Freescale Semiconductor
Application Note
AN2701
Rev. 0, 10/2004
PWM Generation with the HC08
Timer
by: Alban Rampon
8/16-bit Division
East Kilbride
Introduction
All HC08 microcontrollers (MCUs) include at least one timer module (TIM). This module is very useful for
generating or capturing time-dependent signals.
The data sheet describes what the timer module is capable of and how to use it.
This application note gives further explanation of how the TIM behaves in the following situations.
•
Unbuffered pulse width modulation (PWM) signal generation
•
Rolling PWM generation
•
Timer activity during Break and software interrupts
PWM signals are used in many applications from dimmers (duty cycle variation gives more/less light from
LEDs) to IR transceivers (modulation/demodulation).
An example illustrating rolling generation is included, with sample code, in the appendix at the end of this
document.
© Freescale Semiconductor, Inc., 2004. All rights reserved.
Introduction
Principle of PWM Generation Using Timers
The generation of a PWM signal using the dedicated HC08 PWM module is based on hardware
comparisons between register values and a free-running hardware counter. The HC08 TIM offers similar
hardware comparison in the form of output compare circuitry. The contents of channel registers are
continuously compared to the master free-running timer. When a match occurs, a hardware output event
can be configured to take place, and/or an interrupt can then call a service routine. The timer module has
from two to six compare registers and can thus be configured to toggle the port pin on each of two to six
comparison values, with no core overhead.
Each PWM signal requires a dedicated timer channel with output compare capability. Any channel with
this function along with the associated interrupt vector may be used to generate a PWM signal, using the
methods described.
Functional Description
Features of the TIM can include
•
Two to six input capture/output compare channels
– Rising-edge, falling-edge or any-edge input capture trigger
– Set, clear or toggle output compare action
•
Buffered and unbuffered pulse width modulation (PWM) signal generation
•
Programmable TIM clock input
– Seven frequency internal bus clock prescaler selection
– External TIM clock input (4 MHz maximum frequency)
•
Free-running or modulo up-count operation
•
Toggle any channel pin on overflow
•
TIM counter Stop and Reset bits
Block Diagram
Figure 1 shows the structure of the TIM, taken from an MC68HC908GZ60 (two channels represented out
of the six available). The central component of the TIM is the 16-bit counter, which can operate as a freerunning counter or a modulo up-counter. The TIM counter provides the timing reference for the input
capture and output compare functions. The TIM counter modulo registers, TMODH:TMODL, control the
modulo value of the timer counter. Software can read the TIM counter value at any time without affecting
the counting sequence.
In the case of the HC908GZ60, the six timer channels are programmable independently as input capture
or output compare channels. In other devices, the number of channels may vary from two to six; however,
the rest of the module remains identical.
PWM Generation with the HC08 Timer, Rev. 0
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Introduction
TCLK
PTD6/T2CH0
PRESCALER SELECT
INTERNAL
BUS CLOCK
PRESCALER
TSTOP
PS2
TRST
PS1
PS0
16-BIT COUNTER
TOF
TOIE
INTERRUPT
LOGIC
16-BIT COMPARATOR
T2MODH:T2MODL
CHANNEL 0
ELS0B
ELS0A
TOV0
CH0MAX
16-BIT COMPARATOR
T2CH0H:T2CH0L
CH0F
16-BIT LATCH
MS0A
CHANNEL 1
ELS1B
MS0B
ELS1A
TOV1
CH1MAX
16-BIT COMPARATOR
T2CH1H:T2CH1L
CH0IE
CH1F
16-BIT LATCH
MS1A
CH1IE
PTD6
LOGIC
T2CH0
INTERRUPT
LOGIC
PTD7
LOGIC
T2CH1
INTERRUPT
LOGIC
Figure 1. HC08 Generic Timer Module Block Diagram
Timer Counter Prescaler
The timer clock source can be one of the seven prescaler outputs or the timer clock pin (PTD6/T2CH0 on
HC908GZ60). The prescaler generates seven clock rates from the internal bus clock. The prescaler
select bits, PS[2:0], in the timer status and control register select the timer clock source (see Table 1).
