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DSP
Designer is a revolutionary breakthrough to DSP design using Intusoft's
ICAP/4 tool suite to simulate digital control algorithms and other aspects
of DSP design. Special device models provide a visual means to simulate
transfer functions and equivalent assembly code instructions for programming
DSP digital control. Corresponding waveforms for the design including
Bode plots of hidden parameters within Z-transformations can be viewed.
The product is also valuable for verifying DSP assembly language programming
entered through a development board interface running as a DSP idle process.
Coding for real-time signal processing is tedious and error prone. Even when you seemingly get it "right," errors typically creep in. Clearly there is a need for computer automation to free the engineer from detailed micro-code book keeping required by DSP so that better algorithms can be implemented. Simulation using SPICE partially solves the problem. Substituting a SPICE delay line for the z transform, z-1, is the usual approach. But as complexity increases the substitution breaks down, with instabilities above the sampling frequency that are clearly impossible. Even more troublesome is the increased complexity when algebraic "feedback" occurs. |
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Take,
for example, a simple Buck regulator plant model. The output voltage feeds
back to the input, making the current through the inductor proportional
to the difference in output and input voltage.
Simulations are normally
performed for AC, DC and TRAN cases. In order to provide the same capability
in the DSP, Real Time (RT) code
is provided. This links with a serial interface to the DSP to provide
this capability. The most challenging part is the incorporation of a transfer
function analyzer, TFA, inside the DSP. The
TFA uses the single injection modeling
technique inside the ICAP/4 software. IntuScope scripts are used to control
the DSP serial interface, and using the RT code, the automated scripts
make and display the measurement results. A new data acquisition
plugin has been added to IntuScope so
that lab oscilloscope measurements can be transferred by a serial link
into an IntuScope waveform for viewing. |
It's
a well-known fact that traditional programming of assembly instructions
using a DSP development board is very tedious and error prone. Mistakes
in coding are common and often passed through the digital control process
without being detected. Also, hidden variables inside transfer functions
are impossible to analyze. But by simulating the microinstructions
using special device models provided by DSP Designer, the engineer gains
the convenience of a visual approach to the design, is able to view waveforms
of the digital control process, and easily debug this and other aspects
of the design. Once satisfied, the schematic can be used to ascribe
the correct assembly code instructions to be automatically written into
the development board's memory. The automated version of this process
is currently being developed and scheduled for completion in Q1/2009.
Increased ReliabilityCoding errors are eliminated by visually simulating the DSP's digital-control algorithms, and viewing corresponding waveforms. "Hidden" variables normally latent in DSP coding of transfer function analysis are exposed in DSP Designer, using familiar IsSpice4 AC, OP and TRAN analysis, using test code in the DSP, and viewing waveforms with automated communication between the DSP and IntuScope. Saved TimeA
visual design approach using IsSpice4 simulation saves time and tedious
error-prone programming of assembly instructions. Soon DSP Designer’s
automated programming of the DSP’s CPU from IsSpice4 will save even more
time. Parts
can be reduced by building their function into the digital control process;
for example, using Improved Time to MarketThe above advantages equate to faster market entry and higher product reliability. Turnkey Simulation Tool SuiteDSP Designer is but part of today's most advanced and easy to use analog, mixed-signal, and mixed systems simulation toolset, including magnetic design and synthesis software and test synthesis software in select packages. Intusoft's scalable ICAP/4 software offerings and Test Designer are beneficial for any type of design simulation, verification, test and documentation.
