ADUC848BCP8-5 datasheet(92/108 Pages) AD

92 / 108 page

ADuC845/ADuC847/ADuC848

Rev. A | Page 92 of 108

Single-Pin Emulation Mode

Built into the ADuC845/ADuC847/ADuC848 is a dedicated

controller for single-pin in-circuit emulation (ICE). In this mode,

emulation access is gained by connection to a single pin, the EA

pin. Normally on the 8051 standard, this pin is hardwired either

high or low to select execution from internal or external program

memory space. Note that external program memory or execu-

tion from external program memory is not allowed on the

devices. To enable single-pin emulation mode, users need to pull

the EA pin high through a 1 kΩ resistor as shown in Figure 69.

The emulator then connects to the 2-pin header also shown in

Figure 69. To be compatible with the standard connector that

comes with the single-pin emulator available from Accutron

Limited (www.accutron.com), use a 2-pin 0.1-inch pitch

Friction Lock header from Molex (www.molex.com) such as

part number 22-27-2021. Be sure to observe the polarity of this

header. As shown in Figure 69, when the Friction Lock tab is at

the right, the ground pin should be the lower of the two pins

when viewed from the top.

Typical System Configuration

A typical ADuC845/ADuC847/ADuC848 configuration is

shown in Figure 69. Figure 69 also includes connections for a

typical analog measurement application of the parts, namely an

interface to an resistive temperature device (RTD). The

arrangement shown is commonly referred to as a 4-wire RTD

configuration.

Here, the on-chip excitation current sources are enabled to

excite the sensor. The excitation current flows directly through

the RTD generating a voltage across the RTD proportional to its

resistance. This differential voltage is routed directly to one set

of the positive and negative inputs of the ADC (AIN1, AIN2,

respectively in this case). The same current that excited the

RTD also flows through a series resistance, RREF, generating a

ratiometric voltage reference, VREF. The ratiometric voltage

reference ensures that variations in the excitation current do not

affect the measurement system since the input voltage from the

RTD and reference voltage across RREF vary ratiometrically with

the excitation current. Resistor RREF must, however, have a low

temperature coefficient to avoid errors in the reference voltage

overtemperature. RREF must also be large enough to generate at

least a 1 V voltage reference.

The preceding example shows just a single differential ADC

connection using a single reference input pair. The ADuC845/

ADuC847/ADuC848 have the capability of connecting to five

differential inputs directly or ten single-ended inputs (LFCSP

package only) as well as having a second reference input. This

arrangement means that different sensors with different

reference ranges can be connected to the part with the need for

external multiplexing circuitry. This arrangement is shown in

Figure 70. The bridge sensor shown can be a load cell or a

pressure sensor. The RTD is shown using a reference voltage

derived from the RREF resistor via the REFIN± inputs, and the

bridge sensor is shown using a divided down AVDD reference via

the REFIN2± inputs.

ADUC848BCP8-5 Datasheet(PDF) 92 Page - Analog Devices

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