PWM Generation with the HC08 Timer, Rev. 0
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3
PWM Generation
Table 1. Timer Prescaler Values(1)
PS[2:0]
TIM2 Clock Source
000
Internal Bus Clock ÷ 1
001
Internal Bus Clock ÷ 2
010
Internal Bus Clock ÷ 4
011
Internal Bus Clock ÷ 8
100
Internal Bus Clock ÷ 16
101
Internal Bus Clock ÷ 32
110
Internal Bus Clock ÷ 64
111
T2CH0
NOTES:
1. Not all timers allow all possible values; refer to the device data sheet.
PWM Generation
Initialization Procedure
To ensure correct operation when generating unbuffered or buffered PWM signals, follow the procedure
as stated in the data sheet:
1. In the TIMx status and control register (TxSC):
a. Stop the TIMx counter by setting the TIMx stop bit, TSTOP.
b. Reset the TIMx counter and prescaler by setting the TIMx reset bit, TRST.
2. In the TIMx counter modulo registers (TxMODH:TxMODL) write the value for the required PWM
period.
3. In the TIMx channel y registers (TxCHyH:TxCHyL) write the value for the required pulse width.
4. In TIMx channel y status and control register (TxSCy):
c.
d.
e.
Write 0:1 (for unbuffered output compare or PWM signals) or 1:0 (for buffered output compare
or PWM signals) to the mode select bits, MSxB:MSxA.
Write 1 to the toggle-on-overflow bit, TOVx.
Write 1:0 (to clear output on compare) or 1:1 (to set output on compare) to the edge/level
select bits, ELSxB:ELSxA. The output action on compare must force the output to the
complement of the pulse width level.
NOTE
IMPORTANT: Do not use direct assignment instructions, such as MOV
#0x1A, 0x25, or STA 0x25, for channel status and control registers.
PWM Generation with the HC08 Timer, Rev. 0
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PWM Generation
Assignment must be done sequentially (using BSET 4,+ 0x25, for example)
following the steps of the PWM Initialization procedure described above.
Table 2. Mode, Edge, and Level Selection
MSxB
MSxA
ELSxB
ELSxA
x
0
0
0
Mode
Configuration
Pin under port control; initial output level high
Output Preset
x
1
0
0
Pin under port control; initial output level low
0
0
0
1
Capture on rising edge only
0
0
1
0
0
0
1
1
Capture on rising or falling edge
0
1
0
0
Software compare only
0
1
0
1
0
1
1
0
0
1
1
1
1
x
0
1
1
x
1
0
1
x
1
1
Input Capture
Output Compare or
PWM
Capture on falling edge only
Toggle output on compare
Clear output on compare
Set output on compare
Buffered Output
Compare or
Buffered PWM
Toggle output on compare
Clear output on compare
Set output on compare
If this procedure is not followed, the PWM signal generated will not be as expected.
PWM Generation with the HC08 Timer, Rev. 0
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5
PWM Generation
Channel 1: PWM signal generated
Channel 2: Events
Figure 2. Timer Erroneous Initialization
Figure 2 shows what the resulting PWM signal may look like. In this case, the full register has been written
instead of setting the bits in proper order. The first pulse on Channel 2 is just after the direct assignment
to the timer status and control register is complete. The second pulse follows the instruction to start the
timer.
Between these two events, the software initializes other modules. The first pulse of the PWM appears to
be quite long and earlier than expected. Its length also depends on the code executed between the
initialization and the free-running counter startup.
Figure 3 shows the PWM signal being generated correctly when the recommended procedure is followed.
PWM Generation with the HC08 Timer, Rev. 0
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PWM Generation
Channel 1: PWM signal generated
Channel 2: Events
Figure 3. Timer Correct Initialization
PWM Generation Without CPU Load
In many applications, it is useful to be able to generate a PWM signal and then not have to worry about
it. For example, to control LEDs illuminating a dashboard, the duty cycle and frequency do not have to be
constantly changed or monitored.
The HC08 timer can generate such a signal, without loading the MCU. This mode is called unbuffered
PWM generation. After the setup is done, the PWM signal is generated whatever is being executed
(except Reset, Break, and if the timer is stopped), without requiring any interrupt or instruction to be
executed.
The modulo counter value represents the period of the signal. The ratio between the values in the output
compare and modulo registers represents the duty cycle.
This method is simple to use and can be implemented as follows.
If the timer is configured to be reset on output compare and to toggle on overflow, the timer output state
when it starts to count will be logic 0. When the output compare value is reached, the output is already at
logic 0 and will only toggle to logic 1 when the first overflow occurs.
The user should be aware that the first pulse may appear to be missing.
