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Build An Eight-Channel Mixer

Ready to learn several useful PSoC design techniques？ Chris describes a switched capacitor and continuous time modules. He also walks you through an interesting PSoC project： an eight-channel mixer with adjustment knobs.

The Cypress Semiconductor PSoC family of microcontrollers provides a powerful mixed-signal capability with both analog and digital user modules. This allows for some rather unique single-chip solutions. However， a PSoC outsider might find it difficult to get started， especially with designs involving switched-capacitor （SC） analog modules.

SC modules are arguably the most versatile and unique feature of the PSoC， aside from its dynamic reconfiguration capabilities. There are two types of analog modules： SC and continuous time （CT）。 The latter modules are for simple analog functions such as programmable gain amplifiers（PGAs） or comparators. SC modules can be ADCs and DACs of varying resolution， fully configurable op-amps，analog buffers， filters， and more. There is even a "Generic SC" module available，which enables direct access to all switches and capacitance values in the SC micro circuitry. But these modules can be a bit difficult to work with.

BLOCKS AREN‘T CREATED EQUAL
Routing analog signals inside the PSoC can be a tricky process. More often than not， the desired connections are not directly available. An analog PSoC module's input choices are determined by its location. Certain locations have access to the Port 0 input multiplexers； others have direct access to the Port 2 inputs. Some have access to only certain adjacent blocks and their column‘s analog output bus.

CT analog blocks are easy to use. PGAs and comparators go here-no mess， no confusion. All four CT blocks have access to at least four Port 0 input pins （the two center blocks have extra multiplexers， allowing connection to any of the eight Port 0 input pins）， and the column clock has little to no effect on operation. An example CT PGA configuration window is shown in Photo 1. Note that the AnalogBus setting can be enabled if direct PGA output is desired.


All four CT blocks are the same， and have similar designations-"ACBxy，"where x equals the analog row （always 0）and y equals the analog column number（0-3）。 These four blocks are all on the top row of the analog section （see Figure 1）.


SC analog blocks are the subtle bits of genius that grant the PSoC so many capabilities. In fact， the blocks can perform such a variety of real-time functions that they were given a completely customizable user module instead of separate modules for each function. This "Generic SC" module can be found as the only entry under the "Generic" category. SC blocks come in two varieties：Type C and Type D. Originally， in the CY8C25xxx and CY8C26xxx PSoC families， they were Type A and Type B. This is detailed in Cypress‘s application note AN2041， "Understanding Switched Capacitor Blocks." Most， if not all，of it still applies to the newer CY8C27xxx and CY8C29xxx chips. The old Type A blocks have evolved into the new Type C blocks， and the old Type B blocks have evolved into the new Type D blocks. Type C SC blocks are designated"ASCxy，" where x equals the analog row （1 or 2） and y equals the analog column （0-3）。

Type D SC blocks are designated"ASDxy，" where x equals the analog row （1 or 2） and y equals the analog column （0-3）。 Type D blocks include an additional option to allow BCAP to function as a regular capacitor instead of a switched capacitor. Set this BSW to "on" for normal operation. Type C and D SC blocks are arranged in a checkerboard pattern， beginning with ASC10 （see Figure 1）.

Only the outer blocks （ASD20，ASC10， ACB00， ACB01， ACB02，ACB03， ASD13， and ASC23） have direct access to input pins. Restrictions apply for specific pin connections，as well. The remaining SC blocks in the middle have to rely on neighbors for indirect input signals.

Generic SC modules， as I already mentioned， may be used for a variety of purposes. Providing isolated analog blocks with access to just about any analog input is possibly one of the handiest uses.

ADVANCED ANALOG ROUTING
Suppose you want to sample an audio signal with an ADC. The simplest way， with a blank project， is to place the input block to the ADC module in one of the outside blocks so it has direct access to a pin input. This may or may not be possible with other functionality in place. It also doesn‘t provide control over the input signal level. It may be entirely too small or too large to read accurately.

Passing the audio signal through a PGA in one of the top （CT） blocks first enables more versatile placement of the ADC input. It also applies a gain to the signal， which may be adjusted at either design or run time. Unity gain （1.00×）will leave the signal level unaltered.

As you place the ADC input block in various locations， you might notice the different choices available with each placement. An ADC placed in ASD11， for example， has access to signals present in ACB00， ACB01，ASC10， ASC12， and ASC21. Changing the signal input to the ADC during run time involves altering the value in one of the control registers-specifically，ASD11CR1 （bits 7， 6， and 5）。 For details， refer to the PSoC technical reference manual.

The ADC in block ASD11 has easy access to CT PGAs in blocks ACB00 and ACB01. Suppose your design had the resources available for only a single ADC， and you had to cycle through and read signals present in all four CT PGAs up top. There is a missing element.

The block ASC12 has easy access to PGAs in the other two CT blocks ACB02 and ACB03. By configuring this block as an SC PGA with unity gain， the ADC in block ASD11 can access all four CT PGAs with simple register changes. An example of such a setup‘s analog configuration is shown in Figure 2.
点击查看Figure 2

A generic SC module can be configured to act as a PGA and cleanly amplify audio signals. The SC PGA does not have as large a range of gains as the standard CT PGA module， but it is perfect for low and unity gain applications. The standard CT PGA module provides gains from 0.062× to 48×，whereas the SC PGA I created has a range of 0.062× to only 1.938×（with an FCAP value of 16）。 This range gets smaller and finer when FCAP is set to 32 （see Table 1）.

