C508_01 Datasheet, PDF (103/267 Pages) Infineon Technologies AG – 8-Bit CMOS Microcontroller

C508
On-Chip Peripheral Components
6.2.2.3.3 Compare Mode 1
In compare mode 1, the software adoptively determines the transition of the output
signal. lt is commonly used when output signals are not related to a constant signal
period (as in a standard PWM Generation) but must be controlled very precisely with
high resolution and without jitter. In compare mode 1, both transitions of a signal can be
controlled. Compare outputs in this mode can be regarded as high speed outputs which
are independent of the CPU activity.
lf 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, one can choose whether the output signal is 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
the Timer 2 count matches the stored compare value.
Figure 6-19 and Figure 6-20 show functional diagrams of the timer/compare register/
port latch configuration in compare mode 1. In this function, the port latch consists of two
separate latches. The upper latch (which acts as a “shadow latch”) can be written under
software control, but its value will only be transferred to the output latch (and thus to the
port pin) in response to a compare match.
Note that the double latch structure is transparent as long as the internal compare signal
is active. While the compare signal is active, a write operation to the port will then change
both latches. This may become important when driving Timer 2 with a slow external
clock. In this case the compare signal could be active for many machine cycles in which
the CPU could unintentionally change the contents of the port latch.
A read-modify-write instruction will read the user-controlled “shadow latch” and write the
modified value back to this “shadow-latch”. A standard read instruction will read the pin
of the corresponding compare output, as usual.
User’s Manual
6-38
2001-05