Counters

1. c n t 4-bit up-counter t c C 4 1 0 1111 0000 4-bit up-counter c n t ld 4-bit register 4 4 +1 4 4 t c C Counters • N-bit up-counter: N-bit register that can increment (add 1) to its own value on each clock cycle • 0000, 0001, 0010, 0011, ...., 1110, 1111, 0000 • Note how count “rolls over” from 1111 to 0000 • Terminal (last) count, tc, equals1 during value just before rollover • Internal design • Register, incrementer, and N-input AND gate to detect terminal count 1 0 a 0 0101 0100 0011 0010 0001 0000 1110 0001 ... a

2. a3 a2 a1 a0 1 a3 a2 a1 a0 a b a b a b a b +1 t HA HA HA HA n c o s3 s2 s1 s0 c o s c o s c o s c o s r c o s3 s2 s1 s0 I ( b ) ( a ) carries: 0 1 1 0 0 1 1 unused + 1 0 0 1 0 0 Incrementer • Counter design used incrementer • Incrementer design • Could use carry-ripple adder with B input set to 00...001 • But when adding 00...001 to another number, the leading 0’s obviously don’t need to be considered -- so just two bits being added per column • Use half-adders (adds two bits) rather than full-adders (adds three bits)

3. I nputs O utputs a3 a2 a1 a0 c0 s3 s2 s1 s0 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 0 0 1 1 0 0 1 0 0 0 1 0 0 0 0 1 0 1 0 1 0 1 0 0 1 1 0 0 1 1 0 0 0 1 1 1 0 1 1 1 0 1 0 0 0 1 0 0 0 0 1 0 0 1 1 0 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 1 1 0 1 1 0 1 1 0 0 1 1 0 0 0 1 1 0 1 1 1 0 1 0 1 1 1 0 1 1 1 0 0 1 1 1 1 1 1 1 1 1 0 0 0 0 Incrementer • Can build faster incrementer using combinational logic design process • Capture truth table • Derive equation for each output • c0 = a3a2a1a0 • ... • s0 = a0’ • Results in small and fast circuit • Note: works for small N -- larger N leads to exponential growth, like for N-bit adder

4. 1 c n t 8-bit up-counter t c C osc 8 (256 Hz) p (unused) (1 Hz) Counter Example: 1 Hz Pulse Generator Using 256 Hz Oscillator • Suppose have 256 Hz oscillator, but want 1 Hz pulse • 1 Hz is 1 pulse per second -- useful for keeping time • Design using 8-bit up-counter, use tc output as pulse • Counts from 0 to 255 (256 counts), so pulses tc every 256 cycles

5. Down-Counter • 4-bit down-counter • 1111, 1110, 1101, 1100, …, 0011, 0010, 0001, 0000, 1111, … • Terminal count is 0000 • Use NOR gate to detect • Need decrementer (-1) – design like designed incrementer 4-bit down-counter c n t ld 4-bit register 4 4 –1 4 t c C 4

6. 4-bit up/down counter dir x 1 4-bit 2 1 0 4 clr clr c n t ld 4-bit register 4 4 4 4 –1 +1 4 4 4 x 1 2 1 0 t c C Up/Down-Counter • Can count either up or down • Includes both incrementer and decrementer • Use dir input to select, using 2x1: dir=0 means up • Likewise, dir selects appropriate terminal count value

7. 3-bit u p - c ou n t er c n t 1 clk t c c2 c1 c0 (1 H z) 0 0 0 0 0 1 0 1 0 unused x 3 8 d c d i2 i1 i0 d7 d6 d5 d4 d3 d2 d1 d0 Counter Example: Light Sequencer • Illuminate 8 lights from right to left, one at a time, one per second • Use 3-bit up-counter to counter from 0 to 7 • Use 3x8 decoder to illuminate appropriate light • Note: Used 3-bit counter with 3x8 decoder • NOT an 8-bit counter – why not? a lig h ts

