EEE2243 Digital System Design Chapter 4: Datapath Components by Muhazam Mustapha, February 2012

1. EEE2243Digital System DesignChapter 4: Datapath Componentsby Muhazam Mustapha, February 2012

2. Learning Outcome • By the end of this chapter, students are expected to understand the design, operation and block diagram of the following datapath components: • Register and register file • Shifter, counter, incrementer • Comparator • Adder, subtractor, multiplier

3. Chapter Content • Register and Register File • Shifter • Modular Verilog • Counter, Incrementer and Timer • Comparator • Adder, Subtractor, Multiplier

4. Datapath Component

5. Datapath Component Datapath component is a collection of memory and computation circuits that when put together and with proper control, can perform a larger scale of operation In previous chapters we have covered many of the components like: Counter, Decoder and Multiplexer We will go into more details on the components that we already covered, and some new ones Vahid 4.1 pg 167

6. Register & Register File

7. Register A collection of flip-flops used to store data or maintain states in FSM In normal operations registers load on every clock pulse In datapath operation we need to set the register to load only when we want it to I 3 I 2 I 1 I 0 4-bit register D D D D Q Q Q Q clk Q3 Q2 Q1 Q0 load

8. Conditional Parallel Load In datapath operation register load operations are mostly parallel – only communication applications use serial mode load The load operation however, needs to be done when a LOAD signal is on This can be done in BEHAVIORAL approach using a 2-to-1 mux at D input diagram on next slide The LOAD signal will determine either the flip-flop is to be loaded with new data or maintain current data by feedback

9. Conditional Parallel Load Vahid Figure 4.1 pg 169

10. Parallel Register Example a3 a2 a1 a0 I 3 I 2 I 1 I 0 1 ld R0 clk Q3 Q2 Q1 Q0 I 3 I 2 I 1 I 0 I 3 I 2 I 1 I 0 1 ld 1 ld R1 R2 Q3 Q2 Q1 Q0 Q3 Q2 Q1 Q0

11. Shift Register In some applications, datapath operations need to perform serial computation This can be done using shift register Shift register operates by transferring flip-flop’s bit content to neighboring flip-flop in the same register while maintaining the conditional LOAD operation Both SHIFT and LOAD operation are conditional Some shift registers may have parallel LOAD operation as well Vahid pg 173

12. Shift Register shr_in • Implementation: Connect flip-flop output to next flip-flop’s input Datapath Implementation Vahid Figure 4.10 pg 174

13. Rotate Register Bit coming out from one end will go into the other end Exercise: Draw out yourself the datapath implementation of rotate register Vahid Figure 4.11 pg 174

14. More Complicated Register Behavioral control of register by mux can already provide us a good number of features The following is a register with 2 control line: s1 s0 Operation 0 0 Maintain present value Functions: 0 1 Parallel load 1 0 Shift right 1 1 Load zero Vahid Figure 4.13 pg 176

15. Boolean Algebra Approach Register Design For some performance improvement, Boolean algebra design can be used; Try the following: I 3 I 2 I 1 I 0 shr_in I 3 I 2 I 1 I 0 shr_in ld s1 shl_in shl_in s0 shr Q3 Q2 Q1 Q0 shl Q3 Q2 Q1 Q0 c ombi- s1 = ld’*shr’*shl + ld’*shr*shl’ + ld’*shr*shl s0 = ld’*shr’*shl + ld n a tional ci r cuit Vahid pg 177-178

16. Register File In some designs we may require too many registers In such designs we might be using too many wires and may cause congestions or we may exceed the synthesizable limit for no. lines This can be solve by sharing lines – BUS The registers will be accessed through address and only connect to the bus lines when selected There will be many High-Z capable connections Vahid 4.10 pg 225

17. Register File 32 32 W_data R_data 2 2 W_addr R_addr W_en R_en 4x32 register file Vahid Figure 4.78 (modified), 4.79, 4.80

