MAX1455
General Description
The MAX1455 is a highly integrated automotive analog-sensor signal processor for resistive element sensors.The MAX1455 provides amplification, calibration, and temperature compensation that enable an overall per-formance approaching the inherent repeatability of the sensor. The fully analog signal path introduces no quantization noise in the output signal while enabling digitally controlled trimming with integrated 16-bit digi-tal-to-analog converters (DACs). Offset and span are also calibrated using 16-bit DACs, allowing sensor products to be truly interchangeable.
The MAX1455 architecture includes a programmable sensor excitation, a 16-step programmable-gain ampli-fier (PGA), a 768-byte (6144 bits) internal EEPROM,four 16-bit DACs, an uncommitted op amp, and an on-chip temperature sensor. In addition to offset and span compensation, the MAX1455 provides a unique tem-perature compensation strategy that was developed to provide a remarkable degree of flexibility while minimiz-ing testing costs.
The MAX1455 is available in die form, 16-pin SSOP and TSSOP packages.
Customization
Maxim can customize the MAX1455 for high-volume dedicated applications. Using our dedicated cell library of more than 2000 sensor-specific function blocks,Maxim can quickly provide a modified MAX1455 solu-tion. Contact Maxim for further information.
Applications
Pressure Sensors and Transducers Piezoresistive Silicon Sensors Strain Gauges
Resistive Element Sensors Accelerometers Humidity Sensors
MR and GMR Sensors
Outputs
Ratiometric Voltage Output
Programmable Output Clip Limits
Features
o Provides Amplification, Calibration, and Temperature Compensation o Selectable Output Clipping Limits
o Accommodates Sensor Output Sensitivities from 5mV/V to 40mV/V o Single-Pin Digital Programming o No External Trim Components Required o 16-Bit Offset and Span Calibration Resolution o Fully Analog Signal Path
o PRT Bridge Can Be Used for Temperature-Correction Input o On-Chip Lookup Table Supports Multipoint Calibration Temperature Correction o Fast 3.2kHz Frequency Response o On-Chip Uncommitted Op Amp
o Secure-Lock?Prevents Data Corruption
MAX1455
Conditioner
________________________________________________________________Maxim Integrated Products 1
Pin Configuration
Ordering Information
**Dice are tested at T A = +25°C, DC parameters only.
A detailed Functional Diagram appears at end of data sheet.Secure-Lock is a trademark of Maxim Integrated Products, Inc.
For pricing, delivery, and ordering information,please contact Maxim/Dallas Direct!at 1-888-629-4642, or visit Maxim’s website at https://www.wendangku.net/doc/057545775.html,.
M A X 1455
Low-Cost Automotive Sensor Signal Conditioner
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
Supply Voltage, V DD_to V SS .......................................-0.3V, +6V V DD1- V DD2..............................................................-0.3V, +0.6V All Other Pins..................................(V SS - 0.3V) to (V DD_+ 0.3V)Short-Circuit Duration, OUT, BDR, AMPOUT.............Continuous Continuous Power Dissipation (T A = +70°C)
16-Pin SSOP (derate 8.00mW/°C above +70°C).........640mW
Operating Temperature Ranges (T MIN to T MAX )
MAX1455EUE..................................................-40°C to +85°C MAX1455AUE................................................-40°C to +125°C MAX1455C/D...................................................-40°C to +85°C MAX1455EAE ..................................................-40°C to +85°C MAX1455AAE................................................-40°C to +125°C Storage Temperature Range.............................-65°C to +150°C Lead Temperature (soldering, 10s)................................+300°C
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ELECTRICAL CHARACTERISTICS (continued)
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ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(V DD_= +5V, V SS = 0, T A = +25°C, unless otherwise noted.)
