CS5460A datasheet(22/54 Pages) CIRRUS

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CS5460A

DS487F4

EXAMPLE #2: The required number of pulses per

unit energy present at EOUT is specified to be

500 pulses/kW-hr;

given

that

the

maximum

line-voltage is 250 V (RMS) and the maximum

line-current is 20 A (RMS). In such a situation, the

nominal line voltage and current do not determine

the appropriate pulse-rate setting.

Instead, the

maximum line levels must be considered. As be-

fore, the given maximum line-voltage and line-cur-

rent levels are used to determine KV and KI:

KV = 150 mV / 250 V = 0.0006

KI =150 mV / 20A=0.0075

Ω

Again the sensor gains are calculated such that the

maximum line-voltage and line-current levels will

measure as 0.6 in the RMS Voltage Register and

RMS Current Register.

The required Pulse-Rate Register setting is now

determined by using the following equation:

Therefore PR = ~1.929 Hz.

Note that the Pulse-Rate Register cannot be set to

a frequency of exactly 1.929 Hz. The closest set-

ting that the Pulse-Rate Register can obtain is

0x00003E = 1.9375 Hz. To improve the accuracy,

either gain register can be programmed to correct

for the round-off error in PR. This value would be

calculated as

In the last example, suppose a value for MCLK/K

of 3.05856 MHz. When MCLK/K is not equal to

4.096 MHz, the result for ‘PR’ that is calculated for

the Pulse-Rate Register must be scaled by a cor-

rection factor of: 4.096 MHz / (MCLK/K). In this

case the result is scaled by 4.096/3.05856 to get a

final PR result of ~2.583 Hz.

3.2 Pulse Output for Normal Format,

Stepper Motor Format and Mechanical

Counter Format

The duration and shape of the pulse outputs at the

EOUT and EDIR pins can be set for three different

output formats. The default setting is for Normal

output pulse format. When the pulse is set to either

of the other two formats, the time duration and/or

the relative timing of the EOUT and EDIR pulses is

increased/varied such that the pulses can drive ei-

ther an electro-mechanical counter or a stepper

motor. The EOUT and EDIR output pins are capa-

ble

driving

certain

low-voltage/low-power

counters/stepper motors directly. This depends on

the drive current and voltage level requirements of

the counter/motor. The ability to set the pulse out-

put format to one of the three available formats is

controlled by setting certain bits in the Control Reg-

ister.

3.2.1 Normal Format

Referring to the description of the Control Register

in Section 5., Register Descriptions, if both the

MECH and STEP bits are set to ‘0’, the pulse out-

put format at the EOUT and EDIR pins is illustrated

in Figure 10. These are active-low pulses with very

short duration. The pulse duration is an integer

multiple of MCLK cycles, approximately equal to

1/16 of the period of the contents of the Pulse-Rate

tings less than the sampling rate (which is

[MCLK/8]/1024), the pulse duration will remain at a

constant duration, which is equal to the duration of

the pulses when the Pulse-Rate Register is set to

[MCLK/K]/1024. The maximum pulse frequency

from the EOUT pin is therefore [MCLK/K]/16.

When energy is positive, EDIR is always high.

When energy is negative, EDIR has the same out-

put as EOUT. When MCLK/K is not equal to

4.096 MHz, the true pulse-rate can be found by

first calculating what the pulse-rate would be if a

4.096 MHz crystal is used (with K = 1) and then

scaling

the

result

factor

(MCLK/K) / 4.096 MHz.

When set to run in Normal pulse output format, the

pulses may be sent out in “bursts” depending on

both the value of the Pulse-Rate Register as well

as the amount of billable energy that was regis-

tered by the CS5460A over the most recent A/D

sampling period, which is (in Hz): 1 / [(MCLK/K) /

1024]. A running total of the energy accumulation

is maintained in an internal register (not accessible

to the user) inside the CS5460A. If the amount of

energy that has accumulated in this register over

the most recent A/D sampling period is equal to or

greater than the amount of energy that is repre-

500

pulses

kW hr

⋅

------------------

1hr

3600s

--------------

1kW

1000W

------------------

250mV

------------------

250mV

------------------

⋅⋅

⋅

Ign or Vgn

1.929

-------------

1.00441

≅

0x404830

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