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Signal Generator with an ATmega8
This application demonstrates a digital signal generator with an ATMEL ATmega8 with the
The hardware consists of the AVR processor,
the In-System-Programming (ISP) connector, the LCD and diverse external
- Adjustabe frequency range: 0,25 c/s to 8 Mcs/s in 1024 steps
- Wide dynamic range of the frequency without mechanic switching
- Reprogramming of the frequency characteristics possible
- Adjustable pulse-width from 0.00 to 100.00% in 1024 steps
- Independant adjustment of pulse-width and frequency
- Xtal base frequency 16 Mcs/s for stable frequency
- Normal and inverted output
- Polarity change by switch (active low, active high)
- Selectable display: frequency, time, rounds per minute and pulse-width
- Suitable for single- and two-line LCDs with 16 to 40 characters per line
- English or german notation selectable
The processor is attached to the following external components.
The operating voltage of 5 V is supplied via the pins 7 (+5 V)
and 8 (0 V) and blocked by a ceramic capacitor.
Pin 1 (= RESET-pin) is tied to the positive operating voltage.
On the Xtal pins 9 und 10 (XT1, XT2) an Xtal of 16 MHz is
connected. Disconnecting the internal RC clock source and activation of
the Xtal clock generator is done by setting the respective fuses. Both
Xtal inputs are equipped with ceramic capacitors of 22 pF to
improve switching characteristics.
The operating voltage for the AD converter is supplied over a LC network
of a 22 µH coil and a capacitor of 100 nF on pin 20
(AVCC). As reference voltage AVCC is selected by software, so a capacitor
to ground is connected to the AREF (pin 21).
The ISP interface allows programming of the AVR in the completed system
without removing the chip.
ISP uses the port bits 5 (SCK), 4 (MISO), 3 (MOSI) and the RESET on
pin 1. MISO is also used to drive a control signal of the LCD.
Pinning of the 10-pole ISP connector is compatible with the ATMEL-/KANDA-
standard. The LED connected to the ISP connector signals
programming currently active.
As display a standard LCD is used. It is connected via a 14 pole
parallel cable and connectors that fit to the 14-pole connection
of the LCD.
The control port of the LCD is connected to the port bits PB0
(LCD Enable) and PB4 (LCD RS input). LCD Enable is by default
tied to ground over a resistor of 10 k to avoid spurious signals
on that line during phases where the pin PB0 is not (yet)
initialised (during startup and during ISP programming).
LCD-R/W is tied to ground, because the display is not read.
The data port of the LCD is connected to the port D of the AVR.
Data transfer to the LCD uses the 8 bits parallel, no multiplexing
With the exception of the LCD ad the operating voltage supply,
that use separate connections, all other external components
are connected via a single 14-pole connection. This has the
advantage that the processor board can be easily disconnected
and testet separately. The 14-pole cable can be separated to
portions reaching the different external components
The first four cables are connected to the operating voltage
and the first two AD channel inputs of the AVR.
The AD channels ADC0 (Frequency adjust) and ADC1 (Pulse-width)
are connected to the middle of the variable resistors. The
variable resistors are of the 10-turn-type and are connected
to the operating voltage. Their nominal value and linearity
are uncritical, because the real values are displayed on the
Directly on the variable resistors the signal lines to the ADCs
are blocked by capacitors to ground to avoid noise on the
The next five connections of the 14-pole cable are connected
to the port inputs PC2 to PC5. The switch connected to PC5
is only necessary if a single-line LCD is used. Leave this
line open, if you use a two-line LCD.
All switches connect the port inputs with ground, if closed.
The ports have software-activated pullup resistors and are
so held on the operating voltage, if not pulled to ground.
The following three lines of the cable are connected to the
outputs of the timer (normal and inverted). These have to
be soldered to the CINCH output connectors.
The output yield digital signals with the typical
characteristics of the AVR output drivers and are directly
coupled to the output connectors. The outputs are short-circuit
protected, but are not protected against voltages applied
The power supply provides a stabilised operating voltage of
5 V at a current of approximately 20 mA. The
supply current is depending from the variable resistors.
Processor and LCD require below 10 mA, so the whole
generator can also be supplied by batteries.
To use a small transformer with 6 V secondary voltage,
a rectifier bridge with Schottky diodes and a low-drop
regulator is used. With a transformer of 7,5 V
secondary voltage or more these components can be exchanged
to standard components.
The charger capacitor has a relatively high capacity, it
can be reduced.
The two Tantal capacitors suppress instabilities of the
components together on a piece of experimental board is
uncritical. The 14-pole connection to the LCD is located to the
left, the 14-pole connection to the external components
(variable resistors, switches, output connectors) to the
right. For easy mounting the parallel cable can be separated
into portions of four, five, three and two lines. Please don't
forget the two capacitors on the middle of the variable
The 10-pole ISP interface is not used very often, so it is
not accessible on the outside of the casing. The power
supply is located on the upper left of the plastic case.
