DAC registers are loaded from the pre-assigned loca-

tions in the EEPROM.

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.

The typical transfer function for the temp-index is as fol-

lows:

where temp-index is truncated to an 8-bit integer value.

Typical values for the temp-index register are given in

Table 6.

Note that the EEPROM is byte wide and the registers

that are loaded from EEPROM are 16 bits wide. Thus

each index value points to two 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 3). 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.

Communication Protocol

The DIO serial interface is used for asynchronous serial

data communications between the MAX1452 and a

host calibration test system or computer. The MAX1452

will automatically detect the baud rate of the host com-

puter when the host transmits the initialization

sequence. Baud rates between 4800bps and

38,400bps can be detected and used regardless of the

internal oscillator frequency setting. Data format is

always 1 start bit, 8 data bits, 1 stop bit and no parity.

Communications are only allowed when Secure-Lock is

disabled (i.e., CL[7:0] = 00hex) or the UNLOCK pin is

held high.

Initialization Sequence

Sending the initialization sequence shown below

enables the MAX1452 to establish the baud rate that

initializes the serial port. The initialization sequence is

one byte transmission of 01hex, as follows.

1111111101000000011111111

The first start bit 0 initiates the baud rate synchronization

sequence. The 8 data bits 01hex (LSB first) follow this

and then the stop bit, which is indicated above as a 1,

terminates the baud rate synchronization sequence.

This initialization sequence on DIO should occur after a

period of 1ms after stable power is applied to the

device. This allows time for the power-on reset function

to complete and the DIO pin to be configured by

Secure-Lock or the UNLOCK pin.

Reinitialization Sequence

The MAX1452 allows for relearning the baud rate. The

reinitialization sequence is one byte transmission of

FFhex, as follows.

11111111011111111111111111

When a serial reinitialization sequence is received, the

receive logic resets itself to its power-up state and

waits for the initialization sequence. The initialization

sequence must follow the reinitialization sequence in

order to re-establish the baud rate.

Serial Interface Command Format

All communication commands into the MAX1452 follow

a defined format utilizing an interface register set (IRS).

The IRS is an 8-bit command that contains both an

interface register set data (IRSD) nibble (4-bit) and an

interface register set address (IRSA) nibble (4-bit). All

internal calibration registers and EEPROM locations are

accessed for read and write through this interface reg-

ister set. The IRS byte command is structured as fol-

lows:

IRS[7:0] = IRSD[3:0], IRSA[3:0]

Where:

•

IRSA[3:0] is the 4-bit interface register set address

and indicates which register receives the data nib-

ble IRSD[3:0].

•

IRSA[0] is the first bit on the serial interface after the

start bit.

•

IRSD[3:0] is the 4-bit interface register set data.

•

IRSD[0] is the fifth bit received on the serial inter-

face after the start bit.

Special Command Sequences

A special command register to internal logic

(CRIL[3:0]) causes execution of special command

sequences within the MAX1452. These command

sequences are listed as CRIL command codes as

shown in Table 10.

Write Examples

A 16-bit write to any of the internal calibration registers

is performed as follows:

Low-Cost Precision Sensor

Signal Conditioner

12

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