MX844

4

08/25/06

Drawing No. 084423

www.claremicronix.com

MX844

IXYS

APPLICATION OPERATING MODE

At the end of the power-on reset period, pin CS is sensed to determine the operating mode. Placing an

external resistor to VCC on pin CS will cause the MX844 to enter the slave mode. In this mode the

synchronous serial I/O port can be connected to a companion microcontroller for full control of the MX844

from the microcontroller.

If a resistor to GND is connected to CS, then the MX844 enters the master mode at power-up and fetches

commands from a 93C46 serial EEPROM operating in the 64 by 16 mode. In this mode, the MX844

operates as the master and sends clock, data, and select to the EEPROM and reads back the EEPROM

data. This mode is useful when the MX844 operates as a stand-alone data acquisition system with a

fixed pattern of measurement scanning. To facilitate electrically isolated measurements the data can be

sent to a computer's serial port using only one wire connected through a single opto-isolator.

If, neither an EEPROM or microcontroller are connected to the MX844, and CS is connected to VCC,

operation defaults to that of all zeroes in the control register (ADC clock = input clock / 8, input channel

IN1, +/- 50mV range). The asynchronous data output has a valid stream of measurement information.

FUNCTIONAL DESCRIPTION

Voltage Regulator

There are three methods of powering the MX844.

1) An external 4.5 to 5.5V supply is connected to VCC, VCC2, and VIN. Pins DRV and RIN are no-

connect or connected together.

2) VIN is connected to a 6 to 40V supply. DRV is connected to RIN, thus utilizing the internal pass

element.

3) VIN is connected to a 6 to 40V supply. An external PNP transistor such as the MMBTA56 or BCX53 is

used for the pass element. The PNP transistor's emitter, base, and collector are connected to pins

VIN, DRV, VCC respectively. RIN must be connected to VIN.

Power-On Reset

The MX844 contains a power-on reset circuit that resets all the internal flip-flops and initializes the

internal registers to zero. The reset circuit will also generate a reset condition if the voltage at pin VCC

drops to approximately 3.8V.

Oscillator

The oscillator configuration consists of a crystal/resonator connected between pins X1 and X2 and a

capacitor to GND on both pins. The capacitor value depends on the crystal/resonator but is usually

between 15 and 27pF. Alternatively, an external clock may be input to pin X1, with X2 floating. Nominal

DC self-bias at pin X1 is 800mV. In either case the clock is internally AC coupled, therefore clock rates

below 100KHz are not recommended.

Temperature Sensor

The internal temperature sensor is selected for measurement by setting the control register bits for a full

scale of +/-250mV and setting the unipolar bit. The temperature value read from the ADC consists of an

offset value plus the slope. To convert to a standard temperature scale it is necessary to subtract the

constant offset and then multiply by a constant that matches the standard scale desired.

Self-heating of the internal temperature sensor will occur due to the device power dissipation. The

maximum heating will occur when the internal pass element is used in conjunction with a high supply

voltage.