After switching power on the LCD displays a message for about
2.5 seconds, showing the machine's function, the software
version and the copyright, with the format depending from the
available number of characters of the line. The machine the is
ready to operate.
The switch Time changes the display from frequency (switch
open) to time (switch closed). Output of the frequency is in
Hz (cs/s) with two decimals, output of the time is in
microseconds. Both values are rounded and formatted with
If frequency output is selected, the switch Rpm changes
from frequency to rounds per minute (= 60 * f). If the
output of time is selected, this switch is ignored.
The switch Inv inverts the output signals by software
without changing the output connector.
If a single-line LCD is used, the switch Pulse causes
the display to show the pulse-width in % instead of the
frequency/time normally displayed. In case of a two-line
LCD the pulse-width is permanently displayed on line 2.
The digital output signal is available in positive
and inverted form on the two CINCH connectors. To avoid
capacitive effects the lines should be short and not
This section describes the functioning of the processor,
of the ISP interface and the LCD display.
The processor ATmega8 works with an external Xtal and
the respective internal oscillator. Because the processor
is shipped with an internal RC oscillator of 1 Mcs/s,
the respective fuses must be set first to use the Xtal
as clock source. The following fuse combinations must
Reprogramming of these fuses can either be performed
externally on a programming board (e.g. with a STK500
and the ATMEL Studio) or in the completed system with
the ISP interface (e.g. with PonyProg2000).
- CKOPT = 0
- CLKSEL3..CKSEL0 = 1111
- SUT1..SUT0 = 11 or 10
By programming the fuses with an STK500
the Xtal must be connected to the device, otherwise the Mega8
will not respond any more after the fuses are programmed. The
fuses are correctly programmed by selecting one of the two
last options of the device.
When using PonyProg please note that
the fuses are displayed inverted. To have an orientation:
by default CKSEL3..CKSEL0 are 0001 and SUT1..SUT0 are 10.
Read the fuses by pushing the Read-Button first. Of course,
the Xtal should be mounted to the AVR before programming the
fuses. CLKSEL and SUT1 fuses should be programmed like
displayed here, SUT0 can be programmed to either 1 or 0. If
you encounter problems during startup, then program this to
1 (SUT1:SUT0 = 11).
The open switch inputs are at the start pulled to high by
software activation of the internal pull-up resistors. If
the swichtes are closed, the input lines are pulled to
logic zero. The switch Pulse is only needed, if a
single-line LCD is used.
Signal generation is performed with the internal 16-bit
timer/counter TC1 in the Fast-PWM-mode. The graph shows
the function of TC1 in this mode and shows the parameters
that affect the operation (blue).
The clock oscillator, controlled by an external Xtal, is
divided by the TC1 prescaler either by 1, 8, 64, 256 or
1024 and drives the counter. When the counter reaches the
TOP value, which was written by software to the double
register ICR1, the counter is reset with the next clock cycle
and the compare outputs OC1A (Portbit PB1, Pin 15) und OC1B
(Portbit PB2, Pin 16) are activated. The frequency of the
generator so is defined by ICR1. Depending from the
selected value for the pulse-width of the signal, the
compare values in the register pairs COMPA and COMPB are
adjusted. If the counter reaches these compare values, the
respective compare output is deactivated and stays so
until the counter reaches TOP.
The two outputs OC1A and OC1B are, by software, of different
polarity. They produce inverted signals of the same duration.
The switch Inv inverts this polarity by software.
Frequency adjustment is done with the variable resistor
attached to the ADC0 input. The resistor rings a voltage
of between 0.000 and 5.000 V to the ADC0 (port PC0,
pin 23). After conversion, a value between 0x00 and 0x1F
results. This value is used to pick a respective value
for TOP in ICR1 from a table with 1024 values (Lookup-Table,
Include file rectgen_m8_table.inc). Depending from the
value read from ADC0, the prescaler control byte for TC1
is also prepared. Both values are stored in SRAM until
the update of TC1 takes place.
Pulse-width adjustment is done with the second variable
resistor and the voltage reaches ADC1 (port PC1, pin 24),
which is also converted to a value between 0x00 und 0x1F.
The TOP value is multiplied by this conversion value and
then divided by 1024 to yield the compare value, next to
be written to COMPA and COMPB.
During the same cycle, the switch Inv is read and the
resulting control value for the correct polarity of OC1A
and OC1B is determined.
