Capture Mode; Figure 23.4. T2, 3, And 4 Capture Mode Block Diagram - Silicon Laboratories C8051F12 Series Manual

C8051F120/1/2/3/4/5/6/7 C8051F130/1/2/3

23.2.2. Capture Mode

In Capture Mode, Timer 2, 3, and 4 will operate as a 16-bit counter/timer with capture facility. When the
Timer External Enable bit (found in the TMRnCN register) is set to '1', a high-to-low transition on the TnEX
input pin (Timer 3 shares the T2EX pin with Timer 2) causes the 16-bit value in the associated timer (THn,
TLn) to be loaded into the capture registers (RCAPnH, RCAPnL). If a capture is triggered in the counter/
timer, the Timer External Flag (TMRnCN.6) will be set to '1' and an interrupt will occur if the interrupt is
enabled. See Section "11.3. Interrupt Handler" on page 154
figuration of interrupt sources.
As the 16-bit timer register increments and overflows TMRnH:TMRnL, the TFn Timer Overflow/Underflow
Flag (TMRnCN.7) is set to '1' and an interrupt will occur if the interrupt is enabled. The timer can be config-
ured to count down by setting the Decrement Enable Bit (TMRnCF.0) to '1'. This will cause the timer to
decrement with every timer clock/count event and underflow when the timer transitions from 0x0000 to
0xFFFF. Just as in overflows, the Overflow/Underflow Flag (TFn) will be set to '1', and an interrupt will
occur if enabled.
Counter/Timer with Capture mode is selected by setting the Capture/Reload Select bit CP/RLn
(TMRnCN.0) and the Timer 2, 3, and 4 Run Control bit TRn (TMRnCN.2) to logic 1. The Timer 2, 3, and 4
respective External Enable EXENn (TMRnCN.3) must also be set to logic 1 to enable captures. If EXENn
is cleared, transitions on TnEX will be ignored.
2
SYSCLK 12
External Clock
8
(XTAL1)
Crossbar
Tn
EXENn
TnE Crossbar
X

Figure 23.4. T2, 3, and 4 Capture Mode Block Diagram

318
TMRnCF
T T T T D
n n O n C
M M G O E
1 0 n E n
0
1
TCLK
TRn
Rev. 1.4
for further information concerning the con-
Toggle Logic
0xFF 0xFF
TMRnL TMRnH
CP/RLn
C/Tn
EXENn
EXFn
RCAPnL RCAPnH
0
Tn
(Port Pin)
1
TRn
Interrupt
TFn