Initial
capability of DSP Designer will be included in the next ICAP/4 and Test
Designer release in December, 2008 for select packages. This will include
an oscilloscope plug-in for firmware importation of lab waveforms Intusoft
is examining a custom DSP Development board of their own. This will be
comprised of a Dual Buck/Boost synchronus SMPS with Hi voltage inputs
from 12 to 30 VDC, output from 0 to 30 VDC, with a maximum average power
of 200 watts. If approved, it will be targeted for release in January,
2009. The MicroChip dsPICDEM SMPS Buck Development Board was originally used for Intusoft's DSP Designer work, but it did require a number of modifications. A new board from MicroChip is used now, but is not yet released to the public. It is scheduled for a release in January of 2009. The Texas Instruments/Spectrum Digital Board 2812eZdsp kit was also used with DSP Designer. However, there was a piggy-back board that Intusoft attached to it for DSP development. Intusoft is examining a custom DSP Development board of their own. This will be comprised of a Dual Buck/Boost synchronus SPMS with Hi voltage inputs from 12 to 30 VDC, output from 0 to 30 VDC, with a maximum average power of 200 watts. If approved, it will be targeted for release in January, 2009. The Tektronix TDS1012B
oscilloscope with USB interface and the Tektronix TPS2014 with RS232 serial
interface will be supported for waveform importation. Other series 1000
and 2000 scopes should work with the communication interface.
sign
up as a reference account for designing a DSP using this revolutionary
product. We're looking for a few accounts to work closely with.
Receive
the software for free, or at a reduced upgrade cost if you have less than
a “Power Supply Designer” offering. Technical
Articles on DSP Designer. SMPS and Extracting Hidden Variables
The serial interface that connects DSP Designer to an oscilloscope uses the RS232C connection that's included with the MicroChip SMPS Buck evaluation board. Texas Instruments has a similar interface, but uses a different protocol. The objective is to have a low overhead background process that can read and write (peak/poke) into memory. Intusoft's code generator associates symbol name (node names) to DSP memory locations. With the DSP turned on and running at steady state, reading the memory values at these locations and scaling them back into the floating point representation used by IsSpice4, the IsSpice4 OP (operating point values) are acquired. If the code was generated using DSP Designer the scale factor is known, otherwise the user must enter an integer value that DSP uses to represent "one." The signed integer values will be divided by "one" to get the IsSpice4 floating point values. These values need a further conversion by the A/D scale factor to convert the floating-point values into IsSpice4 node voltages. A DSP button will be added to IntuScope's "Add Waveforms" dialog to initialize these values. This information is stored in the registry for each project (SpiceNet drawing name). The DSP will use the IntuScope "Browse" button to access these values. IntuScope gets a new capability to look at vector length 1 variables by loading them into a "plot" data structure that's viewed as a list in the output window. A script is then made to compare the active simulation's OP plot values with the DSP OP values. For transient analysis the DSP is set up for a step load test. That's part of the existing code and hardware built into the DSP SMPS evaluation board. This requires some extra test hardware for a user-designed board. The hardware can be part of an external test setup, but requires a repetitive signal to be sent from a DSP digital port to activate the load switching. The transient vector is built by adding one point per load switching period. The first point is taken just before the load switch. Subsequent points are delayed by the PWM sample, Tsample, so that the entire waveform is collected in (Switch period)/Tsample. For load switching running at 100Hz (Tswperiod = 10ms), and a computation cycle at 100kHz (Tsample=10us), the waveform is 1000 points long and takes 10 seconds to acquire. The DSP background load is small compared to its overall capability. Running the RS232 at 19,200 Baud allows up to 10 test points to be monitored. For AC analysis, a sine-cosine generator is programmed into the DSP firmware. This requires about 15 instructions or about .5 usec for a 30mip instruction rate. Additionally, there are: 4 coefficients that must be set to determine the frequency 4 data words holding sine, cosine, prevous_sine and previous_cosine 1 data word, TFAtime, to control the process and 1 data word, Vsig, to scale the signal level. The memory word, TFAtime, is used as a timer register in the DSP. It is initialized to zero. The DSP doesn't enter the TFA (Transfer Function Analysis) code section until