8. Counter with Parallel Load • Up-counter that can be loaded with external value • Designed using 2x1 mux – ld input selects incremented value or external value • Load the internal register when loading external value or when counting L 4 ld x 1 4-bit 2 1 0 4 c n t ld 4-bit register 4 4 +1 4 t c C

9. 4 ld L 1 c n t t c C clk 4 Counter with Parallel Load 1000 • Useful to create pulses at specific multiples of clock • Not just at N-bit counter’s natural wrap-around of 2N • Example: Pulse every 9 clock cycles • Use 4-bit down-counter with parallel load • Set parallel load input to 8 (1000) • Use terminal count to reload • When count reaches 0, next cycle loads 8. • Why load 8 and not 9? Because 0 is included in count sequence: • 8, 7, 6, 5, 4, 3, 2, 1, 0  9 counts 4-bit down-counter

10. Counter Example: 1 Hz Pulse Generator from 60 Hz Clock • U.S. electricity standard uses 60 Hz signal • Device may convert that to 1 Hz signal to count seconds • Use 6-bit up-counter • Can count from 0 to 63 • Create simple logic to detect 59 (for 60 counts) • Use to clear the counter back to 0 (or to load 0) clr c n t 1 6-bit up counter osc t c C (60 Hz) p (1 Hz)

11. Multiplier – Array Style • Can build multiplier that mimics multiplication by hand • Notice that multiplying multiplicand by 1 is same as ANDing with 1

12. Multiplier – Array Style • Generalized representation of multiplication by hand

13. a3 a2 a1 a0 b0 b1 0 0 b2 + (5-bit) 0 0 b3 A B + (6-bit) * 0 0 0 P + (7-bit) Block symbol p7..p0 Multiplier – Array Style • Multiplier design – array of AND gates pp1 pp2 pp3 pp4

14. 2nd c olumn 3 r d c olumn 4th c olumn 1 0 0 0 1 0 1 0 1 1 0 1 0 1 0 1 0 1 0 1 0 - - - 0 1 1 1 0 1 1 1 0 1 1 1 1 1 0 1 1 0 0 1 1 wi a3 b3 a2 b2 a1 b1 a0 b0 a b wi a b wi a b wi a b wi a3 a2 a1 a0 b3 b2 b1 b0 FS FS FS FS 4-bit subtractor wi w o s3 s2 s1 s0 w o s w o s w o s w o s w o s3 s2 s1 s0 ( b ) ( c ) Subtractor • Can build subtractor as we built carry-ripple adder • Mimic subtraction by hand • Compute borrows from columns on left • Use full-subtractor component: • wi is borrow by column on right, wo borrow from column on left 1st c olumn 1 0 1 0 - 0 1 1 1 1 a

15. Subtractor Example: Color Space Converter – RGB to CMYK • Color • Often represented as weights of three colors: red, green, and blue (RGB) • Perhaps 8 bits each, so specific color is 24 bits • White: R=11111111, G=11111111, B=11111111 • Black: R=00000000, G=00000000, B=00000000 • Other colors: values in between, e.g., R=00111111, G=00000000, B=00001111 would be a reddish purple • Good for computer monitors, which mix red, green, and blue lights to form all colors • Printers use opposite color scheme • Because inks absorb light • Use complementary colors of RGB: Cyan (absorbs red), reflects green and blue, Magenta (absorbs green), and Yellow (absorbs blue)

16. R G B 255 255 255 8 8 8 8 8 8 o CMY - - - t GB R 8 8 8 C M Y Subtractor Example: Color Space Converter – RGB to CMYK • Printers must quickly convert RGB to CMY • C=255-R, M=255-G, Y=255-B • Use subtractors as shown

17. Subtractor Example: Color Space Converter – RGB to CMYK • Try to save colored inks • Expensive • Imperfect – mixing C, M, Y doesn’t yield good-looking black • Solution: Factor out the black or gray from the color, print that part using black ink • e.g., CMY of (250,200,200)= (200,200,200) + (50,0,0). • (200,200,200) is a dark gray – use black ink

