AVR465 datasheet(13/40 Pages) ATMEL

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AVR465

2566A-AVR-07/04

Equation 15.

Interval of Energy Pulse Output.

000

600

CLK

It should be noted that after the meter constant has been set there is only one

variable; active power, P. In order to save instruction cycles, the expression is

reduced to T = N/P, where N is a pre-calculated, constant numerator. The numerator

is calculated during firmware initialization, when MC is read from EEPROM. The

default numerator is as follows:

Equation 16.

Constant Numerator at Default Meter Constant.

250

406

000

024

000

600

000

600

CLK

After the required pulse interval has been calculated the timer/counter (T/C) is

programmed to produce a pulse output with an interval as close as possible to the

one derived. The 16-bit T/C is clocked by a prescaled system clock. The prescaler is

chosen such that even the smallest pulse intervals can be accurately represented.

The problem is to also be able to accurately produce pulses with very long intervals.

For this purpose, the T/C is extended with a firmware counter.

The regular T/C mechanism is illustrated in Figure 6. In this mode of operation the

energy pulse is set on each compare match.

Figure 6.

Basic Timer/Counter Operation.

A compare match with OCR1A sets the OC1A output and a compare match with

OCR1B clears the same. The pulse interval, tEP, is controlled by OCR1A and the

pulse width, tEPH, by OCR1B.

When the calculated energy pulse interval exceeds 16 bits the firmware extended, 16-

bit counter (TC1ext) is activated. The firmware counter value is increased each time

the hardware counter reaches 16-bit maximum, effectively making the extended

counter 32 bits in length. The extended counter mechanism is illustrated in Figure 7.

Regular Mode T/C

Extended Mode T/C

OCR1A

OCR1B

TCNT1

OC1A

(EP)

EPH

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