ATmega128 prototype board

ATmega 128 Prototyping Board

This little project started off with a large graphical VFD. I wanted to make a controller board for that display so it could be controlled from a PC (or other device) via USB. But when I started to draw up the schematics, I ended up with a more generic design that made it possible to use the Atmel ATmega128 (and compatible) in different projects.

The idea behind this page it to provide some help for those who had got a board from me (or made one yourself from my plans). You can find schematics, placement drawings and port usage pictures here as downloadable pdf-file.

There are actually not much to describe about this. A rather simple and small board that allows you to play around with an ATmega128 (verified) or pin-compatible (ATmega1281 or ATmega2561, these are not yet verified). To summarize the design, the following features are avaiable:

Uses 64-pin ATmega128/1281/2561
USB-serial interface via FTDI FT232RL
Selectable power (USB/External/VTarget, external could be 5V or 3.3V)
Selectable clock source (crystal or clock from FT232RL)
All ports are avaiable at pinheaders
Direct interface to GU300 Graphical VFD's (may be removed in future versions)
Port D, E & F are laid out identical to the STK500
Port A, C & G are in a single header, making it possible for future memory expansion

Building the thing

This build requires SMD-mount skills. Best to start with this so You are prepared. The smallest parts are 1206-sized resistors and capacitators. The FT232RL is in a SSOP-28 and the ATmega is a TQFP-64.

First of all, You will need these parts to build this thing:

Value	Position	Type	Note
330 ohm	R1, R4, R5	1206	Value may be adjusted depending on LED selection.
4.7 kohm	R2	1206	Optional - Used at external power only.
6.8 kohm	R7	1206	Adapt value to application.
10 kohm	R3	1206	Optional - Used at external power only.
10 kohm	R6	1206
XX pF	C6, C7	1206	Must be matched with crystal if used.
100 nF	C2 - C5, C8 - C12	1206
22 uF/10 V	C1	EIA 6032	Tantalium type.
XX MHz	X1	HC49	Choose any within ATmega specification.
32.768 kHz	X2		Clock crystal
3 mm LED	D1, D2, D3		Choose any color you like...
ATmega128	U1	TQFP64	Could be replaced with ATmega1281/2561.
FT232RL	U2	SSOP-28
2-pin strap	JP1, JP3, JP4		2.54 mm spacing
4-pin strap	JP2		2.54 mm spacing
Pushbutton	B1	6 x 6 mm	ALPS
6-pin header	J10
10-pin header	J4, J5, J6, J8, J9
26-pin header	J2, J3
USB-B	J1		USB type B PCB connector

The easiest way to build this board is to start with the two IC's, followed by the other SMD parts. Then, mount LED's, reset button and crystal(s). Finish the build by mounting the strapping headers and all pinheaders. This board should be an easy task if you have done SMD before. If not, then search the net - there are some decent guides around (or get someone to help you).

This board allows some different setups regarding power supply and MCU clock. The selection involves selecting some parts to be mounted and some to leave out. Below are the combinations that could be used for power:

Power via USB: Short JP1 (with strap or permanently with a piece of wire), don't mount R2 and R3. If you are using Vtarget power (from programmer), disconnect USB-power (remove J1 or pull the USB-plug).

External power: Mount R3 and R3, leave J1 open (best to not put the strap pin there to avoid mistakes). Connect power source to J7 (observe polarity! There are no protection). You can feed the board with both 5.0 V and 3.3 V as the FT232R runs on both. Don't use Vtarget (power from programmer) when using external power (leave JP4 open).

Vtarget power: This is power taken from the programmer connected via ISP or JTAG. When this mode is used, all other power options must be disconnected. To use this mode, short JP4 and mount R2 and R3 if you are not planning to power the circuit from USB when done with programming. Leave JP1 open and don't connect anything to J7.

To avoid mistakes, there are some simple rules to follow:

1. Don't apply multiple power sources. Choose one and stay with it. If changed (specially if Vtarget have been used), be sure that the board is properly reconfigured.

