ST72E331 datasheet(44/107 Pages) STMICROELECTRONICS

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ST72E331 ST72T331

16-BIT TIMER (Cont’d)

5.4.3.3 Input Capture

In this section, the index,

i, may be 1 or 2 because

there are 2 input capture functions in the 16-bit

timer.

The two input capture 16-bit registers (IC1R and

IC2R) are used to latch the value of the free run-

ning counter after a transition is detected by the

ICAP

i pin (see figure 5).

The IC

iR register is a read-only register.

The active transition is software programmable

through the IEDG

i bit of Control Registers (CRi).

Timing resolution is one count of the free running

counter: (

fCPU/CC[1:0]).

Procedure:

To use the input capture function, select the fol-

lowing in the CR2 register:

– Select the timer clock (CC[1:0]) (see Table 1).

– Select the edge of the active transition on the

ICAP2 pin with the IEDG2 bit (the ICAP2 pin

must be configured as a floating input).

And select the following in the CR1 register:

– Set the ICIE bit to generate an interrupt after an

input capture coming from either the ICAP1 pin

or the ICAP2 pin

– Select the edge of the active transition on the

ICAP1 pin with the IEDG1 bit (the ICAP1pin must

be configured as a floating input).

When an input capture occurs:

– The ICF

i bit is set.

– The IC

iR register contains the value of the free

running counter on the active transition on the

ICAP

i pin (see Figure 6).

– A timer interrupt is generated if the ICIE bit is set

and the I bit is cleared in the CC register. Other-

wise, the interrupt remains pending until both

conditions become true.

Clearing the Input Capture interrupt request (i.e.

clearing the ICF

i bit) is done in two steps:

1. Reading the SR register while the ICF

i bit is set.

2. An access (read or write) to the IC

iLR register.

Notes:

1. After reading the IC

iHR register, the transfer of

input capture data is inhibited and ICF

i will

never be set until the IC

iLR register is also

read.

2. The IC

iR register contains the free running

counter value which corresponds to the most

recent input capture.

3. The 2 input capture functions can be used

together even if the timer also uses the 2 output

compare functions.

4. In One Pulse mode and PWM mode only the

input capture 2 function can be used.

5. The alternate inputs (ICAP1 & ICAP2) are

always directly connected to the timer. So any

transitions on these pins activate the input cap-

ture function.

Moreover if one of the ICAP

i pin is configured

as an input and the second one as an output,

an interrupt can be generated if the user tog-

gles the output pin and if the ICIE bit is set.

This can be avoided if the input capture func-

tion

i is disabled by reading the ICiHR (see note

1).

6. The TOF bit can be used with an interrupt in

order to measure events that exceed the timer

range (FFFFh).

MS Byte

LS Byte

ICiR

iHR

iLR

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