68HC11 Timing System

68HC11 Timing System 

ELET 3144 R. Alba-Flores 1

Programmable Timer 

• The internal programmable timer is a device that is very 

powerful because it allows measuring of time periods, 

frequency, delays, etc., in very efficient and easy ways. 

• Time periods are measured using different internal timing 

devices which includes counters, scalars, flags and 

interrupts. This makes the measurement process more 

precise and easy to implement . 


Time Measuring Methods 

There are different devices that can be used to 

measure time include the following: 

• – Free running counter 

• – Timer overflow flag 

• – Timer overflow Interrupt 

• – Real time interrupt 

• – Output Compare Channels 

• – Input Capture Channels 

• – Pulse Accumulators

68HC11 Timing System 

The 68HC11 timing is controlled by a crystal oscillator 

connected to the XTAL and EXTAL pins on the 68HC11. 

The E clock, which determines the time of each 

instruction cycle is ¼ of the crystal frequency. 

For the Adapt 11 evaluation board the basic clock is 

8MHz and the E clock is 2 MHz. 

Therefore the time of each instruction cycle is 0.5 µsec. 


The Free-Running Counter 

• The output of the E clock is connected to a 16-bit 

counter, called the TCNT register, that is at locations 

$100E and $100F. 

• TCNT is cleared at RESET. 

• This is a read only register. It can not be written but it 

will be incremented by every tick of the E clock 


The Free-Running Counter 


The Prescalar 

• Resolution is the smallest time interval that can be 

measured in a system. 

• In the 68HC11 the TCNT register is incremented once 

every 0.5 µsec. This 0.5 µsec is the resolution of the 

microcontroller system. 

• Because the TCNT register is 16 bits, it takes 65,536 

( 2 16 ) counts to cycle it around once. 

• At the standard clock frequency of 2 MHz, the TCNT 

register requires 32.77 msec to roll over ( 65,536 x 0.5 

µsec). 


The Prescalar 

The 68HC11 contains a prescalar that allows the user to lengthen the 

time of each count by dividing the E clock before it is applied to the 

input of the TCNT register. The available factor are: 1, 4, 8, and 16 and 

can be selected using the bits 0 and bit 1 in the TMSK ($1024) control 

register. 

PR1 PR0 Prescale 

Factor 

Bus Frequency (E Clock) 2 MHz 

Resolution / Overflow 

0 0 1 0.5 µsec / 32.77msec 

0 1 4 2 µsec / 131.1 msec 

1 0 8 4 µsec / 262.1 msec 

1 1 16 8 µsec / 524.3 msec

Timer Overflow 

Every time the TCNT register rolls over from 

FFFF to 0000, the TOF (Timer Over Flow) bit is set. 

The user has no control over TCNT, so TOF will be set 

every 32.77 msec 


Clearing the TOF bit 

Many of the flag bits in the 68HC11 control registers, including the 

TOF, are cleared by writing a 1 into the bit position of the flag. 

Writing a 0 into the bit position has no effect on the flag. 

A routine to clear TOF is: 

SUB1: LDAA #$80 ; Set bit 7 to 1 

STAA $1025 ; Write it to TFLG2 

RTS 

; Return 

A flag can also be cleared using a BCLR instruction, with a 0 in 

the bit position to be cleared. This is because the 68HC11 uses 

the inverse mask to clear the bit. To clear TOF, for example, the 

following instructions would work: 

LDX #$1000 

BCLR $25 , X $7F 

10

Example 1 

• The TCNT register can be used to create time delays. 

A subroutine to cause a time delay for any amount of time is shown 

below. 

• In this subroutine N1 is the number of 32.77ms intervals that the 

time delay requires 

• DIFF is the difference between the time of the intervals and the 

time of the delay. 

• In the subroutine the number of intervals is loaded into register Y . 

• The content of TCNT is added to DIFF and stored in register D. 

• Note that if the sum is too large for D, then Y must be 

incremented by one. 


ORG $2000 ; * This is a General Subroutine for any time delay 

N1: FDB ? ? ; Number of 32.77ms intervals 

DIFF: FDB ? ? ; Difference between the time of the interval and the time of the delay 

TCNT: EQU $100E 

TFLG2: EQU $1025 

; * Starting of the subroutine 

SUB1: LDY N1 ; Y will hold the number of intervals 

JSR SUB2 ; Clear TOF 

LDD TCNT ; Read TCNT and add the Difference, 

ADDD DIFF ; Store the result in D 

BCC LOOP1 ; If the sum of TCNT and DIFF is too large for register D 

INY 

; (i.e. Carry =1 in CCR), then Y must be incremented by one 

LOOP1: CPY #0 

BEQ LOOP2 ; Go to LOOP2 when done with number of intervals 

LOOP3: TST TFLG2 ; Test TOF and wait to be set. 

BPL LOOP3 ; When TOF is set clear it and decrement Y 

JSR SUB2 ; Go to clear TOF 

DEY 

BNE LOOP1 

LOOP2: CPD TCNT ; Wait until TCNT reaches the content 

BHI LOOP2 ; of register D 

RTS 

; end of GENDEL subroutine, return to main program

; * Subroutine to clear TOF 

SUB2: LDAA #$80 

STAA TFLG2 

RTS 

Example 2. 

