Silicon Laboratories C8051F347 - Manual (Page 235)

Rev. 1.3 235
C8051F340/1/2/3/4/5/6/7/8/9/A/B/C/D
21. Timers
Each MCU 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 timer for use with the ADC, SMBus, USB (frame measure-
ments), Low-Frequency Oscillator (period measurements), 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 offer
16-bit and split 8-bit timer functionality with auto-reload.
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 (See SFR Definition 21.3 for pre-scaled clock selection).
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 fre-
quency of up to one-fourth the system clock's frequency can be counted. The input signal need not be peri-
odic, but it should be held at a given level for at least two full system clock cycles to ensure the level is
properly sampled.
21.1. Timer 0 and Timer 1
Each timer is implemented as a 16-bit register accessed as two separate bytes: a low byte (TL0 or TL1)
and a high byte (TH0 or TH1). The Counter/Timer Control register (TCON) is used to enable Timer 0 and
Timer 1 as well as indicate status. Timer 0 interrupts can be enabled by setting the ET0 bit in the IE register
(
Section “9.3.5. Interrupt Register Descriptions” on page 90 ); Timer 1 interrupts can be enabled by
setting the ET1 bit in the IE register ( Section 9.3.5 ). Both counter/timers operate in one of four primary
modes selected by setting the Mode Select bits T1M1-T0M0 in the Counter/Timer Mode register (TMOD).
Each timer can be configured independently. Each operating mode is described below.
21.1.1. Mode 0: 13-bit Counter/Timer
Timer 0 and Timer 1 operate as 13-bit counter/timers in Mode 0. The following describes the configuration
and operation of Timer 0. However, both timers operate identically, and Timer 1 is configured in the same
manner as described for Timer 0.
The TH0 register holds the eight MSBs of the 13-bit counter/timer. TL0 holds the five LSBs in bit positions
TL0.4-TL0.0. The three upper bits of TL0 (TL0.7-TL0.5) are indeterminate and should be masked out or
ignored when reading. As the 13-bit timer register increments and overflows from 0x1FFF (all ones) to
0x0000, the timer overflow flag TF0 (TCON.5) is set and an interrupt will occur if Timer 0 interrupts are
enabled.
Timer 0 and Timer 1 Modes: Timer 2 Modes: Timer 3 Modes:
13-bit counter/timer
16-bit timer with auto-reload 16-bit timer with auto-reload
16-bit counter/timer
8-bit counter/timer with auto-reload Two 8-bit timers with
auto-reload
Two 8-bit timers with
auto-reload Two 8-bit counter/timers (Timer 0 only)