OFFSET DAC DNL
M A X 1455 t o c 01
DAC CODE
D N L (m V )
30k
40k
10k
20k
50k
60k
70k
-2.5
-1.0-1.5-2.0-0.500.51.01.52.02.5 5.02.5
-2.5
-5.0
AMPLIFIER GAIN NONLINEARITY
INPUT VOLTAGE [INP - INM] (mV)
O U T P U T E R R O R F R O M S T R A I G H T L I N E (m V )
-50
-30
-10
10
30
50
OUTPUT NOISE
400μs/div
OUT 10mV/div
INP - INM SHORTED TOGETHER PGA = 0HEX
Note 3:This is the sensor ’s sensitivity normalized to its drive voltage, assuming a desired full-span output of 4V and a bridge voltage of 2.5V.Note 4:Bit weight is ratiometric to V DD .
Note 5:All units production tested at T A = +25°C. Limits over temperature are guaranteed by design.Note 6:Programming of the EEPROM at temperatures below +70°C is recommended.Note 7:For operation above +70°C, limit erase/write cycle to 100.Note 8:All erase commands require 7.1ms minimum time.
Detailed Description The MAX1455 provides amplification, calibration, and temperature compensation to enable an overall perfor-mance approaching the inherent repeatability of the sensor. The fully analog signal path introduces no quantization noise in the output signal while enabling digitally controlled trimming with the integrated 16-bit DACs. The MAX1455 includes four selectable high/low clipping limits set in discrete 50mV steps from 0.1V/4.9V to 0.25V/4.75V. Offset and span can be cali-brated to within ±0.02% of span.
The MAX1455 architecture includes a programmable sensor excitation, a 16-step PGA, a 768-byte (6144 bits) internal EEPROM, four 16-bit DACs, an uncommitted op amp, and an on-chip temperature sensor. The MAX1455 also provides a unique temperature compensation strat-egy that was developed to provide a remarkable degree of flexibility while minimizing testing costs.
The customer can select from 1 to 114 temperature points to compensate their sensor. This allows the lati-tude to compensate a sensor with a simple first-order linear correction or match an unusual temperature curve. Programming up to 114 independent 16-bit EEPROM locations corrects performance in 1.5°C tem-perature increments over a range of -40°C to +125°C.
For sensors that exhibit a characteristic temperature performance, a select number of calibration points can
be used with a number of preset values that define the temperature curve. The sensor and the MAX1455 should be at the same temperature during calibration
and use. This allows the electronics and sensor errors
to be compensated together and optimizes perfor-mance. For applications where the sensor and elec-tronics are at different temperatures, the MAX1455 can
use the sensor bridge as an input to correct for temper-
ature errors.
The single pin, serial DIO communication architecture
and the ability to timeshare its activity with the sensor’s output signal enables output sensing and calibration programming on a single line by parallel connecting
OUT and DIO. The MAX1455 provides a Secure-Lock feature that allows the customer to prevent modification
of sensor coefficients and the 52-byte user-definable EEPROM data after the sensor has been calibrated.
The Secure-Lock feature also provides a hardware override to enable factory rework and recalibration by assertion of logic high on the UNLOCK pin.
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The MAX1455 allows complete calibration and sensor verification to be performed at a single test station. Once calibration coefficients have been stored in the ASIC, the customer can choose to retest in order to verify perfor-mance as part of a regular QA audit or to generate final test data on individual sensors. In addition, Maxim has developed a pilot production test system to reduce time to market. Engineering test evaluation and pilot produc-tion of the MAX1455 can be performed without expending the cost and time to develop in-house test capabilities.Contact Maxim for additional information.
Frequency response can be user adjusted to values lower than the 3.2kHz bandwidth by using the uncom-mitted op amp and simple passive components.The MAX1455 (Figure 1) provides an analog amplifica-tion path for the sensor signal. It uses a digitally con-trolled analog path for nonlinear temperature correction.For PRT applications, analog architecture is available for first-order temperature correction. Calibration and cor-rection are achieved by varying the offset and gain of a PGA and by varying the sensor bridge excitation current or voltage. The PGA utilizes a switched capacitor CMOS technology, with an input-referred offset trimming range of more than ±150mV with an approximate 3μV resolution (16 bits). The PGA provides gain values from 39V/V to 234V/V in 16 steps.
