Auto-Reload Mode; Figure 23.5. Tn Auto-Reload (T2,3,4) And Toggle Mode (T2,4) Block Diagram - Silicon Laboratories C8051F12 Series Manual

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

23.2.3. Auto-Reload Mode

In Auto-Reload Mode, the counter/timer can be configured to count up or down and cause an interrupt/flag
to occur upon an overflow/underflow event. When counting up, the counter/timer will set its overflow/under-
flow flag (TFn) and cause an interrupt (if enabled) upon overflow/underflow, and the values in the Reload/
Capture Registers (RCAPnH and RCAPnL) are loaded into the timer and the timer is restarted. When the
Timer External Enable Bit (EXENn) bit is set to '1' and the Decrement Enable Bit (DCENn) is '0', a falling
edge ('1'-to-'0' transition) on the TnEX pin will cause a timer reload. Note that timer overflows will also
cause auto-reloads. When DCENn is set to '1', the state of the TnEX pin controls whether the counter/timer
counts up (increments) or down (decrements), and will not cause an auto-reload or interrupt event (Timer 3
shares the T2EX pin with Timer 2). See
to count down.
When counting down, the counter/timer will set its overflow/underflow flag (TFn) and cause an interrupt (if
enabled) when the value in the TMRnH and TMRnL registers matches the 16-bit value in the Reload/Cap-
ture Registers (RCAPnH and RCAPnL). This is considered an underflow event, and will cause the timer to
load the value 0xFFFF. The timer is automatically restarted when an underflow occurs.
Counter/Timer with Auto-Reload mode is selected by clearing the CP/RLn bit. Setting TRn to logic 1
enables and starts the timer.
In Auto-Reload Mode, the External Flag (EXFn) toggles upon every overflow or underflow and does not
cause an interrupt. The EXFn flag can be used as the most significant bit (MSB) of a 17-bit counter.
.
2
SYSCLK 12
External Clock
8
(XTAL1)
Tn
Crossbar
EXENn
TnE Crossbar
X

Figure 23.5. Tn Auto-reload (T2,3,4) and Toggle Mode (T2,4) Block Diagram

Section 23.2.1
TMRnCF
D
T T T T
E
n n O n
C
M M G O
E
1 0 n E
n
0
1
TCLK
TRn
Reload
for information concerning configuration of a timer
Toggle Logic
0xFF 0xFF
OVF
TMRnL TMRnH
RCAPnL RCAPnH
Rev. 1.4
0
Tn
(Port Pin)
1
CP/RLn
C/Tn
TRn
EXENn
Interrupt
EXFn
TFn
319