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In order to take advantage of the Display at the BASYS / NEXYS boards do you need a Multiplexed display driver, and its a good way to start using the kit and learn about digital design and VHDL.

The following solution made with standard hardware symbols and not optimal in any way. Your task will be to understand the principles and create your own VHDL based components.

Finally will you be able to make the complete "single" VHDL-component for the driver

•  To better understand the problem, please try the solution first - for the BASYS use:

•                                                                                                    - for the NEXYS use: NEXYS2_muxdisplay.bit

•  Download the zipped project with the design below here: Prob2_Muxdisplay_ver2_start.zip (ISE 9.2i)

Try different combinations of Switches and button presses:

Start with Sw1:Sw0 = 00,  Sw2=1, Sw3=1 - Press Btn0 several times - Press Btn3 once - what happens?

Change to Sw3=0 ..... and later Sw1=1 ... explain what happens.

•  Which update frequency seems to be the best for the purpose?

•  What's the problems with the other?

Clock scale ver.1
 Clock_scale_ver1 combines a 16-bit and a 8-bit counter to form a 24-bit counter - Q(23:0)  = QH(23:16) & QL(15:0) The Test(1:0) inputs = Sw1:Sw0 used to select among 4 different frequencies.
clock_scale_ver1

The standard clock-frequencies of the BASYS boards can be selected among 25MHz, 50MHz and 100MHz with the jumper (please check the documentation).  The NEXYS2 board fixed at 50MHz.

In order to reduce the rather high frequency (say 50MHz) at least 18 F/F needed to form a scale-counter. Two standard counters selected to form a 24-bit counter.

Counter w. 2-bit
 In order to "multiplex" the multiplexed display we need a 2-bit counter.The optimal clock frequency would be 200 Hz (but higher will do as well) The output sequence at S1:S0 will be:  00, 01, 10, 11, 00, 01 .....

 Count2bit

A 2-bit counter needed for the multiplexer / decoder and its driven by the downscaled clock (Clk_200hz).

Note how this counter formed with two Toggle-F/F's and each T-F/F based on a D-F/F and a XOR gate.

Multiplexer 4-bit x 4 and 1-bit x 4

You wont find a 4x4-bit multiplexer as a standard component, hence must it be formed by standard 1-bit multiplexers - and four M4E_1E will do nicely.

The multiplexer for the Decimal Point will however fit nicely into a M4E_1E.

 S1:S0 Y Dp 0 0 Bcd(  3:  0) = A Dp0 0 1 Bcd(  7:  4) = B Dp1 1 0 Bcd(11:  8) = C Dp2 1 1 Bcd(15:12) = D Dp3

Decoder 2 to 4 (active low)
 S1:S0 /D0 /D1 /D2 /D3 0 0 0 1 1 1 0 1 1 0 1 1 1 0 1 1 0 1 1 1 1 1 1 0
D2_4E
Bcd to 7-sement decoder ver.1
bcd27segm_ver1