Using the program in example 1, generate a 100-ms time delay. 

To create a 100-ms delay, we first divide 100 ms by 32.77 ms, the 

timer for TCNT to count around. This gives three cycles (3 X 32.77 

ms = 98.31 ms) plus 1.69 ms, or 3380 counts of the E clock. 

If N1 is set to 3 and DIFF is set to $0D34 (the hex equivalent of 

3380), the program will generate a 100-ms time delay. 

Exercise. Write a program to set PB0 high for 100 ms and then 

low for 100ms. Use the GENDEL subroutine of example 1.

Input Capture Registers 



Input capture registers are a set of three 16-bit registers 

that are connected to the free running counter TCNT 

and are controlled by hardware signals (internally or 

externally generated) 



• The Input Capture registers can be used to trap or 

store the value of the TCNT counter. 

• There are 3 Input Capture Registers in the 68HC11, 

TIC1, TIC2 and TIC3, and they are 16 bit registers. They 

are connected to pins PA2, PA1 and PA0 (Input only 

pins) at Port A. 

• These registers make a capture when an edge occurs 

in one of these input pins. 

• When a capture is made in one of the registers, the time 

of the capture (input capture) is copied from the TCNT 

register into the Input Capture Register (TIC). 


The three 16-bit TIC registers (TIC1, TIC2, TIC3), 

are located at memory locations $1010 to $1015 


The Input Captures are controlled by the edge bits in 

TCTL2 ($1021) in the following way: 

TCTL2 at $1021 

0 0 EDG1B EDG1A EDG2B EDG2A EDG3B EDG3A 

EDGxB EDGxA Configuration 

0 0 Capture disabled 

0 1 Capture on rising edges only 

1 0 Capture on falling edge only 

1 1 Capture on any edge (rising or falling) 


Example 3. 

The time of a falling edge on pin PA1 must be detected. 

How must TCTL2 be set up to do this? 

Sol. 

PA1 is connected to TIC2 (Input Capture Register 2). 

The previous table shows that the edge bits must be 1 

and 0, respectively, to capture a falling edge, thus the 

code: 

LDAA #$08 

STAA $1021 

will set up TCTL2 to capture the time of a falling 

edge on PA1. 


Input Flags and Interrupts. 

Bits 0, 1 and 2 of the TFLG1 register, at $1023, are used to signal 

an input capture on registers 3, 2,and 1, respectively, as shown in 

the following figure, 

TMSK1 at $1022 

OC1I OC2I OC3I OC4I OC5I IC1I IC2I IC3I 

TFLG1 at $1023 

OC1F OC2F OC3F OC4F OC5F IC1F IC2F IC3F 

If a capture is made, the flag will set. The flag can be cleared by 

writing a "1" into its bit position. 


The corresponding interrupt bit is in TMSK1. If this bit is set, a capture 

will cause an interrupt in accordance with the following table: 

Register Pin Location INTERRUPT VECTOR PSEUDO-VECTOR 

TIC1 PA2 $FFEE - $FFEF $00E8-$00EA 

TIC2 PA1 $FFEC - $FFED $00E5-$00E7 

TIC3 PA0 $FFEA - $FFEB $00E2-$00E4 


Example 4. 

Determining the Period of a Square Wave. 

• The following program determines the period of a square wave 

by finding the time between rising edges. 

• The TCTL at $1021 is set up to detect rising edges on PA0. 

• Then the program jumps to SUB2, which clears the input capture 

flag IC3F, and waits until it is set again. 

• The time of its setting is stored in TIC3, at $1014 and $1015. This 

time is saved in memory (at location FIRST) and SUB2 is executed 

again to get the time of the next positive edge. 

• The difference between the times is a measure of pulse width.

; *** This is a program to determine the period of a square wave connected to PA0 

; *** PA0 is connected to TIC3 

TCTL: EQU $21 

TFLG1: EQU $23 

TIC3: EQU $14 

ORG $C100 

FIRST: FDB 

RESULT: FDB 

BEGIN: LDX #$1000 

LDS #$C400 

LDAA #$01 ; Set the edge bits for IC3 

STAA TCTL,X ; to capture on rising edges 

JSR SUB2 ; Clear IC3F and wait until it sets 

LDD TIC3,X ; Save time of first edge 

STD FIRST 

JSR SUB2 ; Clear IC3F and wait until it sets 

LDD TIC3,X ; Get time of next edge 

SUBD FIRST ; Subtract to get the time difference 

STD RESULT 

SWI 

; End of main program 

23

; * Subroutine to clear the input capture flag IC3F, and then wait until it sets again 

SUB2: LDAA #$01 ; Clear the ICF3 

LOOP: 

STAA 

BRCL 

R 

RTS 

END 

TFLG1,X 

TFLG1,X $01 

LOOP 

; Wait until it sets

68HC11 Timing System

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