2. R2 and R3 should not be used if power is taken from USB.

3. There is no polarity protection at the board. Beware of that when connecting to J7. USB or Vtarget does not have the risk as reveres polarity can't be done.

4. 3.3 V power could be used, but only via J7.

Next up are the clock options:

On-board crystal: The design allows the user to mount a standard parallel-resonance crystal with it's two caps (C6 & C7). When this option is used permanently, don't mount JP3 (best to leave the position empty).

Clock taken from FT232R: This is a cool option - you could leave the crystal assembly out and take the clock from the FT232R chip. This is done by shorting JP3. The clock signal is delivered from pin CBUS4 at the FT-chip. By default, this pin aren't used for clocking, so it must be configured. This is done with a tool from FTDI called MProg (see further description below).

The lab-setup: This is just a combination of the two. Mount a 2-pin strapheader at JP3, a small socket for the crystal and the two caps (C6 & C7). To use clock from FT232R, leave the crystal socket empty and short JP3. To use crystal, mount it in the socket and leave JP3 open.

Internal MCU-clock: This option is independent of the options above. It is set by programming the fuse bits in the ATmega128. If you plan to use internal oscillator permanently, then you could leave some parts out: X1, C6, C7 and JP3.

Regarding the rest of the board, just use the pinheaders you want. If you don't use all ports, just skip these if cost is an issue. The GU300-connector (J2) contains the complete port B and partly port D (0 - 4). The complete port D is avaiable at J4. Just note that PD0 - PD4 are duplicated in J2 and J4.

This board is mainly build around the ATmega128 MCU, but it could be used with ATmega1281 and ATmega2561 too as these models are pin-compatible with the 128.

If you don't need the USB-port, then it is ok to skip these parts entirely. They are J1, U2, C3 - C5, D2, D3, R2 - R5 and JP2. A regular serial port could be connected to port E (J5). RXD0 is at pin 1 and TXD0 is at pin 2. Any level conversion has to be done outside the board, using a MAX232 for example. Even if you have the USB fitted, you could use RXD0/TXD0 as a regular com-port by removing the JP4 (power is still avaiable from USB).

A nice feature by having the FT232R at the board is the possiblility to have the clock from it. The board is designed to allow the CBUS4 signal to be connected to XTAL1 of the ATmega. By doing this, you could save a crystal and two caps. You have to program the ATmega to use an external oscillator for this feature. You have totally four different frequencies to choose from: 6 MHz, 12 MHz, 24 MHz and 48 MHz. For a ATmega128, the two lower frequencies could be used as the chip is rated for max 16 MHz. The actual selection of frequency at CBUS4 is done via a tool provided by FTDI (you can find it at their homepage) called MProg. Current version (when this was written) is 3.0a.

Before you can use the tool (or the USB-serial function at all) you must install a Windows driver. This one is also avaiable at the FTDI homepage. You should use the VCP-drivers and they are compatible with all Windows-versions from 2000 and up, including Vista. Running 64 bit? Not a problem - they work for 64 bits versions of XP, Vista and Server 2003 too. The installation of the drivers follows standard Windows procedure. First download the VCP package and store it in a temporary place at the harddisk. Unpack it with WinZIP or similar tool. Now, plug in your ATmega-board into a free USB-port. The "Found new hardware"-guide will pop-up. Deny all request for automatic search of internet and CD/DVD. Instead, point the guide to the directory where you have unpacked the driver files. Finish the installation and reboot the PC if requested.

Add-on board with ethernet and SRAM-expansion

This is a useful upgrade to the ATmega128 prototype board. It adds on three useful features that did not make into the base design:

10 Mps ethernet using the Microchip ENC28J60.
Memory expansion: 32k, 64k, or 128k, the last one by using bank switching (the ATmega can only address 64k).
Direct connection of Hitachi-compatible LCD with contrast adjustment and LED backlighter connection.

Currently, this part is still in development. The schematics design and board layout work is done an the first series of boards have been manufactured and built (see pictures far below). Next step is to write some software that tests through all the functions of the board.

Downloadable files

Useful links

The Pictures (of course)