LogiSim

Basically I am using SubLEq to teach myself LogiSim.

Is it even possible to get anything useful out of a 4 bit SubLEq.

Actually, yes I can get a 4 bit counter working in LogiSim.

74XXX Libraries

You will need some external 74XXX libraries. I used 74xx Libary.circ (https://github.com/kara-abdelaziz/CHUMP-processor/) and logi7400ic.circ (https://github.com/r0the/logi7400). Did not find a library that was complete.

I found and error in logi7400ic.circ for the 74161. The clock input was inverted. I modified my version of the library to fix it. Taking down this error took many hours.

From Concept to Working Simulation

I am impressed, about 15 hours over three days as a LogiSim novice.

Structure

LogiSim encourages the user to design your circuits hierarchically. Both with "bit width" and sub-circuits. As SubLEq is difficult to understand this is a good approach.

Programming SubLEq

Here is the binary counting program I am trying to implement:

Usually address -1 or in the case 0xF serves as Input/Output or output in this case.

Note: A write to the I/O address also write the value to memory.

SubLEq has only one instruction:

1. Mem[B] = Mem[B] - Mem[A] and  jump to C if the result is Less or Equal to zero.

Decoding the Binary Counter Program

Address Comment

0             0,1,3 ;Set up Z (=0) and P (=1)

Usually this is written:

0            0 1 ?    ( ? means the very next address)

3            1 0 ?    ( 1 is the count increment, set Z = -1 )

6            0 F ?   ( Increment the output address 0xF )

9            0 0 3   ( Reset Z = 0 and jump to address 3 )

C            0 0 0  ( Not reachable but: Reset Z = 0 and jump to address 0 )

F            0         ( The I/O address )

I am sure you all got the above and can now program in SubLEq.

Where to Start?

The timing diagram. I can do this because I have a bit of an idea what the sub-system look like:

Lets put this into steps:

Don't be misled, although I present a linear process, it is still iterative. 

The ALU

The ALU is fairly simple:

Subtract: B = B - A

Flag: !LEq

Therefore we need:

• Load A: !ALU_LDA
• Load B: !ALU_LDB
• Read B - A: !ALU_OUT

And because of the type of control logic I am using:

• Clock: CLK
• Reset: RST

Here is my schematic:

Chip Count:

• 2x 74HC173
• 2x 74HC283
• 1x 74HC04
• 1x 74HC125 (the inverter and tri-state output buffers)

The  beauty of LogiSim is that you can test the sub-circuit before proceeding.

Program Counter

The program counter is perhaps the simplest sub-circuit to design. Note: the outputs PC0-3, these are for debuging LED display in the top level:

Note: the 74161 in the logi7400ic.circ library has the clock inverted. I have edited my version of the library.

Memory Address Register (MAR)

The idea behind the MAR is to load a value from memory and then use that value as an address to load an indirect memory value. Thus it is a form a indirect addressing.

Note: the MAR0-3 outputs are for debugging.

Output

Output for SubLEq is a little tricky,...