Silicon Chip – July 2019

78 Silicon chip Australia’s electronics magazine siliconchip.com.au

To program AVR family microproces-

sors, you need a programmer such as

the USBasp (see http://www.fischl.de/usbasp/

for details and drivers). This can be pur-

chased online from many suppliers for

just a few dollars.

Suitable free software is

available for Windows, Linux

and Apple IOS online. This

description will focus on the

Windows version.

You need to install the USBasp

drivers and download suitable pro-

gramming software. For Windows,

this includes

eXtreme Burner (http://extremeelectron-

ics.co.in/avr-tutorials/gui-software-for-

usbasp-based-usb-avr-programmers/),

AVRDUDESS (http://blog.zakkemble.net/

avrdudess-a-gui-for-avrdude/) and

Khazama (http://khazama.com/project/

programmer/).

Plug it in and complete the installation

of the USBasp programmer into your PC.

If you have the option of 3.3V or 5V pro-

gramming levels, select 5V.

Launch the programming software you

downloaded earlier and set the target de-

vice to “ATmega328” or “Atmega 328P”,

depending on your chip. Both may be

used. Now download the HEX file for this

project from the SILICON CHIP website (if

you don’t already have it) and select it as

the file to be used to program the chip in

your software.

Make sure JP1 has not been fitted to

your signal generator board; if it has, re-

move it now. Note that since some of the

ATmega328 pins connect to the AD9850

module, the AD9850 module’s power LED

will still light up and flash while the pro-

grammer is connected and running, de-

spite having removed JP1 and therefore

cut the

power sup-

ply to the mod-

ule.

This is of no concern.

Plug the six-pin connector from

the USBasp programmer into CON3 on

the signal generator PCB, making sure that

pin 1 on the programmer cable lines up

with the pin 1 indicator on the PCB.

Now select “Write FLASH buffer to chip”

or “Write – Flash” to program the ATme-

ga328 with the HEX file. The LEDs on the

USBasp will blink furiously for a minute

or two while the HEX file is loaded into

the ATmega328. A bar graph may be dis-

played in some cases on the PC screen, to

show progress.

You then have to program the ATme-

ga328 internal ‘fuses’. These configure

the operating characteristics of the AT-

mega328 to suit the software being run

on the device.

For this step, insert the following set-

tings into the relevant Fuse page/section

of the programming software, then click

on “Write” to send the data to the fuses:

Low byte: 0xE2

High byte: 0xD9

Extended byte: 0xFF

Lock byte: 0xFF

Since the processor and display are

powered via the programmer, once pro-

gramming is complete, the display will

briefly show the start-up message and then

the initial signal generator screen. At this

point, you can unplug the programming

cable from CON3 and place a shunt on JP1.

Programming the ATmega328 micro

dly, but there are two benefits: this is a
standard part that’s easier to get, and its
shaft will line up perfectly with push-
buttons S1/S2 and the access hole for
trimpot VR1 (if provided).
Alternatively, if you can get your
hands on a 9mm PCB-mounting right-
angle potentiometer, it will be dead
easy to mount to the PCB, as it’s fitted
similarly to RE1.
However, due to the location of the
hole for the 16mm pot’s shaft, its shaft
will sit around 3.5mm lower than S1/
S2 and VR1.

But this is hardly a tragedy. So the

choice is yours.

Now plug in the ATmega328 micro-

controller (IC1), making sure its pin 1

is orientated correctly, to towards the

upper-left corner of the board.

If you haven’t already programmed

it or purchased a programmed chip,

see the panel above detailing the pro-

gramming instructions.

Further testing

Later, we will be attaching REG1 to

the metal case but since we haven’t

built it yet, so for further testing, tem-

porarily attach a flag heatsink or at-

tach it to a spare sheet of metal using

a machine screw and nut.

You can now apply 12V power to

CON1, press S3 and check that you

can control the output frequency, am-

plitude etc (see the operating instruc-

tions below).

Power the unit down before finish-

ing construction.

Fitting the shields

You will notice several holes around

the buffer, attenuator, output and band

select/HPF sections of the board. There

are also lines on the PCB ‘silkscreen’

between these holes. This is where

shield plates can be fitted.

However, you do not need to fit

shields in most of these areas; the only

ones that are critical are those between

the three high-pass filter sections (be-

tween L1 & L2 and L2 & L3).

So you only really need to cut two

shield pieces and mount them using

four posts in the holes provided. These

are shown in red on the PCB overlay

diagram, Fig.5.

Each shield piece should be around

8mm high and cut from 0.5mm tin

plate, or recycled tin cans (a fruit or

Milo tin lid is ideal).

The strips are then mounted to the

board using component leads off-cuts

soldered into the holes shown in red.

This is simple yet effective.

You could fit shields in the other

locations but testing has shown that

it makes virtually no difference to

the device’s performance so I don’t

feel that it’s worth the time and ef-

fort to do so.

Making the enclosure

I couldn’t find a suitable ready-

made box for the signal generator, so

I came up with a relatively easy way

to make one.

It’s a simple folded metal box and

works well, resulting in a unit that is

light but robust, compact and effec-

tively shielded.

Dimensioned drawings of the metal-

work are available on the SILICON CHIP

website – they’re a little too large to

publish here! The two panels are cut

and folded from 0.8mm thick alumin-

ium sheets. The top cover and base

may each be cut from a small 300 x

250mm sheet, making it relatively in-

expensive to build.

This grade of aluminium is light