IEEE Solid-States Circuits Magazine - Summer 2018 - 21
time (e.g., a 2-ns clock distribution delay
at nominal VDD postpones the timingmargin failure to cycle 4).
As illustrated in Figure 6, the salient feature of the autocalibrating
ACD is integrating a tunable-length
delay (TLD) between the PLL and the
global clock distribution to prolong
the path clock-data compensation
during a VDD droop. The TLD prevents
path-timing-margin degradation for
multiple cycles after the VDD droop
occurs to enable a sufficient response
time to detect the droop and adapt
the FCLK. From Figure 6, the other design components consist of a dynamic
variation monitor (DVM) with the clock
connected to clk_root (DVM ROOT), an
autocalibration circuit for DVM ROOT,
an adaptive control unit (ACU), and
a clock divider. The DVM contains a
tunable-path delay inserted between
driving and receiving FFs and continuously measures the impact of dynamic
parameter variations on the path-timing margin every cycle. Since clk_root
remains at a constant FCLK during a
VDD droop, the DVM ROOT monitors
only the data-path-delay change. The
DVMROOT detects the onset of the VDD
droop to trigger an adaptive response.
If a VDD droop induces a DVM ROOT
timing-margin failure, the DVM ROOT
generates an error (error_root), which
PLL
VDD
VDD
D
VDD
VPLL
PLL
clk_root
VDD VDD
Data Path
VDD
clk_leaf
VDD VDD
Clock Distribution
VDD
VDD Droop Occurs
1 ns
clk_root
1.25 ns
1 ns
1.25 ns
clk_leaf
1 ns
D
1.25 ns
1 ns
Cycle 1
1 ns
1.25 ns
1.25 ns
Clock-Data
Compensation
Cycle 2
No Clock-Data Compensation
and Failure Occurs
Cycle 3
FIGURE 5: Conventional clock distribution and conceptual timing diagrams of temporary
clock-data compensation during a hypothetical VDD droop. At cycle 2, VDD immediately droops
and the clock period stretches at the leaf node (clk_leaf) to compensate for the slow datapath delay. At cycle 3, the clock period at clk_leaf compresses, resulting in a timing-margin
failure. (Image from [21].)
clk_root
Frequency
Counter
TLD
clk_tld
error_or_safe_mode
clk_ref
VDD
FF
SYNC
error_sync
Clock
Divider
ACU
FF
clk_div
clk_acd
clk
FF
FF
FF
safe_mode
auto_cal_trig
Autocalibration tde_cfg [20:0]
DVMROOT
DVMLEAF
error_root
error_leaf
DVM
Diagnostic
Logic
FF
clk_leaf
FIGURE 6: An autocalibrating ACD and supporting logic. SYNC: synchronizer. (Image from [21].)
IEEE SOLID-STATE CIRCUITS MAGAZINE
su m m e r 2 0 18
21
IEEE Solid-States Circuits Magazine - Summer 2018
Table of Contents for the Digital Edition of IEEE Solid-States Circuits Magazine - Summer 2018
Contents
IEEE Solid-States Circuits Magazine - Summer 2018 - Cover1
IEEE Solid-States Circuits Magazine - Summer 2018 - Cover2
IEEE Solid-States Circuits Magazine - Summer 2018 - Contents
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IEEE Solid-States Circuits Magazine - Summer 2018 - Cover3
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