C8051F850-C-GM Datasheet, PDF (273/328 Pages) Silicon Laboratories – Low-Cost 8-bit MCU Family with up to 8 kB of Flash

25. Timers (Timer0, Timer1, Timer2 and Timer3)
Each MCU in the C8051F85x/86x family includes four counter/timers: two are 16-bit counter/timers
compatible with those found in the standard 8051, and two are 16-bit auto-reload timers for timing
peripherals or for general purpose use. These timers can be used to measure time intervals, count
external events and generate periodic interrupt requests. Timer 0 and Timer 1 are nearly identical and
have four primary modes of operation. Timer 2 and Timer 3 are also identical and offer both 16-bit and split
8-bit timer functionality with auto-reload capabilities. Timer 2 and Timer 3 both offer a capture function, but
are different in their system-level connections. Timer 2 is capable of performing a capture function on an
external signal input routed through the crossbar, while the Timer 3 capture is dedicated to the low-
frequency oscillator output. Table 25.1 summarizes the modes available to each timer.
Table 25.1. Timer Modes
Timer 0 and Timer 1 Modes
13-bit counter/timer
16-bit counter/timer
8-bit counter/timer with auto-reload
Two 8-bit counter/timers
(Timer 0 only)
Timer 2 Modes
16-bit timer with auto-reload
Two 8-bit timers with auto-reload
Input pin capture
Timer 3 Modes
16-bit timer with auto-reload
Two 8-bit timers with auto-reload
Low-frequency oscillator capture
Timers 0 and 1 may be clocked by one of five sources, determined by the Timer Mode Select bits (T1M–
T0M) and the Clock Scale bits (SCA1–SCA0). The Clock Scale bits define a pre-scaled clock from which
Timer 0 and/or Timer 1 may be clocked.
Timer 0/1 may then be configured to use this pre-scaled clock signal or the system clock. Timer 2 and
Timer 3 may be clocked by the system clock, the system clock divided by 12, or the external oscillator
clock source divided by 8.
Timer 0 and Timer 1 may also be operated as counters. When functioning as a counter, a counter/timer
register is incremented on each high-to-low transition at the selected input pin (T0 or T1). Events with a
frequency of up to one-fourth the system clock frequency can be counted. The input signal need not be
periodic, but it must be held at a given level for at least two full system clock cycles to ensure the level is
properly sampled.
All four timers are capable of clocking other peripherals and triggering events in the system. The individual
peripherals select which timer to use for their respective functions. Table 25.2 summarizes the peripheral
connections for each timer. Note that the Timer 2 and Timer 3 high overflows apply to the full timer when
operating in 16-bit mode or the high-byte timer when operating in 8-bit split mode.
Table 25.2. Timer Peripheral Clocking / Event Triggering
Function
T0
Overflow
UART0 Baud Rate
SMBus0 Clock Rate
X
SMBus0 SCL Low Timeout
PCA0 Clock
X
T1
Overflow
X
X
T2 High
Overflow
X
T2 Low
Overflow
X
T3 High
Overflow
X
T3 Low
Overflow
Rev. 1.0
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