The MAX1455 uses four 16-bit DACs with calibration coefficients stored by the user in an internal 768 x 8EEPROM (6144 bits). This memory contains the follow-ing information, as 16-bit-wide words:?Configuration register ?Offset calibration coefficient table ?Offset temperature coefficient register ?FSO calibration coefficient table ?FSO temperature correction register
?
52 bytes (416 bits) uncommitted for customer pro-gramming of manufacturing data (e.g., serial num-ber and date)
Offset Correction
Initial offset correction is accomplished at the input stage of the signal gain amplifiers by a coarse offset setting. Final offset correction occurs through the use of a temperature-indexed lookup table with one hundred seventy-six 16-bit entries. The on-chip temperature sen-sor provides a unique 16-bit offset trim value from the table with an indexing resolution of approximately 1.5°C from -40°C to +125°C. Every millisecond, the on-chip temperature sensor provides indexing into the offset lookup table in EEPROM and the resulting value is
transferred to the offset DAC register. The resulting volt-age is fed into a summing junction at the PGA output,compensating the sensor offset with a resolution of ±76μV (±0.0019% FSO). If the offset TC DAC is set to zero, then the maximum temperature error is equivalent to 1°C of temperature drift of the sensor, given that the Offset DAC has corrected the sensor every 1.5°C. The temperature indexing boundaries are outside the speci-fied absolute maximum ratings. The minimum indexing value is 00hex, corresponding to approximately -69°C.All temperatures below this value output the coefficient value at index 00hex. The maximum indexing value is AFhex, which is the highest lookup table entry. All tem-peratures higher than approximately +184°C output the highest lookup table index value. No indexing wrap-around errors are produced.
FSO Correction
Two functional blocks control the FSO gain calibration.First, a coarse gain is set by digitally selecting the gain of the PGA. Second, FSODAC sets the sensor bridge cur-rent or voltage with the digital input obtained from a tem-perature indexed reference to the FSO lookup table in EEPROM. FSO correction occurs through the use of a
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Figure 1. Functional Diagram
temperature indexed lookup table with one hundred seventy-six16-bit entries. The on-chip temperature sen-sor provides a unique FSO trim from the table with an indexing resolution approaching one 16-bit value every 1.5°C from -40°C to +125°C. The temperature indexing boundaries are outside the specified absolute maximum ratings. The minimum indexing value is 00hex, corre-sponding to approximately -69°C. All temperatures below this value output the coefficient value at index 00hex. The maximum indexing value is AFhex, which is the highest lookup table entry. All temperatures higher than approxi-mately +184°C output the highest lookup table index value. No indexing wraparound errors are produced.
Linear and Nonlinear Temperature
Compensation Writing 16-bit calibration coefficients into the offset TC and FSOTC registers compensates first-order tempera-ture errors. The piezoresistive sensor is powered by a current source resulting in a temperature-dependent bridge voltage due to the sensor’s temperature coeffi-cient resistance (TCR). The reference inputs of the off-set TC DAC and FSOTC DAC are connected to the bridge voltage. The DAC output voltages track the bridge voltage as it varies with temperature, and by varying the offset TC and FSOTC digital code and a portion of the bridge voltage, which is temperature dependent, is used to compensate the first-order tem-perature errors.
The internal feedback resistors (R ISRC and R STC) for FSO temperature compensation are set to 75k?.
To calculate the required offset TC and FSOTC com-pensation coefficients, two test temperatures are need-ed. After taking at least two measurements at each temperature, calibration software (in a host computer) calculates the correction coefficients and writes them to the internal EEPROM.
With coefficients ranging from 0000hex to FFFFhex and a +5V reference, each DAC has a resolution of 76μV. Two of the DACs (offset TC and FSOTC) utilize the sen-sor bridge voltage as a reference. Since the sensor bridge voltage is approximately set to +2.5V, the FSOTC and offset TC exhibit a step size of less than 38μV.