It is possible to have this pulse earlier by inverting the signal. If the timer sets the output on output
compare (bit ELSxA at logic 1 in the timer status and control register — see Table 2) instead of resetting
it (keeping toggle-on-overflow bits MSxA and MSxB at logic 0), then the greater the duty cycle, the earlier
the first pulse occurs.
PWM Generation with the HC08 Timer, Rev. 0
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7
PWM Generation
Channel 1: PWM signal generated
Channel 2: Events
Figure 4. Set on Output Compare
The major advantage of this method is that it works separately from the CPU activity and the rest of the
software.
However, it also means the timer used for the PWM generation has its modulo counter (i.e. signal period)
fixed for all its other channels. Therefore, generation of multiple signals with different periods will be
impossible on the same module; only different duty cycles will be possible.
Flexible PWM Rolling Generation
Principle
The solution described above is easy to set up and use, but could lack flexibility.
Using the following method will increase the possibility that the timer can generate any PWM signal on
any channel. However, it will also increase the complexity and size of the code.
The principle used here is to not rely on the modulo counter to determine the period, as only one is
available for all channels of the same timer.
References to the master timer modulo register, TxMODH:TxMODL, are avoided by simply adding
consecutive mark (logic 1) and space (logic 0) values to the timer compare register on successive ISR
function calls, as shown in the following equations. Timer roll-over is seamless when using unsigned
integer addition. Using the previous compare value as a reference for generating the next compare value
allows precise output timing, even though the ISR latency may vary.
PWM Generation with the HC08 Timer, Rev. 0
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PWM Generation During Break or Software Interrupt
Bus Frequency
PWM Period (timer ticks) = --------------------------------------------------------------------------------------------Timer Prescaler × PWM Frequency
Eq. 1
Period × Duty Cycle (as an integer percentage)PWM Mark Time (timer ticks) = PWM
-------------------------------------------------------------------------------------------------------------------------------------------100
Eq. 2
PWM Space Time (timer ticks) = PWM Period – PWM Mark Time
Eq. 3
Once the timer channel is configured, the PWM signal can be generated using the timer channel interrupt.
This should be configured to call the channel interrupt service routine (ISR), loading the timer compare
register with its current value added to the length desired (mark or space). This is achieved by identifying
whether the last action was a negative or positive edge (the port pin toggles on output compare), and
loading the compare register with the next appropriate value.
If the port pin is at logic one (or zero), a mark (or space) is to be added.
NOTE
In this case, the overflow flag and interrupt are not relevant as the counter
is used as free-running with roll-over.
The sample code in the appendix at the end of this document gives an example of how to apply this
principle.
Limitations
The MCU must be able to service the timer PWM interrupts in time for the next edge to be configured.
This sets an upper limit on the frequency/resolution of PWM signal that can be reproduced, and will vary
from system to system depending on the MCU application. It is the responsibility of the system designer
to ensure that the core can update the compare registers in sufficient time using the ISR under maximum
core loading conditions.
A secondary limitation is that using the timer module incurs a greater degree of core overhead, as the
timer module has to be serviced at every edge transition (interrupt). This does not happen with the
unbuffered PWM via modulo counter, as the toggling mechanism is independent of the core and code
execution. The described method of PWM generation is likely to be more suited to slower rate PWM
requirements, due to the overhead generated by having to service an interrupt for each edge of each
PWM signal.
PWM Generation During Break or Software Interrupt
The HC08 data sheet states that the timer is stopped during a break. It is important to distinguish a Break
from a software interrupt (SWI) instruction.
The Break Module (BRK) is primarily used in a debugging/development context. For example,
breakpoints in monitor mode are generated using this module. This explains why the debugger stops
program execution when an address match or BRKA bit-mask is performed.
PWM Generation with the HC08 Timer, Rev. 0
Freescale Semiconductor
9
PWM Generation During Break or Software Interrupt
This module could also be used in normal operation to allow for future code enhancements. For example,
on a ROM device with EEPROM (like the HC08AZ60A, the GZ family does not include EEPROM), a
developer can expect either to have to add a routine not available when the part is produced, or to have
to manage future upgrades.
The break instruction would call the non maskable software interrupt (vector $FFFC:$FFFD), and the
interrupt subroutine would reside in EEPROM.
If the program is using the timer to generate a PWM signal, the Break interrupt will hold the timer counter,
to resume after its execution. Therefore, the PWM signal is affected by the interrupt, and this may present
a problem if the Break interrupt execution time is significant.
