MAX1455CSLASHD datasheet(9/25 Pages) MAXIM

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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.

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

memory 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 compen-

sation 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 calibration

and test and the appropriate compensation values stored

in internal EEPROM. The device autoloads the registers

from EEPROM and is ready for use without further con-

figuration 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 registers 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 temperature

sensor to an 8-bit value every 1ms. This digitized 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 x Temperature (°C) + 47.58

where 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

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 memory. All com-

mand inputs to this pin flow into a set of 16 registers,

which form the interface register set (IRS). Additional lev-

els 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 regis-

ters and internal (8-bit-wide) EEPROM locations. Figure

5 shows the relationship between the various serial com-

mands 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 pow-

er-up, or following receipt of the reinitialization command,

is used by the MAX1455 to learn the communication baud

rate. The initialization sequence is a 1-byte transmiss of

01 hex, as follows:

1 1 1 1 1

1 1 1 1 1 1

0 0 0 0 0

MAX1455

Low-Cost Precision Sensor Signal Conditioner

www.maximintegrated.com

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