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ST62T35B/E35B
4.2 TIMER
The MCU features an on-chip Timer peripheral,
consisting of an 8-bit counter with a 7-bit program-
mable prescaler, giving a maximum count of 215.
The peripheral may be configured in three differ-
ent operating modes.
Figure 1 shows the Timer Block Diagram. The ex-
ternal TIMER pin is available to the user. The con-
tent of the 8-bit counter can be read/written in the
Timer/Counter register, TCR, while the state of the
7-bit prescaler can be read in the PSC register.
The control logic device is managed in the TSCR
register as described in the following paragraphs.
The 8-bit counter is decremented by the output
(rising edge) coming from the 7-bit prescaler and
can be loaded and read under program control.
When it decrements to zero then the TMZ (Timer
Zero) bit in the TSCR is set to “1”. If the ETI (Ena-
ble Timer Interrupt) bit in the TSCR is also set to
“1”, an interrupt request is generated as described
in the Interrupt Chapter. The Timer interrupt can
be used to exit the MCU from WAIT mode.
The prescaler input can be the internal frequency
fINT divided by 12 or an external clock applied to
the TIMER pin. The prescaler decrements on the
rising edge. Depending on the division factor pro-
grammed by PS2, PS1 and PS0 bits in the TSCR.
The clock input of the timer/counter register is
multiplexed to different sources. For division factor
1, the clock input of the prescaler is also that of
timer/counter; for factor 2, bit 0 of the prescaler
register is connected to the clock input of TCR.
This bit changes its state at half the frequency of
the prescaler input clock. For factor 4, bit 1 of the
PSC is connected to the clock input of TCR, and
so forth. The prescaler initialize bit, PSI, in the
TSCR register must be set to “1” to allow the pres-
caler (and hence the counter) to start. If it is
cleared to “0”, all the prescaler bits are set to “1”
and the counter is inhibited from counting. The
prescaler can be loaded with any value between 0
and 7Fh, if bit PSI is set to “1”. The prescaler tap is
selected by means of the PS2/PS1/PS0 bits in the
control register.
Figure 2 illustrates the Timer’s working principle.
Figure 21. Timer Block Diagram
DATABUS 8
8
8-BIT
COUNTER
6
5
4
3
2
1
0
PSC
STATUS/CONTROL
REGISTER
b7
b6
b5
b4
b3
b2
b1
b0
TMZ
ETI
TOUT
DOUT
PSI
PS2
PS1
PS0
SELECT
1OF 7
3
LATCH
SYNCHRONIZATION
LOGIC
TIMER
INTERRUPT
LINE
VA00009
:12
fOSC
38