Complex Digital
Waveform Generator
XAPP 008.002 Application Note By BERNIE NEW
Summary
Complex digital waveforms are generated without the need for complex decoding. Instead, fast loadable
counters are used to time individual High and Low periods.
Specifications Xilinx Family
Minimum High/Low Time 44 ns XC3000/XC3100
Maximum High/Low Time >250 µs XC4000
Resolution 4 ns
Demonstrates
Number of Highs and Lows 32
Number of CLBs 40 Fast Loadable Counters
CLB ROMs
Complex digital waveforms with unequally spaced transi- The values stored in the ROM are used to load a preset-
tions are often generated by decoding a counter that table counter that times the duration of individual High
cycles with the same period as the waveform. If precise and Low segments of the complex waveform. A second
placement of edges is required, the counter must be counter is enabled whenever the timer is reloaded, and
clocked at high frequency. This increases the burden on tracks the segment number in the waveform. This is
the decoders; not only must they settle faster, but if the used to address the ROM and access the length of the
period of the waveform remains constant, they must next segment.
become wider. These two requirements are incompati-
The least significant bit of the second counter toggles
ble. Decoders typically become slower as they get wider.
after each cycle of the timer and thus creates the output
In LCA devices, this problem can be overcome by using waveform. This output is guaranteed to be glitch free,
high-speed counters in conjunction with data stored in since it is generated by toggling a flip-flop.
ROM. The data stored in the ROM is not the waveform
In an LCA device, the ROM may be implemented in
itself, but a run-length encoded version of it. A block dia-
the CLBs. Each function generator may be used as a
gram of the waveform generator is shown in Figure 1.
NUMBER OF
ROM HIGHS & LOWS
HIGH/LOW
TIME
PE PE
PRESETTABLE
TC CE COUNTER TC
COUNTER
CLK
OUTPUT WAVEFORM
(LSB)
X1927A
Figure 1. Precision Waveform Generator
8-163
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Complex Digital Waveform Generator
16 x 2-bit ROM or as a 32 x 1-bit ROM. In the XC3000 262 µs. Up to 32 Highs and Lows can be accommodated
series, ROM data may be entered at the schematic level using 32-word ROMs, for total waveform periods of up to
using 16:1 or 32:1 multiplexers to represent ROM bits. 8 ms.
The ROM values are applied to the data inputs of these
The 16-bit timer requires 18 CLBs, and a further six are
multiplexers as hard-wired ones and zeros. CLBMAPs
used in the segment counter. The 32 x 16-bit ROM adds
are used to lock the multiplexers into CLBs. APR incorpo-
16 CLBs, for a total of 40.
rates the ones and zeros into the logic function, and cre-
ates the desired ROM as 4- or 5-input function generator ROM values may be used more than once in a waveform.
look-up tables. To do this, the output of the second counter must be
encoded to the appropriate ROM address. With this tech-
Using a state-skipping technique, the maximum clock rate
nique, any waveform length may be accommodated, pro-
for a presettable counter in an XC3100-series LCA device
vided it comprises a limited number of distinct time
is 270 MHz. This provides for defining the duration of
intervals.
Highs and Lows in 4-ns increments. In such a counter,
the shortest delay is 11 clocks, giving a minimum High Multiple waveforms may also be generated using this
or Low time of 44 ns. While some periods longer than scheme. A single timing counter is used to create a
this are also unavailable, the availability of all periods super-set of transition times for all the waveforms. Individ-
of 30 clocks or greater (≥120 ns) is guaranteed. The ual state machines are then used to create the different
16-bit timer allows maximum High and Low times of waveforms from this timing information.
XAPP 008.002 8-164
