st62t65b STMicroelectronics, st62t65b Datasheet - Page 38

• Manufacturer Part Number: st62t65b
• Description: 8-bit Otp/eprom Mcus With A/d Converter, Auto-reload Timer, Eeprom And Spi
• Manufacturer: STMicroelectronics

38
ST62T55B ST62T65B/E65B
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 2
The peripheral may be configured in three different
operating modes.
21 shows the Timer Block Diagram. The external
TIMER pin is available to the user. The content 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 reg-
ister 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.
Figure 21. Timer Block Diagram
38/74
TIMER
PSC
f
OSC
8
6
5
4
3
2
1
0
:12
SYNCHRONIZATION
LOGIC
SELECT
1 OF 7
15
.
The prescaler input can be the internal frequency
f
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 mul-
tiplexed 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 pres-
caler 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 regis-
ter must be set to “1” to allow the prescaler (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 reg-
ister.
22 illustrates the Timer’s working principle.
INT
DATABUS 8
COUNTER
8-BIT
divided by 12 or an external clock applied to
8
3
TMZ ET I
b7
LATCH
b6
STATUS/CONTROL
TO UT
b 5
REGISTER
DOUT
b4
8
b3
PSI
b 2
P S2
b 1
P S1
VA00009
b0
P S0
INTERRUPT
LINE