C8051F321-GMR Datasheet, PDF (222/250 Pages) Silicon Laboratories – Full Speed USB, 16 k ISP FLASH MCU Family

C8051F320/1
19.3. Timer 3
Timer 3 is a 16-bit timer formed by two 8-bit SFRs: TMR3L (low byte) and TMR3H (high byte). Timer 3 may
operate in 16-bit auto-reload mode, (split) 8-bit auto-reload mode, or USB Start-of-Frame (SOF) capture
mode. The Timer 3 operation mode is defined by the T3SPLIT (TMR3CN.3) and T3SOF (TMR2CN.4) bits.
Timer 3 may be clocked by the system clock, the system clock divided by 12, or the external oscillator
source divided by 8. The external clock mode is ideal for real-time clock (RTC) functionality, where the
internal oscillator drives the system clock while Timer 3 (and/or the PCA) is clocked by an external preci-
sion oscillator. Note that the external oscillator source divided by 8 is synchronized with the system clock.
19.3.1. 16-bit Timer with Auto-Reload
When T3SPLIT (TMR3CN.3) is zero, Timer 3 operates as a 16-bit timer with auto-reload. Timer 3 can be
clocked by SYSCLK, SYSCLK divided by 12, or the external oscillator clock source divided by 8. As the
16-bit timer register increments and overflows from 0xFFFF to 0x0000, the 16-bit value in the Timer 3
reload registers (TMR3RLH and TM3RLL) is loaded into the Timer 3 register as shown in Figure 19.4, and
the Timer 3 High Byte Overflow Flag (TMR3CN.7) is set. If Timer 3 interrupts are enabled, an interrupt will
be generated on each Timer 3 overflow. Additionally, if Timer 3 interrupts are enabled and the TF3LEN bit
is set (TMR3CN.5), an interrupt will be generated each time the lower 8 bits (TMR3L) overflow from 0xFF
to 0x00.
T3XCLK
CKCON
TTTTTTSS
3 3 2 2 1 0CC
MMMMMMA A
HLHL 10
SYSCLK / 12
0
External Clock / 8
1
SYSCLK
0
TR3
1
To ADC
TCLK _{TMR3L TMR3H}
TMR3RLL TMR3RLH
Reload
TF3H
TF3L
TF3LEN
T3SOF
T3SPLIT
TR3
T3XCLK
Figure 19.8. Timer 3 16-Bit Mode Block Diagram
Interrupt
222
Rev. 1.4