18. Register File Timing Diagram 32 32 W_data R_data 2 2 W_addr R_addr W_en R_en 4x32 register file Vahid Figure 4.82

19. Shifter

20. Shifter Shift register makes transfer between memory elements (flip-flop) to achieve shifting effect We can speed up the shift process if we remove the memory element since we don’t have to wait for the clock pulse to actually make the shift happens This fast shifting is required especially if the shifting is done to get the effect of multiplication by powers of 2 Shifter is the datapath component that does this with only multiplexers – no D flip-flop Vahid 4.8 pg 210

21. Shifter i3 i2 i1 i0 i3 i2 i1 i0 i3 i2 i1 i0 inR inL in 2 0 1 2 0 1 2 0 1 2 0 1 in 0 1 0 1 0 1 0 1 shL s0 sh s1 shR q3 q2 q1 q0 Left shifter <<1 q3 q2 q1 q0 Shifter with left shift, right shift, and no shift q3 q2 q1 q0 Symbol Shifter with left shift or no shift

22. Modular Verilog

23. Module Instantiation In Verilog modules can be defined and instantiated Wires can be used to interconnect modules and other components in the design The calling syntax is as follows: Instantiated_Module Name_of_this_instance( .Instantiated_Parameter_1(local_variable_1), .Instantiated_Parameter_2(local_variable_2), ... );

24. Module Instantiation Let’s take the Half-Adder circuit in Chapter 1 to be instantiated in a design of Full-Adder as given in Adder Sub-section of this chapter The Half-Adder code: module HalfAdder(a, b, sum, carry); input a, b; output sum, carry; assign sum = a ^ b; assign carry = a & b;endmodule a sum carry b

25. Module Instantiation The Full-Adder design to make use of the Half-Adder: Wires Half-Adder Instances Wires

26. Module Instantiation module HalfAdder(a, b, sum, carry); input a, b; output sum, carry; assign sum = a ^ b; assign carry = a & b;endmodulemodule FA(a, b, cin, s, cout); input a, b, cin; output s, cout; wire tempSum, finSum, c1, c2; HA1 First (.a(a), .b(b), .sum(tempSum), .carry(c1)); HA1 Second(.a(tempSum), .b(cin), .sum(finSum), .carry(c2)); assign s = finSum; assign cout = c1 | c2;endmodule

27. Counter, Incrementer & Timer

28. Modularized Counter In the previous chapter we designed counter to just count the clock pulses As a datapath component, counter needs control lines so it can be integrated into a bigger design We will see more modularized counter design as a datapath component Vahid 4.9 pg 215

29. Modularized Up Counter c n t 4-bit up-counter t c C 4 1 0 0000 1111 4-bit up-counter c n t ld 4-bit register 4 4 +1 4 4 t c C • Regular up counter can be modularized to have • Registers to keep count value • Incrementer • Terminal count detector – for up counter, it is an AND gate as we assume the terminal count is all ones 0 1 0 0010 0011 0001 0000 0101 0100 0001 1110 ... Vahid pg 216

30. Incrementer Incrementer is an adder whose job is to always add 1 to its input It is natural to design incrementer using FULL adder setting one of the addend to a constant 1 However, a better design will be using cascaded HALF adders with a 1 addend at LSB This can be done since the second addend is basically zero (except at LSB), hence lifting its need as one of full adder inputs which means we can reduce the full adder to half adder This results in a simpler circuit

31. Incrementer a3 a2 0 a1 0 a0 1 0 0 a3 a2 a1 a0 1 a b ci a b ci a b ci a b ci F A F A F A F A a b a b a b a b c o s c o s c o s c o s H A H A H A H A c o s c o s c o s c o s c o s3 s2 s1 s0 c o s3 s2 s1 s0 0011 + 0001 equivalent equivalent 0011 + 1 + 0000

32. Modularized Down Counter • Modular down counter may have • Registers to keep count value • Decrementer • Terminal count detector – for down counter, it is a NOR gate as we assume the terminal count is all zeros 4-bit down-counter c n t ld 4-bit register 4 4 –1 4 t c C 4