For high-accuracy applications (errors less than 0.25%), the first-order offset TC and FSOTC should be compensated with the offset TC and FSOTC DACs, and the residual higher order terms with the lookup table. The offset and FSO compensation DACs provide unique compensation values for approximately 1.5°C of temperature change as the temperature indexes the address pointer through the coefficient lookup table. Changing the offset does not affect the FSO; however,changing the FSO affects the offset due to the nature of
the bridge. The temperature is measured on both the
MAX1455 die and at the bridge sensor. It is recom-mended to compensate the first-order temperature
errors using the bridge sensor temperature.
Typical Ratiometric
Operating Circuit Ratiometric output configuration provides an output that is proportional to the power-supply voltage. This output can
then be applied to a ratiometric ADC to produce a digital
value independent of supply voltage. Ratiometricity is an important consideration for battery-operated instruments, automotive, and some industrial applications.
The MAX1455 provides a high-performance ratiometric output with a minimum number of external components (Figure 2). These external components include the fol-lowing:
?One supply bypass capacitor
?One optional output EMI suppression capacitor
Typical Nonratiometric
Operating Circuit
(5.5VDC < VPWR < 28VDC) Nonratiometric output configuration enables the sensor power to vary over a wide range. A low-dropout voltage regulator, such as the MAX1615, is incorporated in the
circuit to provide a stable supply and reference for
MAX1455 operation. A typical example is shown in Figure 3. Nonratiometric operation is valuable when
wide ranges of input voltage are to be expected and
the system A/D or readout device does not enable ratiometric operation.
Internal Calibration Registers
The MAX1455 has five 16-bit internal calibration regis-
ters (ICRs) that are loaded from EEPROM, or loaded
from the serial digital interface.
Data can be loaded into the ICRs under three different circumstances.
Normal Operation, Power-On Initialization Sequence:
?The MAX1455 has been calibrated, the Secure-Lock byte is set (CL[7:0] = FFhex), and UNLOCK is
low.
?Power is applied to the device.
?The power-on reset (POR) functions have been completed.
?Registers CONFIG, OTCDAC, and FSOTCDAC are refreshed from EEPROM.
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?
Registers ODAC and FSODAC are refreshed from the temperature indexed EEPROM locations.
Normal Operation, Continuous Refresh:
?The MAX1455 has been calibrated, the Secure-Lock byte has been set (CL[7:0] = FFhex), and UNLOCK is low.?Power is applied to the device.
?The POR functions have been completed.
?The temperature index timer reaches a 1ms time period.
?
Registers CONFIG, OTCDAC, and FSOTCDAC are refreshed from EEPROM.
?
Registers ODAC and FSODAC are refreshed from the temperature indexed EEPROM locations.
Calibration Operation, Registers Updated by Serial Communications:
?The MAX1455 has not had the Secure-Lock byte set
(CL[7:0] = 00hex) or UNLOCK is high.?Power is applied to the device.
?The POR functions have been completed.
?
The registers can then be loaded from the serial digital interface by use of serial commands. See the section on serial I/O and commands.
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Figure 2. Basic Ratiometric Output Configuration
Figure 3. Basic Nonratiometric Output Configuration
Internal EEPROM
The internal EEPROM is organized as a 768 by 8-bit memory. It is divided into 12 pages, with 64 bytes per page. Each page can be individually erased. The memo-ry structure is arranged as shown in Table 1. The look-up tables for ODAC and FSODAC are also shown, with the respective temperature index pointer. Note that the ODAC table occupies a continuous segment, from address 000hex to address 15Fhex, whereas the FSODAC table is divided in two parts, from 200hex to 2FFhex, and from 1A0hex to 1FFhex. With the exception of the general-purpose user bytes, all values are 16-bit-wide words formed by two adjacent byte locations (high byte and low byte).
