Motorola DSP56301 - Manual (Page 274)

Operating Modes
9 -12 DSP56301 User’s Manual
9.3.1.4 Timer Event Counter (Mode 3)
In Mode 3, the timer counts external events and issues an interrupt (if interrupt enable bits are
set) when the timer counts a preset number of events. The timer clock signal can be taken
from either the TIO input signal or the prescaler clock output. If an external clock is used, it
must be internally synchronized to the internal clock, and its frequency must be less than the
DSP56301 internal operating frequency divided by 4. The value of the TCSR[INV] bit
determines whether low-to-high (0 to 1) transitions or high-to-low (1 to 0) transitions
increment the counter. If the INV bit is set, high-to-low transitions increment the counter. If
the INV bit is cleared, low-to-high transitions increment the counter.
When the counter matches the value contained in the TCPR, TCSR[TCF] is set and a compare
interrupt is generated if the TCSR[TCIE] bit is set. If the TCSR[TRM] bit is set, the counter is
loaded with the value of the TLR when the next timer clock is received, and the count is
resumed. If the TCSR[TRM] bit is cleared, the counter continues to increment on each timer
clock. This process repeats until the timer is disabled.
Bit Settings Mode Characteristics
TC3 TC2 TC1 TC0 Mode Name Function TIO Clock
0 0 1 1 3 Event Counter Timer Input External
Figure 9-9. Event Counter Mode, TRM = 1
Mode 3 (internal clock): TRM = 1
N = write preload
M = write compare
TE
Clock
(TIO pin or prescale CLK)
TLR
TCPR
TCF (Compare Interrupt if TCIE = 1)
Counter (TCR)
first event
M
0 N N + 1 M N N + 1
N
interrupts every
M - N clock
periods
if clock source
is from TIO pin,
TIO < CPUCLK + 4
NOTE: If INV = 1, counter is clocked on 1-to-0 clock transitions, instead of 0-to-1 transitions.