SDA5250 Datasheet, PDF (81/143 Pages) Siemens Semiconductor Group – TVTEXT 8-Bit Microcontroller

SDA 525x
Timer/Counter 0: Mode Selection
Timer/counter 0 can be configured in one of four operating modes, which are selected
by bit-pairs (M1, M0) in TMOD-register (see page 83).
– Mode 0
Putting timer/counter 0 into mode 0 makes it look like an 8048 timer, which is an 8-bit
counter with a divide-by-32 prescaler. Figure 25 shows the mode 0 operation as it
applies to timer 0.
In this mode, the timer register is configured as a 13-bit register. As the count rolls over
from all 1 s to all 0 s, it sets the timer interrupt flag TF0. The counted input is enabled to
the timer when TR0 = 1 and either GATE = 0 or INT0 = 1. (Setting GATE = 1 allows the
timer to be controlled by external input INT0, to facilitate pulse width measurements.)
TR0 is a control bit in the special function register TCON (see page 84). GATE is
contained in register TMOD (see page 83).
The 13-bit register consists of all 8 bits of TH0 and the lower 5 bits of TL0. The upper 3
bits of TL0 are indeterminate and should be ignored. Setting the run flag (TR0) does not
clear the registers.
– Mode 1
Mode 1 is the same as mode 0, except that the timer/counter 0 register is being run with
all 16 bits.
– Mode 2
Mode 2 configures the timer/counter 0 register as an 8-bit counter (TL0) with automatic
reload, as shown on see page 83. Overflow from TL0 not only sets TF0, but also reloads
TL0 with the contents of TH0, which is preset by software. The reload leaves TH0
unchanged.
– Mode 3
Timer/counter 0 in mode 3 establishes TL0 and TH0 as two separate counters. The logic
for mode 3 on timer 0 is shown in Figure 27. TL0 uses the timer 0 control bits: C/T,
GATE, TR0, INT0 and TF0. TH0 is locked into a timer function (counting machine cycles)
and takes over the use of TR1 and TF1 from timer 1. Thus, TH0 now controls the “timer
1” interrupt.
Mode 3 is provided for applications requiring an extra 8-bit timer or counter. With timer
0 in mode 3, the processor can operate as if it has three timers/counters. When timer 0
is in mode 3, timer 1 can be turned on and off by switching it out of and into its own mode
3, or can still be used in any application not requiring an interrupt.
Semiconductor Group
81
1998-04-08