C8051F52x-53x
19.2.2. 8-bit Timers with Auto-Reload
When T2SPLIT is set, Timer 2 operates as two 8-bit timers (TMR2H and TMR2L). Both 8-bit timers oper-
ate in auto-reload mode as shown in Figure 19.5. TMR2RLL holds the reload value for TMR2L; TMR2RLH
holds the reload value for TMR2H. The TR2 bit in TMR2CN handles the run control for TMR2H. TMR2L 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 2 Clock Select bits (T2MH and T2ML in CKCON) select either SYSCLK or
the clock defined by the Timer 2 External Clock Select bit (T2XCLK in TMR2CN), as follows:
T2MH
T2XCLK TMR2H Clock Source
T2ML
T2XCLK TMR2L Clock Source
0
0
1
0
1
X
SYSCLK / 12
External Clock / 8
SYSCLK
0
0
1
0
1
X
SYSCLK / 12
External Clock / 8
SYSCLK
The TF2H bit is set when TMR2H overflows from 0xFF to 0x00; the TF2L bit is set when TMR2L overflows
from 0xFF to 0x00. When Timer 2 interrupts are enabled (IE.5), an interrupt is generated each time
TMR2H overflows. If Timer 2 interrupts are enabled and TF2LEN (TMR2CN.5) is set, an interrupt is gener-
ated each time either TMR2L or TMR2H overflows. When TF2LEN is enabled, software must check the
TF2H and TF2L flags to determine the source of the Timer 2 interrupt. The TF2H and TF2L interrupt flags
are not cleared by hardware and must be manually cleared by software.
CKCON
T T T T T T S S
3 3 2 2 1 0 C C
T2XCLK
M M M M M M A A
Reload
TMR2RLH
H L H L
1 0
SYSCLK / 12
0
1
0
External Clock / 8
TCLK
TF2H
TF2L
TF2LEN
TMR2H
Interrupt
TR2
1
T2SPLIT
TR2
Reload
TMR2RLL
T2XCLK
SYSCLK
1
0
TCLK
TMR2L
Figure 19.5. Timer 2 8-Bit Mode Block Diagram
194
Rev. 0.3
SILICON [ SILICON ]