33. Decrementer Decrementer is a subtractor whose job is to always minus 1 from its input Decrementer can be designed using full adder by setting one of the addend to all 1-s This is true because subtracting by 1 is equivalent to adding with 2’s complement of 1, and 2’s complement of 1 is a binary number with all 1-s There is a way to design decrementer using HALF SUBTRACTOR (HS), but we are not going into detail about it as it is not so behavioral design friendly as HS is not well accepted as datapath component a3 a2 1 a1 1 a0 1 0 1 a b ci a b ci a b ci a b ci F A F A F A F A c o s c o s c o s c o s c o s3 s2 s1 s0

34. Up/Down Counter To make it a more useful counter in datapath circuit, we might want to add more control to the counter One possible control is to control the direction of counting – either up or down This can be done by adding DIR (direction) control line DIR will be used to multiplex the: feedback input to register – either from incrementer or decrementer terminal count detecter – either from AND or NOR gate

35. Up/Down Counter 4-bit up/down counter dir x 1 4-bit 2 1 0 4 4-bit register 4 4 4 4 –1 +1 4 4 4 x 1 2 1 0 t c C Vahid pg 217

36. Timer Timer is just a counter that is clocked by a KNOWN clock frequency Normally it is an up counter, with additional controls of: count (CNT) – only count if CNT is 1 clear (CLR) – synchronously reset timer if CLR is 1 Vahid pg 222

37. Comparator

38. Equality Comparator Comparator is a datapath component that test the values of its two inputs for equality, less than or greater condition Equality comparator just test for equality Obviously equality test can be done by comparing the equality of the two inputs bit-by-bit Comparison can be done bit-by-bit by 2-input XNOR gates, then combine the output by an AND Vahid 4.4 pg 191

39. Equality Comparator a3 b3 a2 b2 a1 b1 a0 b0 0 0 1 1 1 1 0 1 a3 a2 a1 a0 b3 b2 b1 b0 4-bit equality comparator eq ( b ) eq ( a ) • Example comparing 0110 to 0111: 1 1 1 0 0

40. Magnitude Comparator Magnitude comparator gives all possible equality, less than or greater than condition Algorithm to compare A and B starting at MSB: If the bit are the same repeat to compare next LSB Else if bit A is 1 and bit B is 0 then stop by giving condition A > B Else if bit A is 0 and bit B is 1 then stop by giving condition A < B 1011 1011 1011 1001 1001 1001 Equal A=1011 B=1001 So A > B Equal Unequal Vahid pg 192

41. Cascade Comparator From the algorithm in the previous slide, we can formulate one stage of the bit-by-bit comparison using the following circuit: b a from previous stage to next stage out_gt in_gt out_lt in_lt out_eq in_eq

42. Cascade Comparator At each stage: out_gt = in_gt + (in_eq * a * b) A>B (so far) if already determined in higher stage, or if higher stages equal but in this stage a=1 and b=0 out_lt = in_lt + (in_eq * a * b) A<B (so far) if already determined in higher stage, or if higher stages equal but in this stage a=0 and b=1 out_eq = in_eq * (a XNOR b) A=B (so far) if already determined in higher stage and in this stage a=b too a3 b3 a2 b2 a1 b1 a0 b0 a b a b a b a b Igt in_gt out_gt in_gt out_gt in_gt out_gt in_gt out_gt AgtB Ieq in_eq out_eq in_eq out_eq in_eq out_eq in_eq out_eq AeqB in_lt out_lt in_lt out_lt in_lt out_lt in_lt out_lt AltB Ilt Stage 3 Stage 2 Stage 1 Stage 0