With the parameters mentioned, the TC1 counter runs
free without additional software overhead. In order
to update these parameters the 8-bit timer/counter TC0
is used. The TC0 prescaler divides the system clock
by 1024 and overflows after 256 prescaler pulses
(@16Mcs/s: every 16,384 ms). TC0 interrupts and
decrements a register, from 30. If this reaches zero
(after 492 ms), the AD converter is connected to
channel 0 and the first conversion is started.
The ADC runs with a clock that is derived from the
system clock and divided by 128. After the first ADC
result is complete, the ADC interrupts the CPU. The
interrupt service routine reads the result to a double
register, sets a flag bit, muxes channel 1 to the ADC
and starts the second conversion. If the result of the
second conversion interrupts the CPU, this result is
read, the ADC is switched off and a signal flag is set.
The conversion of the ADC values and the update of TC1
is done asynchroneous in the main program loop, after
the signal flag had been set by the interrupt service
routine. The ADC values are converted and TC1 is
programmed with the new parameters. After programming
the TC1, the LCD is updated and the main program loop
ends until the next TC0 overflow interrupt wakes up
The LCD is connected to the 8-bit data port and the two
conrol lines E(nable) and R(egister)S(elect). The
R(ead)/W(rite)-line is permanently on Write, the whole
timing control is done in correct timing loops.
After program start and after a wait time for the internal
init of the LCD the LCD is set to the following modes:
Then, for 2.5 seconds a message is displayed.
- 8-Bit interface
- Single or two line operation (depending from the
values in the software)
- no display shift
- cursor off
- blinking off
After each update of the TC1 timer the LCD is also updated.
First the CTC value in ICR1 is multiplicated by the prescaler
value (1, 8, 64, 256, 1024). I displaying of the frequency
is selected, the clock frequency, multiplied by 100, is
divided by this value to yield an integer representing the
frequency with a resolution of 0.01 cs/s. If displaying of
the time is selected (switch Time closed), the CTC value
in ICR1 is multiplied by the prescaler value, and then
multiplied by the factor 25.600.000.000 / clock
(@16MHz: 1.600) to yield the time in microseconds (*100).
If a two-line LCD is used, the value of frequency or time
is displayed on line 1. In case of a single-line display,
this is displayed only if the switch Pulse is open.
The pulse-width is calculated by multiplying the compare
value in COMPA or COMPB by 10,000 and dividing by the CTC
value in ICR1. The resulting integer is the pulse-width
in % with a resolution of 0.01%. This value is written
on line 2 (on a two-line LCD) or written on line 1 (of a
single-line LCD, if switch Pulse is closed).
The display is updated approximately 2 times per second.
At higher frequencies the frequency and pulse-width cannot
be adjusted to accurate values due to the limited resolution
of the 16-bit counter. This is recognised by the last two
decimals being displayed. Due to the fact that the displayed
numbers are calculated from the real values used in TC1,
these numbers are correct.
The ISP interface is for updating the software in circuit.
The data and clock signals MOSI, MISO and SCK are accessed
on the 1o-pole connector. Via the RESET line (port PC6,
pin 1) the ISP interface brings the ATmega8 to the
programming mode. After releasing RESET, the AVR restarts.
The programming LED indicates an active programming cycle,
is only active in case of programming. If not build in,
or if a 6-pin connector is used instead of the 10-pin
standard, this does not change functioning.
Te software is written exclusively in assembler language and
divided into three different packages. Prior to assembling
the source code a number of parameters has to be adjusted
to optimise the hardware.
The frequency table in the file rectgen_m8_table.inc
has 1024 words for converting the selected
voltage to the CTC value for ICR1. The values were
calculated with a Spreadsheet
and exported as an Include-Text-file.
If you make changes in that table, please note that
this may have consequences for the prescaler values. In
that case you will have to readjust the prescaler values
in the routine Convert in the file rectgen_m8_v1.asm
(current values: 392, 225, 60 and 3).
If changing the clock frequency of the Xtal oscillator
the constant clock will have to be changed. In that case
also the 5-byte-constants cDivFx and cDivUx have to be
changed accordingly. These cannot be calculated by the
assembler due to overflow problems. Note that changes
in the clock frequency are automatically changing the
LCD timing loops, no additional adjusts in the driver
routine are necessary.
The constants cLcdLw and cLcdLn define the connected
LCD. Inproper settings might cause strange effects on
The constant cLcdMicro defines the micro character of
the connected LCD. The default is not a micro but a u,
because some displays do not have greek characters and
do not use 0xE4 for that character.
The constant cEn enables english thousands- and
The switches Time, Rpm, Inv and Pulse can be placed to
different pins, this can be re-defined with the constants
pbTime, pbRpm, pbInv and pbPwm.
The switches dbgon and dbghx are for debugging only. For
correct function of the software these must be set to
The commented source code is available in HTML-format
and in zipped form as assembler source code:
Assembler source code.
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