Figure 5 shows how the PWM signal on Channel 1 is irregular and delayed by the activity of the interrupt
(Channel 2 high when Break interrupt is being executed).
Channel 1: PWM signal generated
Channel 2: High during Break ISR execution
Figure 5. PWM Generation During a Break
Note that the user also must allow status bits to be changed (by modifying the BFCR_BCFE bit at $FE03),
for them to be updated. For further details, please refer to the System Integration Module (SIM) Chapter
in the device data sheet.
The Break interrupt shares its vector with the software interrupt (SWI) and the data sheet states that the
Break function fetches an SWI. However, the SWI instruction does not 'freeze' any function of the MCU.
A PWM signal configured with toggle-on-overflow without using interrupt (unbuffered PWM) will not be
influenced in any way by an SWI (see Figure 6).
PWM Generation with the HC08 Timer, Rev. 0
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Conclusion
Channel 1: PWM signal generated
Channel 2: High during SWI ISR
Figure 6. PWM Generation During and SWI
To summarize, when this interrupt has to be used in normal operation, it is advisable to use an SWI
instruction, instead of a Break, to minimize the impact on the microcontroller operations (timer and other
modules).
NOTE
The Break (BRK) and Software Interrupt (SWI) share the same interrupt
vector. Therefore, the effect really depends on how the ISR is called and
not on its content.
Conclusion
The HC08 timer is easy to use and can be used in a myriad of applications. However, the user must take
care to set it up properly to ensure that the desired function is achieved.
Depending on the user’s priority (code size, complexity, resource utilization, for example), it will be
possible to find a solution to suit the application.
PWM Generation with the HC08 Timer, Rev. 0
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11
APPENDIX
APPENDIX
Program Flowchart
Channel ISR called
N
Increase the
channel register
of a space value
Port Pin at
Logic 1?
Y
Increase the
channel register
of a mark value
Sample Code
/******************************************************************************
*
COPYRIGHT (c) FREESCALE SEMICONDUCTOR 2004
*
ALL RIGHTS RESERVED
* FILE NAME: AN2701_timerISR.c
*
* PURPOSE: HC908GZ60 Sample Code for the Timer Interrupt Sub Routine
*
*******************************************************************************
*******************************************************************************
** THIS CODE IS ONLY INTENDED AS AN EXAMPLE FOR THE METROWERKS COMPILER AND **
**THE MMDS0508/EM08QA24 EVB AND HAS ONLY BEEN GIVEN A MIMIMUM LEVEL OF TEST. **
** IT IS PROVIDED 'AS SEEN' WITH NO GUARANTEES AND NO PROMISE OF SUPPORT.
**
*******************************************************************************
*******************************************************************************
*
* DOCUMENTATION SOURCE: EKB Apps
*
* TARGET DEVICE: HC908GZ60
*
* COMPILER: Metrowerks for HC08
VERSION: v5.2.1149
*
PWM Generation with the HC08 Timer, Rev. 0
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APPENDIX
* DESCRIPTION: This ISR describe how to use rolling PWM generation, not to tie
*
all channels from a Timer to the same period.
*
* AUTHOR: A.RAMPON
LOCATION: EKB
LAST EDIT DATE: 15/08/04
*
* UPDATE HISTORY
* REV
AUTHOR
DATE
DESCRIPTION OF CHANGE
* -----------------------------------* 1.0
A.Rampon
11/08/04
Initial Release
******************************************************************************/
#ifndef __AN2701_TIMER_ISR_C
#define __AN2701_TIMER_ISR_C
/* if this H file not included, include */
#include "hc08gz60.h"
/**********************************************************************
Function Name : _PWMTimerChan0ISR
Engineer
: A.Rampon
Date
: 01/04/04
Parameters
: NONE
Returns
: NONE
Notes
: Interrupt service routine for Rolling PWM Generation on Channel 0.
**********************************************************************/
#pragma TRAP_PROC
void _PWMTimerChan0ISR(void)
{
/* if channel is set to generate rising edge */
/* On MC908AZ60A, TBCH0 is PTF4 */
/* If after toggled output is set, */
/* a mark is beginning */
if (PTF4 == 1)
{
/* set up timer compare for mark time */
/* relative to the last transition */
TBCH0 += Mark[0];
}
else
{
/* set up timer compare for space time */
/* relative to the last transition */
TBCH0 += Space[0];
}
}
PWM Generation with the HC08 Timer, Rev. 0
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13
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AN2701
Rev. 0, 10/2004