Rainbow Electronics ATAR862-3 - Manual (Page 44)

44
ATAR862-3
4556B–4BMCU–02/03
Timer 2 8-/12-bit Timer for:
• Interrupt, square-wave, pulse and duty cycle generation
• Baud-rate generation for the internal shift register
• Manchester and Biphase modulation together with the SSI
• Carrier frequency generation and modulation together with the SSI
Timer 2 can be used as an interval timer for interrupt generation, as signal generator or
as baud-rate generator and modulator for the serial interface. It consists of a 4-bit and
an 8-bit up counter stage which both have compare registers. The 4-bit counter stages
of Timer 2 are cascadable as a 12-bit timer or as an 8-bit timer with 4-bit prescaler. The
timer can also be configured as an 8-bit timer and separate a 4-bit prescaler.
The Timer 2 input can be supplied via the system clock, the external input clock (T2I),
the Timer 1 output clock, the Timer 3 output clock or the shift clock of the serial inter-
face. The external input clock T2I is not synchronized with SYSCL. Therefore, it is
possible to use Timer 2 with a higher clock speed than SYSCL. Furthermore, with that
input clock the Timer 2 operates in the power-down mode SLEEP (CPU core -> sleep
and OSC-Stop -> yes) as well as in the POWER-DOWN (CPU core -> sleep and OSC-
Stop -> no). All other clock sources supplied no clock signal in SLEEP. The 4-bit counter
stages of Timer 2 have an additional clock output (POUT).
Its output has a modulator stage that allows the generation of pulses as well as the gen-
eration and modulation of carrier frequencies. The Timer 2 output can modulate with the
shift register data output to generate Biphase- or Manchester code.
If the serial interface is used to modulate a bitstream, the 4-bit stage of Timer 2 has a
special task. The shift register can only handle bitstream lengths divisible by 8. For other
lengths, the 4-bit counter stage can be used to stop the modulator after the right bitcount
is shifted out.
If the timer is used for carrier frequency modulation, the 4-bit stage works together with
an additional 2-bit duty cycle generator like a 6-bit prescaler to generate carrier fre-
quency and duty cycle. The 8-bit counter is used to enable and disable the modulator
output for a programmable count of pulses.
For programming the time interval, the timer has a 4-bit and an 8-bit compare register.
For programming the timer function, it has four mode and control registers. The compar-
ator output of stage 2 is controlled by a special compare mode register (T2CM). This
register contains mask bits for the actions (counter reset, output toggle, timer interrupt)
which can be triggered by a compare match event or the counter overflow. This archi-
tecture enables the timer function for various modes.
The Timer 2 has a 4-bit compare register (T2CO1) and an 8-bit compare register
(T2CO2). Both these compare registers are cascadable as a 12-bit compare register, or
8-bit compare register and 4-bit compare register.
For 12-bit compare data value: m = x +1 0 £ x £ 4095
For 8-bit compare data value: n = y +1 0
£
y
£
255
For 4-bit compare data value: l = z +1 0
£
z
£
15