8-Bit Timers With Auto-Reload; Figure 21.9. Timer 3 8-Bit Mode Block Diagram - Silicon Laboratories C8051F341 Product Manual

C8051F340/1/2/3/4/5/6/7/8/9/A/B/C/D

21.3.2. 8-bit Timers with Auto-Reload

When T3SPLIT is '1' and T3CE = '0', Timer 3 operates as two 8-bit timers (TMR3H and TMR3L). Both 8-bit
timers operate in auto-reload mode as shown in Figure 21.5. TMR3RLL holds the reload value for TMR3L;
TMR3RLH holds the reload value for TMR3H. The TR3 bit in TMR3CN handles the run control for TMR3H.
TMR3L is always running when configured for 8-bit Mode.
Each 8-bit timer may be configured to use SYSCLK, SYSCLK divided by 12, or the external oscillator clock
source divided by 8. The Timer 3 Clock Select bits (T3MH and T3ML in CKCON) select either SYSCLK or
the clock defined by the Timer 3 External Clock Select bit (T3XCLK in TMR3CN), as follows:
T3MH T3XCLK
0 0
0 1
1 X
The TF3H bit is set when TMR3H overflows from 0xFF to 0x00; the TF3L bit is set when TMR3L overflows
from 0xFF to 0x00. When Timer 3 interrupts are enabled, an interrupt is generated each time TMR3H over-
flows. If Timer 3 interrupts are enabled and TF3LEN (TMR3CN.5) is set, an interrupt is generated each
time either TMR3L or TMR3H overflows. When TF3LEN is enabled, software must check the TF3H and
TF3L flags to determine the source of the Timer 3 interrupt. The TF3H and TF3L interrupt flags are not
cleared by hardware and must be manually cleared by software.
T3XCLK
SYSCLK / 12
0
External Clock / 8
1

Figure 21.9. Timer 3 8-Bit Mode Block Diagram

250
TMR3H Clock Source
SYSCLK / 12
External Clock / 8
SYSCLK
CKCON
T T S S
T T T T
3 3 C C
2 2 1 0
M M M M M M A A
H L H L 1 0
0
TR3
1
SYSCLK
1
0
Rev. 1.3
T3ML T3XCLK
0 0
0 1
1 X
Reload
TMR3RLH
TCLK
TMR3H
Reload
TMR3RLL
TCLK
TMR3L
TMR3L Clock Source
SYSCLK / 12
External Clock / 8
SYSCLK
To ADC
TF3H
Interrupt
TF3L
TF3LEN
T3CE
T3SPLIT
TR3
T3CSS
T3XCLK