L9805 datasheet(34/103 Pages) STMICROELECTRONICS

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L9805

16-BIT TIMER (Cont’d)

16-bit read sequence: (from either the Counter

The user must read the MSB first, then the LSB

value is buffered automatically.

This buffered value remains unchanged until the

16-bit read sequence is completed, even if the

user reads the MSB several times.

After a complete reading sequence, if only the

CLR register or ACLR register are read, they re-

turn the LSB of the count value at the time of the

read.

An overflow occurs when the counter rolls over

from FFFFh to 0000h then:

– The TOF bit of the SR register is set.

– A timer interrupt is generated if:

– TOIE bit of the CR1register is set and

– I bit of the CCR register is cleared.

If one of these conditions is false, the interrupt re-

mains pending to be issued as soon as they are

both true.

Clearing the overflow interrupt request is done by:

1. Reading the SR register while the TOF bit is

set.

2. An access (read or write) to the CLR register.

Notes: The TOF bit is not cleared by accesses to

ACLR register. This feature allows simultaneous

use of the overflow function and reads of the free

running counter at random times (for example, to

measure elapsed time) without the risk of clearing

the TOF bit erroneously.

The timer is not affected by WAIT mode.

In HALT mode, the counter stops counting until the

mode is exited. Counting then resumes from the

previous count (MCU awakened by an interrupt) or

from the reset count (MCU awakened by a Reset).

5.2.3.2 External Clock

The external clock (where available) is selected if

CC0=1 and CC1=1 in CR2 register.

The status of the EXEDG bit determines the type

of level transition on the external clock pin EXCLK

that will trigger the free running counter.

The counter is synchronised with the falling edge

of the internal CPU clock.

At least four falling edges of the CPU clock must

occur between two consecutive active edges of

the external clock; thus the external clock frequen-

cy must be less than a quarter of the CPU clock

frequency.

LSB is buffered

Read MSB

At t0

Read LSB

Returns the buffered

LSB value at t0

At t0 +

∆t

Other

instructions

Beginning of the sequence

Sequence completed

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