL and the output filter on SDA to operate in HS-mode, thus allowing transfers at up to 2.17 MHz. After the HS-mode master code is issued, the master transmits a two-wire slave address to initiate a data transfer operation. The bus continues to operate in HS-mode until a stop condition occurs on the bus. Upon receiving the stop condition, the TMP461 device switches the input and output filters back to fast mode operation.

8.6.1.1 Pointer Register

Figure 17 shows the internal register structure of the TMP461 device. The 8-bit pointer register is used to address a given data register. The pointer register identifies which of the data registers must respond to a read or write command on the two-wire bus. This register is set with every write command. A write command must be issued to set the proper value in the pointer register before executing a read command. Table 4 describes the pointer register and the internal structure of the TMP461 registers. The power-on reset (POR) value of the pointer register is 00h (0000 0000b).

Figure 17. Internal Register Structure

8.6.1.2 Local and Remote Temperature Registers

The TMP461 device has multiple 8-bit registers that hold temperature measurement results. The eight most significant bits (MSBs) of the local temperature sensor result are stored in register 00h, and the four least significant bits (LSBs) are stored in register 15h (the four MSBs of register 15h). The eight MSBs of the remote temperature sensor result are stored in register 01h, and the four LSBs are stored in register 10h (the four MSBs of register 10h). The four LSBs of both the local and remote sensor indicate the temperature value after the decimal point (for example, if the temperature result is 10.0625°C, then the high byte is 0000 1010 and the low byte is 0001 0000). These registers are read-only and are updated by the ADC each time a temperature measurement is completed.

When the full temperature value is needed, reading the MSB value first causes the LSB value to be locked (the ADC does not write to it) until the LSB value is read. The same thing happens upon reading the LSB value first (the MSB value is locked until it is read). This mechanism assures that both bytes of the read operation are from the same ADC conversion. This assurance remains valid only until another register is read. For proper operation, read the high byte of the temperature result first. Read the low byte register in the next read command; if the LSBs are not needed, the register can be left unread. The power-on reset value of all temperature registers is 00h.

8.6.1.3 Status Register

The status register reports the state of the temperature ADC, the temperature limit comparators, and the connection to the remote sensor. Table 5 lists the status register bits. The status register is read-only and is read by accessing pointer address 02h.

The BUSY bit = 1 if the ADC is making a conversion. This bit is set to 0 if the ADC is not converting.

The LHIGH and LLOW bits indicate a local sensor overtemperature or undertemperature event, respectively. The RHIGH and RLOW bits indicate a remote sensor overtemperature or undertemperature event, respectively. The HIGH bit is set when the temperature exceeds the high limit in alert mode and therm mode and the low bit is set when the temperature goes below the low limit in alert mode. The OPEN bit indicates an open-circuit condition on the remote sensor. When pin 7 is configured as the ALERT output, the five flags are NORed together. If any of the five flags are high, the ALERT interrupt latch is set and the ALERT output goes low. Reading the status register clears the five flags, provided that the condition that caused the setting of the flags is not present anymore (that is, the value of the corresponding result register is within the limits, or the remote sensor is connected properly and functional). The ALERT interrupt latch (and the ALERT pin correspondingly) is not reset by reading the status register. The reset is done by the master reading the temperature sensor device address to service the interrupt, and only if the flags are reset and the condition that caused them to be set is no longer present.

The RTHRM and LTHRM flags are set when the corresponding temperature exceeds the programmed THERM limit. These flags are reset automatically when the temperature returns to within the limits. The THERM output goes low in the case of overtemperature on either the local or remote channel, and goes high as soon as the measurements are within the limits again. The THERM hysteresis register (21h) allows hysteresis to be added so that the flag resets and the output goes high when the temperature returns to or goes below the limit value minus the hysteresis value.

When pin 7 is configured as THERM2, only the high limits matter. The LHIGH and RHIGH flags are set if the respective temperatures exceed the limit values, and the pin goes low to indicate the event. The LLOW and RLOW flags have no effect on THERM2 and the output behaves the same way when configured as THERM.

8.6.1.4 Configuration Register

The configuration register sets the temperature range, the ALERT/THERM modes, and controls the shutdown mode. The configuration register is set by writing to pointer address 09h, and is read by reading from pointer address 03h. Table 6 summarizes the bits of the configuration register.

MASK1 (bit 7) of the configuration register masks the ALERT output. If MASK1 is 0 (default), the ALERT output is enabled. If MASK1 is set to 1, the ALERT output is disabled. This configuration applies only if the value of ALERT/THERM2 (bit 5) is 0 (that is, pin 7 is configured as the ALERT output). If pin 7 is configured as the THERM2 output, the value of the MASK1 bit has no effect.