The MAX1455 compensates for sensor offset, FSO, and temperature errors by loading the internal calibration registers with the compensation values. These com-pensation values can be loaded to registers directly through the serial digital interface during calibration or loaded automatically from EEPROM at power-on. In this way, the device can be tested and configured during cal-ibration and test and the appropriate compensation val-ues stored in internal EEPROM. The device autoloads the registers from EEPROM and is ready for use without fur-ther configuration after each power-up. The EEPROM is configured as an 8-bit-wide array so each of the 16-bit registers is stored as two 8-bit quantities. The Configuration register, FSOTCDAC, and OTCDAC regis-ters are loaded from the preassigned locations in the EEPROM. Table 2 is the EEPROM ODAC and FSODAC lookup table memory map.
The ODAC and FSODAC are loaded from the EEPROM lookup tables using an index pointer that is a function of temperature. An ADC converts the integrated tem-perature sensor to an 8-bit value every 1ms. This digi-tized value is then transferred into the Temp-Index register. Table 3 lists the registers.
The typical transfer function for the temp-index is as fol-lows:
temp-index = 0.69 ?Temperature (°C) + 47.58where temp-index is truncated to an 8-bit integer value.Typical values for the Temp-Index register are given in Table 4.
Note that the EEPROM is 1 byte wide and the registers that are loaded from EEPROM are 16 bits wide. Thus,each index value points to 2 bytes in the EEPROM.
Maxim programs all EEPROM locations to FFhex with the exception of the oscillator frequency setting and Secure-Lock byte. OSC[2:0] is in the Configuration register (Table 5). These bits should be maintained at the factory-preset values. Programming 00hex in the
Secure-Lock byte (CL[7:0] = 00hex) configures the DIO as an asynchronous serial input for calibration and test purposes.
MAX1455 Digital Mode
A single-pin serial interface provided by the DIO accesses the MAX1455’s control functions and memo-ry. All command inputs to this pin flow into a set of 16registers, which form the interface register set (IRS).Additional levels of command processing are provided by control logic, which takes its inputs from the IRS. A bidirectional 16-bit latch buffers data to and from the 16-bit Calibration registers and internal (8-bit-wide)EEPROM locations. Figure 5 shows the relationship between the various serial commands and the MAX1455 internal architecture.
Communication Protocol
The DIO serial interface is used for asynchronous serial data communications between the MAX1455 and a host calibration test system or computer. The MAX1455 auto-matically detects the baud rate of the host computer when the host transmits the initialization sequence. Baud rates between 4800 and 38400 can be detected and used. The data format is always 1 start bit, 8 data bits,and 1 stop bit. The 8 data bits are transmitted LSB first,MSB last. A weak pullup resistor can be used to maintain logic 1 on the DIO pin while the MAX1455 is in digital mode. This is to prevent unintended 1 to 0 transitions on this pin, which would be interpreted as a communication start bit. Communications are only allowed when the Secure-Lock byte is disabled (i.e., CL[7:0] = 00HEX ) or UNLOCK is held high. Table 8 is the control location.
Initialization Sequence
The first Command Byte sent to the MAX1455 after power-up, or following receipt of the reinitialization command, is used by the MAX1455 to learn the com-munication baud rate. The initialization sequence is a 1-byte transmission of 01 hex, as follows:
The start bit, shown in bold above, initiates the baud rate synchronization. The 8 data bits 01hex (LSB first) follow this and then the stop bit, also shown in bold above. The MAX1455 uses this sequence to calculate the time inter-val for a 1-bit transmission as a multiple of the period of its internal oscillator. The resulting number of oscillator clock cycles is then stored internally as an 8-bit number (BITCLK). Note that the device power supply should be stable for a minimum period of 1ms before the initializa-tion sequence is sent. This allows time for the POR func-tion to complete and DIO to be configured by the Secure-Lock byte or UNLOCK.
11111010000000111111
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M A X 1455
All communication commands into the MAX1455 follow the format of a start bit, 8 command bits (command byte), and a stop bit. The Command Byte controls the
is also used to enable the MAX1455 analog output and to place output data (serial digital output) on DIO. Table 10 shows a full listing of CRIL commands.
