C517A Datasheet, PDF (36/72 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

C517A
Compare Modes
The compare function of a timer/register combination operates as follows : the 16-bit value stored
in a compare or compare/capture register is compared with the contents of the timer register; if the
count value in the timer register matches the stored value, an appropriate output signal is generated
at a corresponding port pin and an interrupt can be generated.
Compare Mode 0
In compare mode 0, upon matching the timer and compare register contents, the output signal
changes from low to high. lt goes back to a low level on timer overflow. As long as compare mode
0 is enabled, the appropriate output pin is controlled by the timer circuit only and writing to the port
will have no effect. Figure 14 shows a functional diagram of a port circuit when used in compare
mode 0. The port latch is directly controlled by the timer overflow and compare match signals. The
input line from the internal bus and the write-to-latch line of the port latch are disconnected when
compare mode 0 is enabled.
Port Circuit
Compare Register
Circuit
Compare Reg.
16 Bit
Comparator
16 Bit
Timer Register
Timer Circuit
Compare
Match
Timer
Overflow
Internal
Bus
Write to
Latch
Read Latch
V^V_CC^DD
S
Q
Port
D _Port
Pin
Latch
CLK
R
Q
Read Pin
MCS02661
Figure 14
Port Latch in Compare Mode 0
Compare Mode 1
If compare mode 1 is enabled and the software writes to the appropriate output latch at the port, the
new value will not appear at the output pin until the next compare match occurs. Thus, it can be
choosen whether the output signal has to make a new transition (1-to-0 or 0-to-1, depending on the
actual pin-level) or should keep its old value at the time when the timer value matches the stored
compare value.
In compare mode 1 (see figure 15) the port circuit consists of two separate latches. One latch
(which acts as a "shadow latch") can be written under software control, but its value will only be
transferred to the port latch (and thus to the port pin) when a compare match occurs.
Semiconductor Group
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