Assembling the Handy Board, Part 2

Memory Circuitry


This is the second step in the Handy Board assembly. In this assembly sequence, the 32K static memory is installed and tested.

For this assembly sequence, collect the following parts:


Memory Power Circuit

The memory chip has its own power circuit that provides it with power even when main board power is turned off. This is necessary to keep the memory from losing its contents when the board is off.

Assembly

Install C4*, C5, and U14*.

Testing

Apply power to the board via the charge jack, and turn the board off. Measure the voltage at pin 28 of U2 (the voltage supply for the RAM chip). Any ground may be used as reference (e.g., the pin labelled GND on the U4 connector). The power to the RAM should be between 4.9v and 5.1v, whether main board power is switched on or off.

Debugging

Make sure power exists on the main +5v supply when the board is turned on (e.g., the +5v pin of the U4 connector). Make sure there is power to the input side of U14, which is the pin closest to C3.

Check that U14 is installed correctly---the flatted side should face toward the SPRK1 component. Check polarity on C4---the + terminal should be nearer to the top edge of the board.


Memory Addressing Latch

U3, the 74HC373, is known as a transparent latch. This chip is used in the memory circuit to temporarily store, or latch, the lower eight bits of the address being selected from the memory. The 6811 first generates the low address bits, instructs U3 to store them, and then generates the high address bits and performs the memory read or write operation.

In the Handy Board component layout, U3 is mounted underneath the memory chip itself. This construction trick saves board space, and since U3 and the memory chip are highly interconnected, works out in terms of the board's printed wiring.

The only downside to this method is that U3 must be soldered directly to the board. Vertical clearance would be a problem if a socket were used. This should not cause any great consternation; as long as U3 is soldered carefully (and in the correct orientation!) there should not be any difficulty.

Assembly

Install U3* (74HC373). This chip is soldered directly to the main board---no socket is used. Since U3 is soldered directly to the board, make absolutely sure that the notch indicating pin 1 of the chip lines up with the notch on the printed component outline.

Get DIP3, the 28--pin socket for the memory chip. Using an X-acto knife or diagonal cutters, remove the center plastic support from the socket so that the socket may be installed around U3. Install the socket its mounting holes, aligning the notch on the socket with the notch on the printed component outline. Solder it in place.

Testing and Debugging

There are no testing and debugging procedures for this assembly step.

Memory Chip and Decoding Gates

In addition to U3, the latch chip, the memory requires a couple gates from U7, a package of four NAND gates. (Logic gates for interfacing digital circuits are generically known as ``glue logic'' in digital design.)

In this assembly step the remainder of the circuitry for proper functioning of the memory is installed.

Assembly

Install DIP5, the 14--pin socket for U7. Align the notch in the socket with the notch in the printed component outline.

Place U7* (74HC132) in the socket, aligning its notch with the one on the socket.

Place U2* (62256-12LP), the memory chip, in its socket. Make sure to align the notch on the chip with the notch on the socket/component placement marking.

Testing

Testing requires the use of custom memory-test software that is not yet prepared. For now, just move ahead to the next assembly section; the test will be running Interactive C itself.

Debugging

Debugging requires the use of the custom software mentioned above. Please write to handyboard@media.mit.edu for debugging assistance.
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Fred Martin / MIT Media Laboratory / fredm@media.mit.edu / Sat Jan 6 17:16:28 1996