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DIO is configured as a digital output by writing a Read IRS (RDIRS) command (5 hex) to the CRIL location. On receipt of this command, the MAX1455 outputs a byte of data, the contents of which are determined by the IRS pointer (IRSP[3:0]) value at location IRSA[3:0] = 8hex. The data is output as a single byte, framed by a start bit and a stop bit. Table 11 lists the data returned for each IRSP address value.
Once the RDIRS command has been sent, all connec-tions to DIO must be three-stated to allow the MAX1455 to drive the DIO line. Following receipt of the RDIRS command, the MAX1455 drives DIO high after 1 byte time. The MAX1455 holds DIO high for a single bit time and then asserts a start bit (drives DIO low). The start bit is then followed by the data byte and a stop bit. Immediately following transmission of the stop bit, the MAX1455 three-states DIO, releasing the line. The MAX1455 is then ready to receive the next command sequence 1 byte time after release of DIO.MAX1455 sends the data byte when all devices on the
DIO line are three-stated. It is recommended that a
weak pullup resistor be applied to the DIO line during
these time intervals to prevent unwanted transitions (Figure 4). In applications where DIO and analog out-
put (OUT) are not connected, a pullup resistor should
be permanently connected to DIO. If the MAX1455 DIO
and analog outputs are connected, then do not load
this common line during analog measurements. In this situation, perform the following sequence:
1)Connect a pullup resistor to the DIO/OUT line,
preferably with a relay.
2)Send the RDIRS command.
3)Three-state the user connection (set to high imped-
ance).
4)Receive data from the MAX1455.
5)Activate the user connection (pull DIO/OUT line high).
6)Release the pullup resistor.
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Figure 4 shows an example transmit/receive sequence with the RDIRS command (59hex) being sent and the MAX1455 responding with a byte value of 10hex.
Internal Clock Settings
Following initial power-up, or after a power reset, all of the calibration registers within the MAX1455 contain 0000hex and must be programmed. Note that in analog
mode, the internal registers are automatically refreshed from the EEPROM.
When starting the MAX1455 in digital mode, pay spe-cial attention to the 3 CLK bits: 3MSBs of the Configuration register. The frequency of the MAX1455internal oscillator is measured during production testing and a 3-bit adjustment (calibration) code is calculated
Figure 5. MAX1455 Serial Command Structure and Hardware Schematic
161hex (EEPROM upper configuration byte).
The MAX1455 internal clock controls timing functions, including the signal path gain, DAC functions, and com-munications. It is recommended that, while in digital mode, the Configuration register CLK bits be assigned the values contained in EEPROM (upper configuration byte). The 3 CLK bits represent a two’s-complement number with a nominal clock adjustment of 9% per bit. Table 12 shows the codes and adjustment available. Any change to the CLK bit values contained in the Configuration register must be followed by the MAX1455 baud rate learning sequence (reinitialize and initialize commands). To maximize the robustness of the communication system during clock resetting only, change the CLK bits by 1LSB value at a time. The rec-ister CLK bits is, therefore, as follows. (Use a minimum
baud rate of 9600 during the setting procedure to pre-
vent potential overflow of the MAX1455 baud rate counter with clock values near maximum.)
The following example is based on a required CLK
code of 010 binary:
1)Read the CLK bits (3MSBs) from EEPROM location
161hex. CLK = 010 binary.
2)Set the CLK bits in the Configuration register to 001
binary.
3)Send the reinitialize command, followed by the ini-
tialize (baud rate learning) command.
4) Set the CLK bits in the Configuration register to 010
binary.
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tialize (baud rate learning) command.The frequency of the internal oscillator can be checked at any time by reading the value of BITCLK[7:0]. This 8-bit number represents the number of internal oscillator cycles corresponding to 1 cycle (1 bit time) of the com-munications baud rate.
