of University California at Berkeley College of Engineering Department of Electrical Engineering and Computer Sciences
CS 150 Fall 1999
Lab 7 Wire-Wrap and SRAMS
J. Wawrzynek and N. Weaver Later revisions by R. Fearing, X. Zhang, and B. Choi
1 Objectives
For this lab, you will 1. 2. 3. 4. Learn to use Wire-Wrap. Gain more practice in digital design Learn some strategies to test circuits Wire an SRAM to the Xilinx on your Design Demonstration Board and test it.
2 Prelab
Following the directions in Section 3, use Wire-Wrap to connect a W24512AK, an 64K 8 SRAM to the Xilinx chip on your Design Demonstration Board. Use the pinouts in Figure 2 and a Wrap-ID from Page 5. Design the control logic for the circuit described in Section 4.1.
3 Wire-Wrap
Wire-Wrap is a prototype construction technique where 30-gauge wire Wire-Wrap wire" is twisted around a square post to make a connection. These connections are physically strong, reliable, electrically sound, and quickly made.
Wrapping End Unwrapping End
Hole for Pin
Hole for Wire
Slot for Wire
Wire Stripper
Figure 1: A Wire-Wrap tool not to scale A Wire-Wrap tool consists of three parts: A central hexagonal rod with a wire stripper in the center the little black notch. The wrapping end: a black circular rod with a hole in the center and an o -center hole leading to a side slot. The unwrapping end: a shorter circular rod with a hole in the center. 1
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Lab 7 | Wire-Wrap and SRAMS
To make a Wire-Wrap connection, 1. Plan the route of your wire. Leave little slack, perhaps a half inch. Wires should lie at against the board. 2. Cut the wire 2.5 inches longer than the route you chose. 3. Strip an inch of insulation o each end of the wire using the WireWrap tool's stripper: thread the wire through the circular hole in the hexagonal handle, push it into the stripper's notch, and pull. 4. Insert a stripped end of the wire into the o -center hole in the end of the wrapping end of the tool. The wire should be visible in the slot on the side, although it should not hang out of this slot. 5. Put the wrapping end of the tool on the pin to be connected. The pin should t easily in the center hole. 6. Gently, without pushing down or lifting up, twist the tool clockwise about twelve revolutions. This should wrap a few turns of insulated wire around the bottom, followed by the stripped wire. There should be a single layer of wire, with no spaces between turns.
October 6, 1999
If you make a mistake connecting a wire, remove it completely and start again. Place the unwrapping end of the tool on the pin and turn counter-clockwise. The wire should slide o . An unwrapped wire should not be re-wrapped. Use di erently-colored wire to group signals. For example, make the data lines in this lab green, the address lines orange, and the control signals red. It is easy to forget that when you Wire-Wrap, the chip's pins appear as the mirror image of the usual pinout diagram. The best solution is to use a Wrap-ID, something that ts over the Wire-Wrap pins with the correctly mirrored pinout written on it. Such a Wrap-ID is printed on Page 5|cut it out, and carefully push it down on the pins before you start wiring your W24512AK. A few hints on wirewrapping: Always use Wrap-IDs. We will always provide you with Wrap-IDs, but if you ever need to wire wrap something and you don't have one, make one. Be neat. Although it is di cult to produce perfectly pristine connections, you should keep the wires as at and as neet as possible. Messy wirewrap is much harder to debug, and is less reliable. Check for wire fragments. It is quite possible to have a little piece of wire fall down and short out two pins. Look carefully at the wires to make sure this does not happen. Inspect each connection after you make it, to insure it is to the right place and that the connection is solid. If you have a digital multimeter1 , use it to check each connection after you make it.
4 SRAMs
An SRAM Static RAM is a standard form of digital storage. It is generally faster, simpler, and more expensive then DRAM Dynamic RAM. SRAMS are nice when one needs to store a moderate but not excessive amount of data, and needs to retrieve it quickly. The W24512AK you use is a high speed SRAM. It takes only 15 nanoseconds for an address to produce output data, and comes in a standard DIP package. Their electrical interface is simple:
1 If
have. you don't, inexpensive ones are available at Radio Shack. You probably should buy one, they are good things to
October 6, 1999
Lab 7 | Wire-Wrap and SRAMS
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eight data pins, for input and output address input pins. The W24512AK, a 216 = 65536-byte part, has 16 address pins. an active-low and an active high enable chip inputs CE1 and CE2, and an active-low output enable input OE an active-low write enable WE. power VCC and ground GND pins In normal operation CE2 should be tied high. The CE1 input should be low to enable the chip. If WE is high and OE is low, the output lines always re ect the contents of the location speci ed by the address lines: change the address and the output will change. If WE is low, the data at the address will change to re ect the data input.
NC NC A14 A12 A7 A6 A5 A4 A3 A2 A1 A0 IO0 IO1 IO2 GND
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17
VCC A15 CE2 WE A13 A8 A9 A11 OE A10 CE1 IO7 IO6 IO5 IO4 IO3
D0 D1 D2 D3 D4 D5 D6 D7 WE OE A0 A1 A2
72 70 69 68 67 66 65 62 14 41 10 7 5
A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15
4 3 84 83 82 81 80 79 78 77 71 60 61
Figure 2: W24512AK Pinouttop view and pins on the Xilinx
4.1 Testing the SRAM Wiring
There are generally two ways to test a memory, by producing speci c patterns, or by a brute force approach. A set of speci c patterns would test each wire a line at a time, to insure the output is correct. Brute force simply checks all possible values. For this lab, we recommend the brute force approach, as it is easier to implement. We provide a bit le Wvlib cs150 lab7.bit which tests the SRAM , but you should also build your own version. In addition to the wire wraps made to connect the SRAM, you need to connect the spare button to pin 58 and ground, so that you can use it in your design. We provide a datapath for your test circuit, but no control logic. Attach the library Wvlib Lab7 lib Lab7 and add the schematic Wvlib Lab7 lab7block.SCH. The datapath is fairly straightforward. It consists of a 16 bit counter and an 8 bit LFSR. The lower 16 bits of the counter address the SRAM, and the LFSR data is output to written into the SRAM write cycle when the CE1.L, CE2.H, and WE.L signals are asserted. When the WE.L signal is deasserted and OE.L, CE1.L, CE2.H are asserted, the contents of the SRAM are read out read cycle, and compared with the contents of the LFSR. These counters and ip ops have a synchronous reset, so reset must be high and the clock must come to reset them. Your controller should write every memory location, and then read back all memory locations. It should clock the LFSR and the counter, to increment both the address and the LFSR. To write data,
W24512AK-15
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Lab 7 | Wire-Wrap and SRAMS
October 6, 1999
OE.L should rst be deasserted high, and after the address is stable, then on the NEXT clock cycle, assert the WE.L line low. Once all the memory is full, both the counter and LFSR should be reset, and OE.L should be asserted low, then the data in the SRAM should be read back out of memory, and compared with the LFSR value. If the data is di erent at any...