18. R G B 8 8 8 Y R G B R GB t o CMY C M Y 8 8 8 8 C M Y MIN 8 MIN 8 K - - - 8 8 8 8 C2 M2 Y2 K Subtractor Example: Color Space Converter – RGB to CMYK • Call black part K • (200,200,200): K=200 • (Letter “B” already used for blue) • Compute minimum of C, M, Y values • Use MIN component designed earlier, using comparator and mux, to compute K • Output resulting K value, and subtract K value from C, M, and Y values • Ex: Input of (250,200,200) yields output of (50,0,0,200) K o CM t GB R

19. Representing Negative Numbers: Two’s Complement • Negative numbers common • How represent in binary? • Signed-magnitude • Use leftmost bit for sign bit • So -5 would be: 1101 using four bits 10000101 using eight bits • Better way: Two’s complement • Big advantage: Allows us to perform subtraction using addition • Thus, only need adder component, no need for separate subtractor component!

20. 1 9 2 8 3 7 4 6 5 5 6 4 7 3 8 2 9 1 Ten’s Complement • Before introducing two’s complement, let’s consider ten’s complement • But, be aware that computers DO NOT USE TEN’S COMPLEMENT. Introduced for intuition only. • Complements for each base ten number shown to right – Complement is the number that when added results in 10

21. - - Ten’s Complement • Nice feature of ten’s complement • Instead of subtracting a number, adding its complement results in answer exactly 10 too much • So just drop the 1 – results in subtracting using addition only

22. Two’s Complement is Easy to Compute: Just Invert Bits and Add 1 • Hold on! • Sure, adding the ten’s complement achieves subtraction using addition only • But don’t we have to perform subtraction to have determined the complement in the first place? e.g., we only know that the complement of 4 is 6 by subtracting 10-4=6 in the first place. • True – but in binary, it turns out that the two’s complement can be computed easily • Two’s complement of 011 is 101, because 011 + 101 is 1000 • Could compute complement of 011 as 1000 – 011 = 101 • Easier method: Just invert all the bits, and add 1 • The complement of 011 is 100+1 = 101 -- it works! Q: What is the two’s complement of 0101? A: 1010+1=1011 (check: 0101+1011=10000) a Q: What is the two’s complement of 0011? A: 1100+1=1101

23. A B N-bit A B 1 Adder cin S Two’s Complement Subtractor Built with an Adder • Using two’s complement A – B = A + (-B) = A + (two’s complement of B) = A + invert_bits(B) + 1 • So build subtractor using adder by inverting B’s bits, and setting carry in to 1

24. Arithmetic-Logic Unit: ALU • ALU: Component that can perform any of various arithmetic (add, subtract, increment, etc.) and logic (AND, OR, etc.) operations, based on control inputs • Motivation: • Suppose want multi-function calculator that not only adds and subtracts, but also increments, ANDs, ORs, XORs, etc.

25. 32 32 W_data R_data 4 4 W_addr R_addr W_en R_en × 16 32 register file Register Files • MxN register file component provides efficient access to M N-bit-wide registers • It has one data input and one data output, and allows us to specify which internal register to write and which to read a a

26. 32 32 W_data R_data 2 2 W_addr R_addr W_en R_en 4x32 register file Register File Timing Diagram • Can write one register and read one register each clock cycle • May be same register

27. 32 C load d0 huge mux r eg0 32 i0 × 4 16 t oo much 32-bit fanout x 16 1 4 i3-i0 d D 32 c ongestion d15 load r eg15 e i15 load 32 s3-s0 Register-File Example: Above-Mirror Display • 16 32-bit registers that can be written by car’s computer, and displayed • Use 16x32 register file • Simple, elegant design • Register file hides complexity internally • And because only one register needs to be written and/or read at a time, internal design is simple OLD design a