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ST7285C Datasheet(HTML) 37 Page - STMicroelectronics

Part No. ST7285C
Description  8-BIT MCU FOR RDS WITH 48K ROM, 3K RAM, ADC, TWO TIMERS, TWO SPIs, I2C AND SCI INTERFACES
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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ST7285C Datasheet(HTML) 37 Page - STMicroelectronics

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ST7285C
16-BIT TIMER (Cont’d)
When the counter rolls over from FFFFh to 0000h,
the Timer Overflow flag (TOF) of the Timer Status
Register (TSR) is set. A timer interrupt is then gen-
erated if the TOIE enable bit of the Timer Control
Register (TCR1) is set, provided the I bit of the
CCR is cleared. If one of these conditions is false,
the interrupt remains pending to be issued as soon
as they are both true. The interrupt request is
cleared by reading TSR while TOF is set, followed
by an access (read or write) to the LSB of the
Counter Register.
The TOF flag is not affected by accesses to the Al-
ternate Counter Register. This feature allows si-
multaneous use of the overflow function and reads
of the free running counter at random times (for
example, to measure elapsed time) without risking
to clear the TOF flag erroneously. Accesses to the
timer without the intention of servicing the TOF
flag should therefore be performed to the Alternate
Counter Register while only the TOF service rou-
tine accesses the Counter Register.
The free running counter can be reset under soft-
ware control, by writing to the LSB of the Counter
Register or of the Alternate Counter Register. The
counter and the prescaler are then configured to
their reset conditions. This reset also completes
any 16-bit access sequence. All flags and enable
bits are unchanged.
The value in the counter registers repeats every
131,072, 262,144, or 524,288 internal processor
clock cycles, depending on the clock control option
selected in TCR2. As shown in the timing dia-
grams, the counter increment is triggered by a fall-
ing edge of the CPU clock.
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 External Clock
When the external clock is selected by setting the
relevant clock control bits in TCR2, the counter
clocks on each external clock rising edge, if
EXEDG in TCR2 is set, or the falling edges if reset,
and is synchronised with the falling edge of the in-
ternal 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.
The parasitic pulses generated by the EXTEDG
and EXTCLKE transitions are filtered during two
clock periods, so the manipulation of the external
clock control bits must occur before or after two in-
ternal clock periods.


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