The shutdown bit (SD, bit 6) enables or disables the temperature-measurement circuitry. If SD = 0 (default), the TMP461 device converts continuously at the rate set in the conversion rate register. When SD is set to 1, the TMP461 device stops converting when the current conversion sequence is complete and enters a shutdown mode. When SD is set to 0 again, the TMP461 resumes continuous conversions. When SD = 1, a single conversion can be started by writing to the one-shot start register; see the One-Shot Start Register section for more information.

ALERT/THERM2 (bit 5) sets the configuration of pin 7. If the ALERT/THERM2 bit is 0 (default), then pin 7 is configured as the ALERT output; if this bit is set to 1, then pin 7 is configured as the THERM2 output.

The temperature range is set by configuring RANGE (bit 2) of the configuration register. Setting this bit low (default) configures the TMP461 device for the standard measurement range (–40°C to +127°C); temperature conversions are stored in the standard binary format. Setting bit 2 high configures the TMP461 device for the extended measurement range (–64°C to +191°C); temperature conversions are stored in the extended binary format (see Table 1).

The remaining bits of the configuration register are reserved and must always be set to 0. The power-on reset value for this register is 00h.

8.6.1.5 Conversion Rate Register

The conversion rate register (read address 04h, write address 0Ah) controls the rate at which temperature conversions are performed. This register adjusts the idle time between conversions but not the conversion time itself, thereby allowing the TMP461 power dissipation to be balanced with the temperature register update rate. Table 7 lists the conversion rate options and corresponding time between conversions. The default value of the register is 08h, which gives a default rate of 16 conversions per second.

8.6.1.6 One-Shot Start Register

When the TMP461 device is in shutdown mode (SD = 1 in the configuration register), a single conversion is started by writing any value to the one-shot start register, pointer address 0Fh. This write operation starts one conversion and comparison cycle on either both the local and remote sensors or on only one or the other sensor, depending on the LEN and REN values configured in the channel enable register (read address 16h, write address 16h). The TMP461 device returns to shutdown mode when the cycle completes. The value of the data sent in the write command is irrelevant and is not stored by the TMP461 device.

8.6.1.7 Channel Enable Register

The channel enable register (read address 16h, write address 16h) enables or disables the temperature conversion of remote and local temperature sensors. LEN (bit 0) of the channel enable register enables/disables the conversion of local temperature. REN (bit 1) of the channel enable register enables/disables the conversion of remote temperature. Both LEN and REN are set to 1 (default), this enables the ADC to convert both local and remote temperatures. If LEN is set to 0, the local temperature conversion is disabled and similarly if REN is set to 0, the remote temperature conversion is disabled.

Both local and remote temperatures are converted by the internal ADC as a default mode. Channel Enable register can be configured to achieve power savings by reducing the total ADC conversion time to half for applications that do not require both remote and local temperature information.

8.6.1.8 Consecutive ALERT Register

The Consecutive ALERT register (read address 22h, write address 22h) controls the number of out-of-limit temperature measurements required for ALERT to be asserted. Table 8 summarizes the values of the consecutive ALERT register. The number programmed in the consecutive ALERT applies to both the remote and local temperature results. When the number of times that the temperature result consecutively exceeds the high limit register value is equal to the value programmed in the consecutive ALERT register, ALERT is asserted. Similarly, the consecutive ALERT register setting is also applicable to the low-limit register.

8.6.1.9 η-Factor Correction Register

The TMP461 device allows for a different η-factor value to be used for converting remote channel measurements to temperature. The remote channel uses sequential current excitation to extract a differential V_BE voltage measurement to determine the temperature of the remote transistor. Equation 1 shows this voltage and temperature.

Equation 1.

The value η in Equation 1 is a characteristic of the particular transistor used for the remote channel. The power-on reset value for the TMP461 device is η = 1.008. The value in the η-factor correction register can be used to adjust the effective η-factor according to Equation 2 and Equation 3.

Equation 2.

Equation 3.

The η-factor correction value must be stored in twos complement format, yielding an effective data range from –128 to 127. The η-factor correction value is written to and read from pointer address 23h. The register power-on reset value is 00h, thus having no effect unless a different value is written to it. The resolution of the η-factor register is 0.000483.

8.6.1.10 Remote Temperature Offset Register

The offset register allows the TMP461 device to store any system offset compensation value that may result from precision calibration. The value in the register is stored in the same format as the temperature result, and is added to the remote temperature result upon every conversion. Combined with the η-factor correction, this function allows for very accurate system calibration over the entire temperature range.

8.6.1.11 Manufacturer Identification Register

The TMP461 device allows for the two-wire bus controller to query the device for manufacturer and device IDs to enable software identification of the device at the particular two-wire bus address. The manufacturer ID is obtained by reading from pointer address FEh. The TMP461 device reads 55h for the manufacturer code.