899 Densely Packed Decimal :: Quicker, easier and cheaper to make your own chip!

How it works

This project converts between three formats for representing values from 0 to 999:

Binary, wherein decimal values 0-999 are encoded respectively as 10-bit binary values 0000000000-1111100111.
Binary-Coded Decimal (BCD), wherein each decimal digit 0-9 is encoded separately as a 4-bit binary value 0000-1001.
Densely-Packed Decimal (DPD), a compressed form of BCD which only occupies 10 bits.

Asynchronous mode

When RESET is LOW, this project functions in asynchronous mode. Changes to the input are immediately reflected in the output without the use of a clock signal. Input format and output format are specified separately, and only half the output is available at any moment.

#	Dedicated Input	Dedicated Output	Bidirectional I/O
0	Input D8	Output Q0/Q8	Input D0
1	Input D9	Output Q1/Q9	Input D1
2	Input D10	Output Q2/Q10	Input D2
3	DPD input/D11	Output Q3/Q11	Input D3
4	BCD input	Output Q4/—	Input D4
5	A0	Output Q5/—	Input D5
6	DPD output	Output Q6/—	Input D6
7	BCD output	Output Q7/ERR	Input D7

For binary input, set dedicated inputs 4 and 3 LOW, dedicated inputs 1 and 0 to the most significant 2 bits, and bidirectional pins 7 down to 0 to the least significant 8 bits.
For BCD input, set dedicated input 4 HIGH, dedicated inputs 3 down to 0 to the hundreds digit, bidirectional pins 7 down to 4 to the tens digit, and bidirectional pins 3 down to 0 to the ones digit.
For DPD input, set dedicated input 4 LOW, dedicated input 3 HIGH, dedicated inputs 1 and 0 to the most significant 2 bits, and bidirectional pins 7 down to 0 to the least significant 8 bits.
For binary output, set dedicated inputs 7 and 6 LOW. Set A0 (dedicated input 5) HIGH for the most significant 2 bits and LOW for the least significant 8 bits on the dedicated outputs.
For BCD output, set dedicated input 7 HIGH. Set A0 (dedicated input 5) HIGH for the hundreds digit and LOW for the tens and ones digits on the dedicated outputs.
For DPD output, set dedicated input 7 LOW and dedicated input 6 HIGH. Set A0 (dedicated input 5) HIGH for the most significant 2 bits and LOW for the least significant 8 bits on the dedicated outputs.

Synchronous mode

When RESET is HIGH, this project functions in synchronous mode. Input is latched when /WE (dedicated input 7) is LOW and CLK is pulsed. Output is driven when /OE (dedicated input 6) is LOW. Input format and output format are specified using the same two inputs, and the entire output is available at once.

#	Dedicated Input	Dedicated Output	Bidirectional I/O
0	Input D8	Output Q8	Input D0/Output Q0
1	Input D9	Output Q9	Input D1/Output Q1
2	Input D10	Output Q10	Input D2/Output Q2
3	Input D11	Output Q11	Input D3/Output Q3
4	BCD mode	—	Input D4/Output Q4
5	DPD mode	—	Input D5/Output Q5
6	/OE (output enable)	—	Input D6/Output Q6
7	/WE (write enable)	ERR	Input D7/Output Q7

Inputting binary format

Set /WE and /OE (dedicated inputs 7 and 6) HIGH.
Set DPD and BCD (dedicated inputs 5 and 4) LOW.
Set dedicated inputs 1 and 0 and bidirectional pins 7 down to 0 to the 10 bits of the binary representation.
Set /WE LOW and pulse CLK.

Inputting BCD format

Set /WE and /OE (dedicated inputs 7 and 6) HIGH.
Set DPD (dedicated input 5) LOW and BCD (dedicated input 4) HIGH.
Set dedicated inputs 3 down to 0 and bidirectional pins 7 down to 0 to the 12 bits of the BCD representation.
Set /WE LOW and pulse CLK.

Inputting DPD format

Set /WE and /OE (dedicated inputs 7 and 6) HIGH.
Set DPD (dedicated input 5) HIGH and BCD (dedicated input 4) LOW.
Set dedicated inputs 1 and 0 and bidirectional pins 7 down to 0 to the 10 bits of the DPD representation.
Set /WE LOW and pulse CLK.

Outputting binary format

Set /WE (dedicated input 7) HIGH and /OE (dedicated input 6) LOW.
Set DPD and BCD (dedicated inputs 5 and 4) LOW.
Read the binary representation from dedicated outputs 1 and 0 and bidirectional pins 7 down to 0.

Outputting BCD format

Set /WE (dedicated input 7) HIGH and /OE (dedicated input 6) LOW.
Set DPD (dedicated input 5) LOW and BCD (dedicated input 4) HIGH.
Read the BCD representation from dedicated outputs 3 down to 0 and bidirectional pins 7 down to 0.

Outputting DPD format

Set /WE (dedicated input 7) HIGH and /OE (dedicated input 6) LOW.
Set DPD (dedicated input 5) HIGH and BCD (dedicated input 4) LOW.
Read the DPD representation from dedicated outputs 1 and 0 and bidirectional pins 7 down to 0.

How to test

The test.py file tests conversion in all directions.

External hardware

Whatever input and output devices you want.

899 Densely Packed Decimal