Embedded Systems Hardware: Storage Elements; Finite State Machines; Sequential Logic

fig_03_08 Finite state machine (FSM): High-level view Moore machine: output is a function of the present state only Mealy machine: output is a function of the present stare and the inputs

fig_03_15 fig_03_16 Verilog—shift registers; behavioral and structural (por=power on reset)

fig_03_20, 3_21, 3_22 Linear feedback shift register (for providing random numbers, e.g.); Note: pullUp needed to prevent floating Reset pin on D flipflops What goes in “Feedback Logic” block? CHOICE IS NOT ARBITRARY, DEPENDS ON PROPERTIES OF POLYNOMIALS OVER THE FINITE FIELD {0,1,,xor,*} (e.g., 1+1=0). Table of correct values for an n-bit register is given in Appendix of Hamblen.

fig_03_23,3_24 “Dividers”: slow clock down, e.g. Simple divide-by-2 example

fig_03_25, 03_26, 03_27 Example: Asynchronous divide- by-4 counter [asynchronous 2-bit binary upcounter; ripple counter] Note: asynchronous because flip-flops are changed by different signals Note: if 1 st stage output appears at time t0 + m, nth stage output appears at time t0 + nm; so this configuration is good for dividing the signal but using it as a ripple counter is prone to static and dynamic hazards Both outputs change:

fig_03_28, 03_29 Synchronous dividers and counters (preferred): Example: 2-bit binary upcounter Inputs: D A = not A D B = A xor B

fig_03_30, 03_31, 03_32, 03_33 Johnson counter (2-bit): shift register + feedback input; often used in embedded applications; states for a Gray code; thus states can be decoded using combinational logic; there will not be any race conditions or hazards

fig_03_34 3-stage Johnson counter: --Output is Gray sequence—no decoding spikes --not all 2 3 (2 n ) states are legal—period is 2n (here 2*3=6) --unused states are illegal; must prevent circuit from ever going into these states

Making actual working circuits: Must consider --timing in latches and flip-flops --clock distribution --how to test sequential circuits (with n flip- flops, there are potentially 2 n states, a large number; access to individual flipflops for testing must also be carefully planned)

fig_03_36, 03_37 Timing in latches and flip-flops: Setup time: how long must inputs be present and stable before gate or clock changes state? Hold time: how long must input remain stable after the gate or clock has changed state? Metastable oscillations can occur if timing is not correct Setup and hold times for a gated latch enabled by a logical 1 on the gate

fig_03_38 Example: positive edge triggered FF; 50% point of each signal

fig_03_39, Propagation delay: minimum, typical, maximum values--with respect to causative edge of clock: Latch: must also specify delay when gate is enabled:

fig_03_41, 03_42 Timing margins: example: increasing frequency for 2-stage Johnson counter –output from either FF is …. assume t PDLH = 5-16ns t PDLH =7-18ns t su = 16ns

Case 1: L to H transition of Q A Clock period = t PDLH + t su + slack 0  t PDLH + t su If t PDLH is max, Frequency F max = 1/ [5 + 16)* ]sec = 48MHz If it is min, F max = 31.3 MHz Case 2: H to L transition: Similar calculations give F max = 43.5 MHz or 29.4 MHz Conclusion: F max cannot be larger than 29.4 MHz to get correct behavior

Clocks and clock distribution: --frequency and frequency range --rise times and fall times --stability --precision

fig_03_43 Clocks and clock distribution: Lower frequency than input; can use divider circuit above Higher frequncy: can use phase locked loop:

fig_03_44 Selecting portion of clock: rate multiplier

fig_03_46 Note: delays can accumulate

fig_03_47 Clock design and distribution: Need precision Need to decide on number of phases Distribution: need to be careful about delays Example: H-tree / buffers

fig_03_48 Testing: Scan path is basic tool

fig_03_56 Testing fsms: Real-world fsms are weakly connected, i.e., we can’t get from any state S1 to any state S2 (but we could if we treat the transition diagram as an UNDIRECTED graph) Strongly connected: we can get from a state S initial to any state Sj; sequence of inputs which permits this is called a transfer sequence Homing sequence: produce a unique destination state after it is applied Inputs: I test = Ihoming + Itransfer Finding a fault: requires a Distinguishing sequence Example: Strongly connected Weakly connected

fig_03_57 Basic testing setup:

fig_03_58

fig_03_59 Example: machine specified by table below Successor tree

fig_03_63 Example: recognize 1010

fig_03_65 Scan path

fig_03_66 Standardized boundary scan architecture Architecture and unit under test