43. Cascade Comparator Example comparing 1011 to 1001: = 1 1 0 0 1 0 1 1 a3 b3 a2 b2 a1 b1 a0 b0 a b a b a b a b 0 0 in_gt out_gt in_gt out_gt in_gt out_gt in_gt out_gt A gtB 1 1 in_eq out_eq in_eq out_eq in_eq out_eq in_eq out_eq A eqB 0 in_lt out_lt in_lt out_lt in_lt out_lt in_lt out_lt A ltB 0 S tage3 S tage2 S tage1 S tage0 ( a ) = 1 1 0 0 1 0 1 1 a3 b3 a2 b2 a1 b1 a0 b0 a b a b a b a b 0 in_gt out_gt in_gt out_gt in_gt out_gt in_gt out_gt A gtB 0 1 in_eq out_eq in_eq out_eq in_eq out_eq in_eq out_eq A eqB 1 0 in_lt out_lt in_lt out_lt in_lt out_lt in_lt out_lt A ltB 0 S tage3 S tage2 S tage1 S tage0 ( b )

44. Cascade Comparator Example comparing 1011 to 1001 (continue): > 1 1 0 0 1 0 1 1 a3 b3 a2 b2 a1 b1 a0 b0 a b a b a b a b 1 0 in_gt out_gt in_gt out_gt in_gt out_gt in_gt out_gt A gtB 0 1 in_eq out_eq in_eq out_eq in_eq out_eq in_eq out_eq A eqB 0 0 in_lt out_lt in_lt out_lt in_lt out_lt in_lt out_lt A ltB S tage3 S tage2 S tage1 S tage0 ( c ) 1 1 0 0 1 0 1 1 a3 b3 a2 b2 a1 b1 a0 b0 a b a b a b a b 1 0 in_gt out_gt in_gt out_gt in_gt out_gt in_gt out_gt A gtB 0 1 in_eq out_eq in_eq out_eq in_eq out_eq in_eq out_eq A eqB 0 0 in_lt out_lt in_lt out_lt in_lt out_lt in_lt out_lt A ltB S tage3 S tage2 S tage1 S tage0 ( d )

45. Comparator Application Minimum of 2 numbers: MIN A B 8 8 8 8 8 8 A B M in A B I 1 I 0 s 0 Igt AgtB C 8-bit 8-bit magnitude comparator 1 Ieq AeqB 2x1 mux 8 0 Ilt AltB 8 ( b ) C ( a )

46. Adder, Subtractor & 2’s Complement, Multiplier

47. Half Adder Half adders add two 1 bit inputs, then generate 1 bit sum and 1 bit carry I nputs O utputs a b c s 0 0 0 0 0 1 0 1 1 0 0 1 1 1 1 0 a b a b Half Adder c s c s Truth Table Block Diagram Circuit Vahid 4.3 pg 181

48. Full Adder Full adders add two 1 bit inputs and one carry-in then generate 1 bit sum and 1 bit carry-out I nputs O utputs a b ci c o s 0 0 0 0 0 0 0 1 0 1 0 1 0 0 1 0 1 1 1 0 a b ci 1 0 0 0 1 1 0 1 1 0 1 1 0 1 0 1 1 1 1 1 s c o Circuits Truth Table Full adder a b ci Block Diagram Full Adder co s

49. Larger Adder – Ripple Carry Arbitrary no. bit adder can be constructed by cascading half adder and full adder as follows: This is called carry ripple technique as the carry is being transferred from one adder to the next one a3 b3 a2 b2 a1 b1 a0 b0 a b ci a b ci a b ci a b F A F A F A H A c o s c o s c o s c o s c o s3 s2 s1 s0 OR a3 b3 a2 b2 a1 b1 a0 b0 0 a b ci a b ci a b ci a b ci F A F A F A F A c o s c o s c o s c o s c o s3 s2 s1 s0

50. Cascading Adders a7 a6 a5 a4 b7 b6 b5 b4 a3 a2 a1 a0 b3 b2 b1 b0 a3 a2 a1 a0 b3 b2 b1 b0 a3 a2 a1 a0 b3 b2 b1 b0 4-bit adder 4-bit adder ci ci c o s3 s2 s1 s0 c o s3 s2 s1 s0 s3 s2 s1 s0 c o s7 s6 s5 s4 a7.. a0 b7.. b0 8-bit adder ci c o s7.. s0