Erasing and Writing to the EEPROM
The internal EEPROM must be erased (bytes set to FFhex) prior to programming the desired contents. The MAX1455 is supplied in a nominally erased state except byte 161hex and byte 16Bhex. The 3MSBs of byte 161hex contain the internal oscillator calibration setting. Byte 16Bhex is set to 00hex to allow serial com-munication regardless of the UNLOCK status.
When erasing the EEPROM, first save the 3MSBs of byte 161hex. Following erasure, these 3 bits must be rewritten, together with the Secure-Lock byte value of 00hex. Failure to do this may cause the part to stop communicating. Do not remove power from the device before rewriting these values.
The internal EEPROM can be entirely erased with the ERASE command or partially erased with the PageErase command (Table 10). It is necessary to wait 7.1ms after issuing an erase or PageErase command.Any attempt to communicate with the part or to interrupt power before 7.1ms have elapsed may produce inde-terminate states within the EEPROM.
First load the required page number (Table 1) into the IRS location IEEA[3:0]. Then send a CRIL PageErase command (79hex).
To write a byte to EEPROM: Load IRS locations IEEA[9:8], IEEA[7:4], and IEEA[3:0] with the byte address (Address[9:0]). Load IRS locations DH R[7:4]and DH R[3:0] with the 8 data bits to be written (Data[7:0]). Send the EEPROM WRITE command to CRIL (19hex).
To read a byte from EEPROM:
1)Load IRS locations IEEA[9:8], IEEA[7:4], and
IEEA[3:0] with the byte address (Address[9:0]). 2)Send a READ EEPROM command to the CRIL reg-ister (49hex); this loads the required EEPROM byte into DHR[7:0]. 3)Load IRS location IRSP[3:0] with 00hex (return
DHR[7:0]). 4)Send the READ IRSP command to the CRIL register
(59hex).
Multiplexed Analog Output
The MAX1455 provides the facility to output analog sig-nals while in digital mode through the read analog (RdAlg) command. One byte time after receiving the RdAlg command, the internal analog signal determined by the ALOC[3:0] register (Table 13) is multiplexed to the MAX1455 OUT. The signal remains connected to OUT for the duration set by the ATIM[3:0] register. The
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timing basis. See Table 14 for details. At the end of the period determined by ATIM[3:0], the analog signal is disconnected from the analog output and OUT resumes a three-state condition. The MAX1455 can receive further commands on DIO 1 byte after resum-ing a three-state condition on OUT. Figure 6 shows the timing of this scheme.The MAX1455 DIO is three-state for the duration that
the analog output is active. This is to allow OUT and
DIO to be connected in parallel. When DIO and OUT
are connected in parallel, the host computer must also
three-state its communications connection to the
MAX1455. This requirement produces periods when all connections to the DIO are three-stated simultaneously, making it necessary to have a weak pullup resistor applied to DIO during these periods.
A continuous output mode is available for the analog output and is selected by setting ATIM[3:0] to Fhex.
This mode may only be used when DIO and OUT are separate. While in this mode and following receipt of
the RdAlg command, or any other command, DIO
three-states for a period of 32,769 byte times. Once this period has elapsed, DIO enters receive mode and accepts further command inputs. The analog output is always active while in continuous mode.
Note:The internal analog signals are not buffered
when connected to OUT. Any loading of OUT while one
of these internal signals is being measured is likely to produce measurement errors. Do not load OUT when reading internal signals such as BDR, FSOTC, etc.
Communication Command Examples
A selection of examples of the command sequences for various functions within the MAX1455 follows.
Example 1.Change the baud rate setting and check communications. If the communication with the
MAX1455 is lost due to a system baud rate change
before sending the reinitialization command, apply a power reset to guarantee the initialization condition.
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Figure 6. Analog Output Timing
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Sensor Compensation Overview
Compensation requires an examination of the sensor
performance over the operating pressure and tempera-ture range. Use a minimum of two test pressures (e.g.,
zero and full span) and two temperatures. More test pressures and temperatures result in greater accuracy.A typical compensation procedure can be summarized as follows:
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