ST72T311 ST Microelectronics, ST72T311 Datasheet - Page 37

• Manufacturer Part Number: ST72T311
• Description: 8-BIT MCU WITH 8 TO 16K OTP/EPROM
• Manufacturer: ST Microelectronics

16-BIT TIMER (Cont’d)
16-bit read sequence: (from either the Counter
Register or the Alternate Counter Register).
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.
Whatever the timer mode used (input capture, out-
put compare, one pulse mode or PWM mode) 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:
If one of these conditions is false, the interrupt re-
mains pending to be issued as soon as they are
both true.
At t0
At t0 + t
Beginning of the sequence
Sequence completed
– TOIE bit of the CR1 register is set and
– I bit of the CC register is cleared.
Read LSB
Read MSB
instructions
Other
Returns the buffered
LSB value at t0
LSB is buffered
Clearing the overflow interrupt request is done in
two steps:
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).
4.3.3.2 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 EXT-
CLK 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.
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