ST7LITE3 datasheet(62/167 Pages) STMICROELECTRONICS

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ST7LITE3

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DUAL 12-BIT AUTORELOAD TIMER 3 (Cont’d)

■ Long input capture

Pulses that last between 8µs and 2s can be meas-

ured with an accuracy of 4µs if fOSC = 8MHz in the

following conditions:

– The 12-bit AT3 Timer is clocked by the Lite Timer

(RTC pulse: CK[1:0] = 01 in the ATCSR register)

– The ICS bit in the ATCSR2 register is set so that

the LTIC pin is used to trigger the AT3 Timer cap-

ture.

– The signal to be captured is connected to LTIC

– Input Capture registers LTICR, ATICRH and

ATICRL are read

This configuration allows to cascade the Lite Timer

and the 12-bit AT3 Timer to get a 20-bit input cap-

ture value. Refer to Figure 43.

Figure 43. Long Range Input Capture Block Diagram

Notes:

1. Since the input capture flags (ICF) for both tim-

ers (AT3 Timer and LT Timer) are set when signal

transition occurs, software must mask one inter-

rupt by clearing the corresponding ICIE bit before

setting the ICS bit.

2. If the ICS bit changes (from 0 to 1 or from 1 to

0), a spurious transition might occur on the input

capture signal because of different values on LTIC

and ATIC. To avoid this situation, it is recommend-

ed to do as follows:

– First, reset both ICIE bits.

– Then set the ICS bit.

– Reset both ICF bits.

– And then set the ICIE bit of desired interrupt.

3. How to compute a pulse length with long input

capture feature.

As both timers are used, computing a pulse length

is not straight-forward. The procedure is as fol-

lows:

– At the first input capture on the rising edge of the

pulse, we assume that values in the registers are

as follows:

LTICR = LT1

ATICRH = ATH1

ATICRL = ATL1

Hence ATICR1 [11:0] = ATH1 & ATL1

Refer to Figure 44 on page 63.

LTIC

ATIC

ICS

12-bit Input Capture Register

OFF

fcpu

fLTIMER

12-bit Upcounter1

12-bit AutoReload Register

8-bit Input Capture Register

8-bit Timebase Counter1

fOSC/32

LTICR

CNTR1

ATICR

ATR1

8 LSB bits

12 MSB bits

LITE TIMER

12-Bit ARTIMER

cascaded

bits

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