PS700 datasheet(5/38 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21760F-page 5

PS700

3.0

OPERATIONAL DESCRIPTION

3.1

A/D and Accumulator/Timer

Operation

3.1.1

A/D CONVERSION CYCLE

When the A/D converter is enabled and active, it

repeatedly performs a cycle of 1 to 8 conversions as

programmed by the user through 8 A/D Control

registers. These registers determine the input source,

resolution, reference voltage source and sequence of

conversions during an A/D converter cycle. During the

cycle, the A/D logic accesses each register in

sequence and performs the conversion specified by the

bits within the register. This register contains an enable

bit, a resolution field, a select bit for a single-ended or

differential reference and a select field for the analog

input mux. The results from each conversion are stored

in one of eight corresponding16-bit result registers.

If the “enable” bit is set within a control register, a

conversion will be performed. If it is disabled, that

conversion will be skipped and the logic will move on to

the next register. In this manner, the user can specify a

sequence of conversions that will be performed during

each A/D cycle.

As stated above, the input source for each of the

registers is programmable. The 3-bit mux field within

each control register selects one of seven possible

input sources for the A/D conversion. The list of input

sources is as follows:

• Charge/Discharge Current (voltage from SR pin to

GND)

• Internal Temperature Sensor

• External Thermistor (constant current source on

NTC pin)

• Battery Pack Voltage

• VC1 Voltage

• VC2 Voltage

• A/D Offset (conversion performed with input

shorted internally to determine offset error

associated with the converter)

However, the accumulator/timer functions are “hard

wired” to specific A/D Result registers. For this reason,

the control/result registers are given names which

indicate their primary intended usage (see Table 3-1).

TABLE 3-1:

A/D CONTROL/RESULT REGISTERS

The 3-bit “resolution” field in each A/D Control register

determines the resolution of the conversion, from a

minimum of 9 bits (8 bits plus sign bit) to a maximum of

16 bits (15 bits plus sign bit). The time required to

complete the conversion is a function of the number of

bits of resolution (n) selected. The conversion time can

be calculated as follows:

TADC = 30.52

µs * 2n

where:

“n” is the number of bits of resolution selected

The “Ref” bit selects the magnitude of the reference

voltage. Either a ±340 mV or a ±170 mV reference is

available. The ±170 mV reference is typically used for

current readings and the ±340 mV reference for all

other measurements.

The value of the LSB can be expressed as a function of

the resolution selected as follows:

A/D LSB = 680/340 mV/2n

where:

“n” is the number of bits of resolution selected

The result value is given in sign/magnitude format (i.e.,

a sign bit with a 15-bit magnitude).

A/D Register #

Control Register

Result Register

Intended Input Source

Ictrl

Ires

Battery Pack Current (via sense resistor)

ITctrl

ITres

Internal Temperature Sensor

ETctrl

ETres

External Temperature Sensor

VPctrl

VPres

Battery Pack Voltage (VC1 to GND)

VC1ctrl

VC1res

Battery Cell Voltage (VC1 to VC2)

VC2ctrl

VC2res

Battery Cell Voltage (VC2 to GND)

OFFSctrl

OFFSres

Internal A/D Offset Voltage (A/D input automatically

internally shorted to GND)

AUXctrl

AUXres

Any

Magnitude

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