mk68901 STMicroelectronics, mk68901 Datasheet - Page 13

• Manufacturer Part Number: mk68901
• Description: Multi.function Peripheral
• Manufacturer: STMicroelectronics

A is reprogrammed for another mode, interrupts will
again occur on the transition, as normally defined by
the edge bit. Note that, like changing the edge bit,
placing the timer into or taking it out of the pulse
width mode can produce a transition on the signal
to the interrupt channel and may cause an interrupt.
If measuring consecutive pulses, it is obvious that
one must read the contents of the timer and then rei-
nitialize the main counter by writing to the timer data
register. If the timer data register is written while the
pulse is going to the active state, the write operation
may result in an indeterminate value being written
into the main counter. If the timer is written after the
pulse goes active, the timer counts from the pre-
vious contents, and when it counts through H”01”,
the correct value is written into the timer. The pulse
width then includes counts from before the timer
was reloaded.
In the event count mode, the prescaler is disabled.
Each time the control input on TAI or TBi makes an
active transition as defined by the associated Inter-
rupt Channel’s edge bit, a count pulse will be gene-
rated, and the main counter will decrement. In all o-
ther respects, the timer functions as previously des-
cribed. Altering the edge bit while the timer is in the
event count mode can produce a count pulse. The
interrupt channel associated with the input (I3 for I4
for TAI) is allowed to function normally. To count
transitions reliably, the input must remain in each
state (1/O) for a length of time equal to four periods
of the timer clock ; thus signals of a frequency up to
one fourth of the timer clock can be counted.
The manner in which the timer output pins toggle
states has previously been described. All timer out-
puts will be forced low by a device RESET. The out-
put associated with Timers A and B will toggle on
each Time Out pulse regardless of the mode the ti-
mers are programmed to. In addition, the outputs
from Timers A and B can be forced low at any time
by writing a ”1” to the reset location in TACR and
Figure 16 : USART Data Register.
TBCR, respectively. The output will be forced to the
low state during the WRITE operation, and at the
conclusion of the operation, the output will again be
free to toggle each time a Time Out pulse occurs.
This feature will allow waveform generation.
During reset, the Timer Data Registers and the main
counters are not reset. Also, if using the reset option
on Timers A or B, one must make sure to keep the
other bits in the correct state so as not to affect the
operation of Timers A and B.
USART
Serial Communication is provided by a full-duplex
double-buffered USART, which is capable of either
asynchronous or synchronous operation. Variable
word length and start/stop bit configurations are
available under software control for asynchronous
operation. For synchronous operation, a Sync Word
is provided to establish synchronization during re-
ceive operations. The Sync Word will also be repea-
tedly transmitted when no other data is available for
transmission. Moreover, the MK68901 allows strip-
ping of all Sync Words received in synchronous o-
peration. The handshake control lines RR (Receiver
Ready) and TR (Transmitter Ready) allow DMA o-
peration. Separate receive and transmit clocks are
available, and separate receive and transmit status
and data bytes allow independent operation of the
transmit and receive sections.
The USART is provided with three Control/Status
Registers and a Data Register. The USART Data
Register form is illustrated in figure 16. The pro-
grammer may specify operational parameters for
the USART via the Control Register, as shown in fig-
ure 17. Status of both the Receiver and Transmitter
sections is accessed by means of the two Status Re-
gisters, as shown in figures 18 and 19. Data written
to the Data Register is passed to the transmitter,
while reading the Data Register will access data re-
ceived by the USART.
MK68901
V000362
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