Timers

1
Timers

• Timers can be used for
• timing
• event counting
• pulse width measurement
• pulse generation
• frequency multiplication
• There are 8 Timers (T0 T7)
• T0 and T1 CAPCOM1 CAPture and COMpare
• T7 and T8 CAPCOM2
• T2, T3, T4, T5 and T6 General Purpose Timers
  (GPT)

2
General Purpose Timers (GPTs)

• The five GPTs are 16-bit timers that are grouped
  into the two timer blocks GPT1 and GPT2.
• Block GPT1 contains 3 timers/counters with a
  maximum resolution of 16 TCL(TCL 50ns for fCPU
  20MHz)
• Block GPT2 contains 2 timers/counters with a
  maximum resolution of 8 TCL and a 16-bit
  Capture/Reload register (CAPREL).

3
GPT1 Block
From a programmers point of view, the GPT1 block
is composed of a set of SFRs as summarized
below. Those portions of port and direction
registers which are used for alternate functions
by the GPT1 block are shaded.
4
Timers T2, T3 and T4

• Four basic modes
• Timer
• Gated timer
• Counter
• Incremental interface mode for incremental
  encoders
• All timers can count up or down
• When a timer register overflows from FFFFH to
  0000H (when counting up), or when it underflows
  from 0000H to FFFFH (when counting down), its
  interrupt request flag (T2IR, T3IR orT4IR) in
  register TxIC will be set.
• Each timer has an alternate input function pin
  (TxIN) associated with it which serves as the
  count input in counter mode, or as the gate
  control in gated timer mode.

x refers to the timer number
5
Timer 2, 3 and 4
Physical microcontroller pin
TxUD External Timer Up/Down control TxIN
External clock input T3OUT External output pin
from Timer3 fcpu CPU clock input to prescaler
(which divides by 8 ,16, 32, 1024)
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T2, T3 and T4 Timers (contd.)

• T3 is the called core timer
• The count direction (Up/Down) may be programmed
  via software or may be dynamically altered by a
  signal at an external control input pin.
• Each overflow/underflow of core timer T3 is
  latched in the toggle Flip-Flop T3OTL and may be
  indicated on an alternate output function pin. T2
  and T4 do not have this feature.
• Some timer modes require the use of more than one
  timer. For example The auxiliary timers T2 and
  T4 may additionally be concatenated with the core
  timer, or used as capture or reload registers for
  the core timer.

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Timer Control Register
T3CON Timer 3 Control Register (bit addressable)
8
T3 in Timer mode
The 3 TxI bits in TxCON determine the precaler
factor as shown in the table below.
n 3..10
Note the x in TxUD, TxUDE, TxI etc. refers to the
timer number
9
T3 in Timer mode
UP(0)/DOWN(1)
10
Timer 3 generating a fixed delay
// Initialise timer 3 for Timer mode, prescaler
101 (256), timer stopped // count down, disable
external up/down, T3OTL output disabled T3CON
0x0085 //00000 000 10 000 101 // timer
period prescaler / fcpu 256/20MHz
12.8uS // delay required 400000uS. Timer value
400000/12.8 31250 // Clear T3 interrupt
request bit, load timer value and run timer T3IR
0 T3 31249 // 1 less than number of counts
required T3R 1 / T3 is now decremented by
hardware every 12.8uS, after 31250 decrements the
interrupt request flag is set. The program should
poll/test this flag. / while (T3IR 0) //
wait until interrupt request is set
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Example - Timer delay
include ltstdio.hgt / standard I/O .h-file
/ include ltreg167.hgt / special function
registers for 167s / /
/ / Use Timer 3 to
generate a delay of 0.4 seconds/ /
/ void main
(void) / initialize the
serial interface / ifndef MCB167 / do not
initialize with Monitor-166 / P3 0x0400
/ SET PORT 3.10 OUTPUT LATCH (TXD) / DP3
0x0400 / SET PORT 3.10 DIRECTION CONTROL(TXD
O/P)/ DP3 0xF7FF / RESET PORT 3.11
DIRECTION CONTROL(RXD i/p)/ S0TIC 0x80 /
SET TRANSMIT INTERRUPT FLAG / S0RIC 0x00
/ DELETE RECEIVE INTERRUPT FLAG / S0BG
0x40 / SET BAUDRATE TO 9600 BAUD / S0CON
0x8011 / SET SERIAL MODE / endif . . . see
next slide, which goes here. . .
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//Initialise timer 3 for timer mode,
prescaler256, timer stopped //count down,
disable external up/down T3CON
0x0085 //0000000010 000 101 while (1) //do
this endlessly! printf("\nPress a key to
start timer\n") _getkey() // special Keil
function to wait for a key // Clear T3
interrupt request bit, load timer value and run
timer T3IR 0 T3 31249 T3R
1 printf("Timing started\n") // Now wait
for T3 interrupt request bit to be set to 1 //
This should take 31250 x 12.8uS 400000uS
400ms 0.4S // based on a 20MHz cpu clock
frequency while( T3IR 0) T3R 0 //Stop
timer //When loop terminates we know the
required delay has elapsed printf("400
milliseconds has elapsed\n")
13
Gated Timer Mode
This mode is used for measuring the period of
pulses
n 3..10
Timer mode bits TxM
gate open
TxIN provides the gating of the clock
14
Counter mode
15
Timer T2 and T4
16
Concatenation of Timers
Concatenation of Core Timer T3 and an Auxiliary
Timer