AT89LP6440 datasheet(65/194 Pages) ATMEL

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3706A–MICRO–9/09

AT89LP6440 - Preliminary

Figure 12-6. Timer 2 Waveform: Auto-Reload Mode (DCEN = 1)

12.3.3

Dual Slope Counter

When Timer 2 Auto-Reload Mode uses Count Mode 2 (T2CM

1-0 = 10B) or Count Mode 3

(T2CM

1-0 = 11B), the timer operates in a dual slope fashion. The timer counts up from MIN to

TOP and then counts down from TOP to MIN, following a sawtooth waveform as shown in Fig-

ure 12-7. The EXF2 bit is set/cleared by hardware to reflect the current count direction (Up =

0and Down = 1). The value of TOP stored in RCAP2H and RCAP2L is double-buffered such that

a new TOP value takes affect only after an underflow. The only difference between Mode 2 and

Mode 3 is when the interrupt flag is set. In Mode 2 TF2 is set once per count period when the

timer underflows at MIN. In Mode 3 TF2 is set twice per count period, once when the timer over-

flows at TOP and once when the timer underflows at MIN. The interrupt service routine can

check the EXF2 bit to determine if TF2 was set at TOP or MIN. These count modes are provided

to support variable precision symmetrical PWM in the CCA. DCEN has no effect when using

dual slope operation. The Timer 2 overflow rate for this mode is given in the following equation:

MAX

MIN

BOTTOM

T2CM1-0 = 00B, DCEN = 1

MAX

MIN

TOP

T2CM1-0 = 01B, DCEN = 1

T2EX

TF2 Set

EXF2

Auto-Reload Mode:

DCEN = 0, T2CM = 10B

Time-out Period

RCAP2H RCAP2L

}

Oscillator Frequency

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TPS

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