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Using Timers to Measure Time
8-3
Timers
The individual bits of TMOD have the following functions:
7 6 5 4 3 2 1 0 Reset Value
SFR 8E
H
0 0 T2M T1M T0M MD2 MD1 MD0 01
H
T2M (bit 5)—Timer 2 Clock Select. This bit controls the division of the system
clock that drives Timer 2. This bit has no effect when the timer is in baud rate
generator or clock output modes. Clearing this bit to 0 maintains 80C32 com-
patibility. This bit has no effect on instruction cycle timing.
0: Timer 2 uses a divide by 12 of the crystal frequency.
1: Timer 2 uses a divide by 4 of the crystal frequency.
T1M (bit 4)—Timer 1 Clock Select. This bit controls the division of the system
clock that drives Timer 1. Clearing this bit to 0 maintains 8051 compatibility.
This bit has no effect on instruction cycle timing.
0: Timer 1 uses a divide by 12 of the crystal frequency.
1: Timer 1 uses a divide by 4 of the crystal frequency.
T0M (bit 3)—Timer 0 Clock Select. This bit controls the division of the system
clock that drives Timer 0. Clearing this bit to 0 maintains 8051 compatibility.
This bit has no effect on instruction cycle timing.
0: Timer 0 uses a divide by 12 of the crystal frequency.
1: Timer 0 uses a divide by 4 of the crystal frequency.
MD2, MD1, MD0 (bits 2-0)—Stretch MOVX Select 2−0. These bits select the
time by which external MOVX cycles are to be stretched. This allows slower
memory or peripherals to be accessed without using ports or manual software
intervention. The RD
or WR strobe will be stretched by the specified interval,
which will be transparent to the software except for the increased time to exe-
cute the MOVX instruction. All internal MOVX instructions on devices contain-
ing MOVX SRAM are performed at the 2 instruction cycle rate.
MD2 MD1 MD0
Stretch
Value
MOVX Duration
RD or WR
Strobe
Width
(SYS CLKs)
RD or WR
Strobe
Width (µs)
at 12MHz
0 0 0 0 2 Instruction Cycles 2 0.167
0 0 1 1 3 Instruction Cycles
(default)
4 0.333
0 1 0 2 4 Instruction Cycles 8 0.667
0 1 1 3 5 Instruction Cycles 12 1.000
1 0 0 4 6 Instruction Cycles 16 1.333
1 0 1 5 7 Instruction Cycles 20 1.667
1 1 0 6 8 Instruction Cycles 24 2.000
1 1 1 7 9 Instruction Cycles 28 2.333