STK500 Expansion Board - Technical Details
The ECROS Technology STK500 Expansion Board is a high-quality printed circuit board using through-hole plating and a lead-free gold finish. All interesting pads are marked with a white silk-screen as shown in the picture below:
The prototyping area is approximately 2" by 3.7" and is a field of individual plated-through holes. At the right and left sides, signals from the STK500 expansion connectors are brought to clearly labeled pads. There are ground pads in all four corners for the attachment of test instrument ground leads.
Power, Ground and Decoupling
Power and ground rails completely surround the prototyping area. They are brought to pads labeled VCC and GND every 0.2" along the top and bottom edges. For power supply decoupling, there are places for two ceramic capacitors (C2 and C3) and one electrolytic capacitor (C1). Suitable parts are supplied in the (optional) kit. Whether you need these capacitors or not will depend on your application. If you need even more decoupling, you can use the VCC and GND pads.
JTAG debug interface
With ATmega16 and ATmega32 MCUs installed in the STK500, their JTAG interface pins are brought to a header suitable for connecting a JTAG ICE. This enables on-chip debugging to be performed. The header is at the left edge of the Expansion Board (see picture, above) and is marked "ATmega16/32 JTAG". As well as the Atmel JTAG ICE, this header is compatible with other ICE interfaces, such as the ECROS Technology AVR ICE-Cube. Note that because the JTAG pins are assigned to different port bits, this header cannot be used with the ATmega162. (An adapter is available for this application, but cannot be used in conjunction with the STK500 Expansion Board.)
On both EXPAND0 and EXPAND1 of the STK500, many low-numbered pins are not interesting for add-on circuits. They are associated with high-voltage programming. The Expansion Board does not connect to these signals, so the headers are 30-pin rather than 40-pin. This actually makes the board easier to connect to and detach from the STK500. As pins 1 and 2 are grounds, you may want to connect them and have a total of 12 ground connections. To do this, either add a couple of two-pin headers (not supplied in the kit) or carefully cut off two pins from the supplied connectors. Install these as pins 1 and 2 at the very top of the board. Install the 28-pin headers as low as possible on the board so that pins 11 and 12 are not populated. These are not interesting signals and are not connected on the Expansion Board. On the other hand, 8 grounds are probably enough and the simplest thing is to just solder the supplied 30-pin headers in the obvious place.
Crystal and Load Capacitors
Atmel decided to make the crystal connections to the MCU available at expansion ports. The Expansion Board provides a location for a standard crystal and two load capacitors. These are on the right edge of the board, marked Y1, C4 and C5. If you put a crystal here, take into consideration the very long traces connecting EXPAND1 to the MCU sockets and also to the PORTE/AUX header of the STK500. Read Atmel's STK500 User Guide regarding the use of crystals with the target MCU. Rather surprisingly, tests have uncovered no problems using crystals up to 15.36 MHz in a crystal socket on the Expansion Board.
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