Timer Control Register (Tcr) - Motorola MC6805R Series Advance Information

becomes the input to the timer. The external TIMER pin can, in this mode, be used to count exter-
nal events as well as external frequencies for generating periodic interrupts.
5.2.5 Timer Control Register (TCR)
I
TCR 7 1 TCR6\ TCR51 TCR41 TCR3*
I
TCR21 TCR1
I
TCRO
I
$009
*Write only (read as zero)
TCR7 - Timer interrupt request bit: indicates the timer interrupt when it is a logic one.
1- Set whenever the counter decrements to zero, or under program control.
0-
Cleared on external reset, power-on reset, or program control (write).
TCR6 - Timer interrupt mask bit: inhibits the timer interrupt to the processor, when this bit is a
logic one.
1 - Set on external reset, power-on reset, or program control.
0- Cleared under program control.
TCR5 - External or internal bit: selects the input clock source to be either the external TIMER pin
or the internal clock (unaffected by RESET).
1 - Select external clock source. Set to a logic one on external reset, power-on reset, or
program control.
0- Select internal clock source (phase two). Cleared under program control.
TCR4 - External enable bit: control bit used to enable the external TIMER pin (unaffected by
RESET).
1- Enable external TIMER pin. Set on external reset, power-on reset, or program control.
0-
Disable external TIMER pin. Cleared under program control.
TCR5
o
1
1
1
TCR4
o
1
o
1
Internal clock to timer
AND of internal clock and TIMER pin to timer
Input to timer disabled
TIMER pin to timer
TCR3 - Timer prescaler reset bit: writing a one to this bit resets the prescaler to zero. A read of
this location always indicates a zero (unaffected by RESET),
TCR2, TCR1, and TCRO - Prescaler address bits: decoded to select one of eight outputs of the
prescaler (set to all ones by RESET),
PRESCALER
TCR2 TCR1 TCRO Result
TCR2 TCR1 TCRO Result
0 0
0
+1
1
0 0 +16
0 0 1 +2
1 0 1
+32
0 1 0 +4
1 1
0
+64
0 1 1
+8
1 1
1 + 128
5-4