the TFAtime word is set to 0x1 using the serial interface. When the DSP sees this condition in the idle loop, it initializes and starts the sine-cosine generator. The generator will run forever in this state. When TFAtime >= 0x2, the data accumulation phase starts. Then at the end of each Tsample interrupt, the sum of Vout*sine and Vout*cosine is accumulated and TFAtime is incremented. This computation is done after setting the PWM duty ratio so that it doesn't add to the computational delay; it just uses some of the "spare" time at the end of the A/D interrupt. When TFAtime =TFAperiod, the data accumulation stops. When the idle loop sees TFAtime==TFAperiod, it sets TFAtime=0 and sends the accumulated data (Vout) back to IntuScope using the RS232 interface. Scope then scales the data and computes Vin=Vout+Vsig for the TFA frequency that was previously initialized. Scope proceeds to the next frequency and repeats the process. This is all done in a new IntuScope script, so when its finished a Bode Plot will be automatically generated. The user will be required to insert the Vout test point by adding Vsig=Ksig*sine to the data at Vout and placing it a Vin, just as it's done in SpiceNet to set up a single voltage injection GFT. But it is not necessary to measure Vin because it can be computed using IntuScope as Vout+Vsig. For the default using DSP Designer as the code generator, Vout will be the computed PWM duty ratio before limiting, and Vin will be presented to the PWM limiter. There are a few fine
points that must be understood when programming the TFA. Second, it's necessary
to let the transient residues settle out after changing the frequency.
Third, Vsig must be small enough to prevent Duty Ratio limiting. If Vsig is made constant for the entire frequency sweep, it may be too small to get accurate data at lower frequencies. Good control system design has a loop gain that decreases 6dB per octave, so that the Vin signal strength can be estimated for the "best" deadbeat controller. The following calculations show how to make the duty ratio signal constant at about .1 p-p for this case: First, sine is scaled 2.14 and the controller is scaled 8.8 so the sine needs to be scaled 2^8/2^14 = 2^-6
Design and analysis of control system are usually performed in the frequency domain, whereby the time domain process of convolution is replaced by a simple multiplication in frequency domain. In DSP design sampled data systems use a similar concept using a unit delay as the basic building block. The s-plane mapping into discrete z-plane is achieved by:
where
in general
The equation can be re-arranged to solve for Vout and is implemented using the direct programming method as shown in the following block diagram with the unit time delay (UTD) implemented using a IsSpice4 transmission line. This implementation allows for IsSpice4 analysis of the time domain difference equations, including both transient and ac analysis, and of course Bode plots.
A new model, Zdelay, has been developed, which has superior convergence and memory usage performance over the traditional UTD transmission line IsSpice4 model. This new model is implemented with Intusoft's Code Model SDK tool. The model is highly customizable with several user specified parameters. These parameters include start up delay, sampling time, propagation time, initial condition, and high and low output limits. For DSP and general sample and hold applications, Zdelay becomes the model of choice due to its customizability and superior performance over transmission line based models. As an example, consider the following sine-cosine waveform generator in Figure 1, which is implemented with the new Zdelay model. Note that the initial conditions are necessary to kick-start the circuit. This is easily achieved by specifying the necessary initial condition (IC) values on the delay models.
Figure 1, Sine-cosine waveform generator 2KHz generated sine and cosine waveforms are shown below in Figure 2.
Figure 2, 2KHz generated sine and cosine waveforms
Consider
a Buck regulator operating in Continuous Conduction Mode(CCM) with common
input and output grounds as shown below in Figure 3. Note that the schematic
is shown with UTD models, which are eventually replaced by Zdelay models
described earlier. IsSpice4 is capable of modeling continuous time functions
by using a variable time step. That capability will be used to make a
continuous time analog model. The continuous time model is then transformed
to a z-domain based model by the zero order hold. The z-domain equations
are then re-arranged into the familiar Infinite Impulse Response (IIR)
DSP equations.
Figure 3, Buck regulator operating in Continuous Conduction Mode (CCM) |
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=
represents a unit delay of nT
seconds. S-domain transfer functions are then transformed into ratios
of 
=
