UPC EETAC Bachelor's Degree in Telecommunications Systems and in Network Engineering EEL

Chapter 3 problems

- B3.17 -

2-digit BCD adder / subtractor (µC - C)

Products

B3.16

B3.18

1. Specifications

Design a 2-digit adder-subtractor for unsigned BCD numbers (Add_Subt_BCD_2digit) using a PIC18F46K22. We limit the circuit to 2-digit because the limited number of microcontroller pins.

Symbol Adder 2-digits BCD

Fig. 1. Symbol and some truth table values.

The same project is proposed in D1.17 as a combinational circuit based on logic gates.

Pin assignment option #1:

RU(3..2) ---> RA(4..3) RU(3..2) ---> RD(5..4)  
RT(3..2) ---> RC(7..6) RT(1..0) ---> RB(7..6) OP ---> RA5
BT(3..2) ---> RC(5..4) BT(1..0) ---> RB(5..4)
AU(3..2) ---> RB(3..2) AU(1..0) ---> RC(3..2) Cout ---> RA1
BU(3..2) ---> RB(1..0) BU(3..2) ---> RC(1..0)  
AT(3..0) ---> RD(3..0) Error ---> RD7 S ---> RE1

 Pin assignment option #2:

AT(3..2) ---> RC(7..6) AT(1..0) ---> RB(7..6) OP ---> RA2
BT(3..2) ---> RB(5..4) BT(1..0) ---> RC(5..4)
AU(3..2) ---> RC(3..2) AU(1..0) ---> RB(3..2) Cout ---> RA5
BU(3..2) ---> RC(1..0) BU(3..2) ---> RB(1..0)  
RT(3..0) ---> RD(3..0) Error ---> RD4 S ---> RE1
RU(3..2) ---> RA(1..0) RU(1..0) ---> RD(7..6)  

Pin assignment option #3:

AT(3..2) ---> RB(7..6) AT(1..0) ---> RC(7..6) OP ---> RA1
RT(3..2) ---> RB(5..4) RT(1..0) ---> RC(5..4)
RU(3..2) ---> RE(1..0) RU(3..2) ---> RA(3..2)  
AU(3..2) ---> RC(3..2) AU(1..0) ---> RB(3..2) Cout ---> RA5
BU(3..2) ---> RC(1..0) BU(3..2) ---> RB(1..0)  
BT(3..0) ---> RD(7..4) Error ---> RD3 S ---> RE2