Timer Input Mode; Timer Input Mode 2; Timer Input Mode 4 - Motorola MC6805R Series Advance Information

External
Input
NOTES:
Cleared by
TCR3
Write Read
Interrupt
Software Functions
1. The prescaler and 8-bit counter are clocked on the rising edge of the internal clock (phase two) or external input.
2. The counter is written to during data strobe (OS) and counts down continuously.
Figure 5-2. MC6805R3/ MC6805U3 Timer Block Diagram
The prescaler is a 7-bit divider which is used to extend the maximum length of the timer. Bit 0, bit 1,
and bit 2 of the TCR are programmed to choose the appropriate prescaler output which is used as
the counter input. The processor cannot write into or read from the prescaler; however, its contents
are cleared to all zeros by the write operation into TCR when bit 3 of the written data equals one,
which allows for truncation-free counting.
The timer input can be configured for three different operating modes, plus a disable mode,
depending on the value written to the TCR4 and TCR5 control bits. Refer to 5.2.5 Timer Control
Register (TCR) for further information.
5.2.1 Timer Input Mode 1
If TCR5 and TCR4 are both programmed to a zero, the input to the timer is from an internal clock
and the external TIMER input is disabled. The internal clock mode can be used for periodic interrupt
generation, as well as a reference in frequency and event measurement. The internal clock is the in-
struction cycle clock.
5.2.2 Timer Input Mode 2
With TCR5= 0 and TCR4= 1, the internal clock and the TIMER input pin are ANDed to form the
timer input Signal. This mode can be used to measure external pulse widths. The external timer in-
put pulse simply turns on the internal clock for the duration of the pulse widths.
5.2.3 Timer Input Mode 3
If TCR5= 1 and TCR4= 0, then all inputs to the timer are disabled.
5.2.4 Timer Input Mode 4
If TCR5= 1 and TCR4= 1, the internal clock input to the timer is disabled and the TIMER input pin
5-3