SC PGAs have the unique capability of mixing two inputs and controlling their gains independently. An SC PGA can also invert its non-inverting input. The ratio between FCAP and either ACAP or BCAP controls the gain applied to either the A or B inputs， as shown in the following equations：

Empirically， an analog column clock of 4 MHz was found to be optimum for an audio SC PGA. Faster clocks resulted in distortion because the SC blocks do not perform quite as well near their upper limit of 8 MHz.

The parameters necessary to create an SC PGA intended for use with AC signals （or audio） from a generic SC module are shown for the two types of SC blocks in Photos 2a and 2b. To pass just one signal， set the unused channel （A or B） capacitance to 0. If your signals are not referenced to analog ground （AGND = VCC/2）， then a different reference setting may be selected for ArefMux （just like the reference setting of a CT PGA).


CREATE AN 8-CHANNEL MIXER

Now that you have the capability to route analog signals anywhere at will， you can create a single PSoC project to perform a function not typically thought of as possible： the eight-channel mixer.

Using both CT and SC PGA modules，all eight simultaneously routable analog input signals （four CT inputs from Port 0 and four end-column SC inputs from Port 2） can be mixed into three outputs： Left， Right，and Mono. The basic mixer schematic and internal routing is shown in Figure 3.
点击查看Figure 3

The left output consists of a mix of input channels La， Lb， Lc， and Ld. The right output consists of a mix of input channels Ra， Rb， Rc， and Rd. The mono output is a mix of all eight inputs.

Lc， Ld， Rc， and Rd are routed through CT PGAs before entering the mix， and can therefore have a preamp gain of up to 48× applied to them. These channels are ideal for use with microphones or small-signal input sources.

This mixer design operates passively without logic. The only code required is to initialize the CT PGA modules. The SC PGA modules do not need start commands when their power level is set in the device editor to low，medium， or high.

If you can preset all of your gains，then you are done！ However， you might want some sort of real-time control over the gains.

DIGITALLY TIME POT INPUTS
 One relatively simple way to provide real-time control over the mixer‘s gains involves three buttons and a serial display terminal. This does not lend itself to a user-friendly interface-knobs or sliders would make much more sense. Typically， potentiometer inputs are read with an ADC in the analog domain；however， there is an interesting way to utilize the remaining digital resources of the PSoC to implement adjustment potentiometers for up to 16 adjustments simultaneously （see Figure 4）.

An 8-bit timer module captures one of four input lines， which is multiplexed to four control lines in a keypad-matrix style. This allows timing of up to 16 signals，which will be our potentiometers（or knobs， sliders， and more）。 The potentiometer‘s resistance controls how long the RC circuit takes to charge up， transforming a voltage measurement into a time measurement （easily measurable in the digital realm）。 The PSoC's digital High-Z inputs switch at VCC/2 （behaving exactly as an analog comparator referenced to ground with the trip point at 0.500）， which works out great for this.

To measure a specific line， after the input is connected to the timer module‘s capture input， all four capacitors are discharged. Then， the timer module is reset and started just before the capacitors are released and allowed to begin charging. When the selected line charges up above VCC/2， the capture input triggers an interrupt-during which， the timed value is converted into an appropriate R/C gain ratio and written to the channel's gain register.

If any of the eight audio input lines are left floating， the PSoC will allow some noise in from the potentiometer scanning process. At extremely high gains， some digital noise might still leak in even without floating inputs.

A simple solution to both is to use a pin as a "Scan Enable" line. Scanning the potentiometers is performed only while P2[7] is high； it ceases when P2[7] is low. A button might be labeled"Hold to adjust or push to update." A switch might be labeled "Down to lock out changes and Up to adjust."

There was another situation caused by leakage in this application. The bank select outputs were originally driven as Open Drain High， which should theoretically mix without diodes in the potentiometer scanning circuitry. The bank selects were driven high to select a particular bank， and released to float otherwise. Unfortunately， when floating，the bank outputs sharing Port 1 with the inputs were affected by the various charging capacitors. This resulted in multiple gains being adjusted unintentionally.

For this reason， the bank selects are now strong outputs. They are driven high to select a bank， and driven low otherwise. To ensure that the low drives do not affect the potentiometer readings，blocking diodes （D1， D2， D3， and D4） are now required on these lines.

A flowchart describing the operation of this adjustable eight-channel mixer project is shown in Figure 5.

A COUPLE OF NOTES
By enabling the Agnd bypass in the global settings and selecting external Agnd on P2[4]， a few hundred microamps are available to bias the AC-coupled inputs. The two 100-μF bypass capacitors （C14 and C15） are arranged in series with the supply to facilitate a quick initial charge-up. The ACAP and BCAP settings in the device editor have no effect because they are set in the software.
 
The option also exists to convert one of the eight channels into an intercom line， which would make this firmware perfect for mixing at the individual level in the studio. This would enable musicians to control each relevant channel level in their headphones and simultaneously maintain communication with the control room. Just some simple configuration changes in the PSoC Designer，and this may be achieved （disconnect PGA_Rd from SC_Rcd， and directly connect PGA_Rd to Analog Output 3 to isolate it from the mix）。 Actually， don‘t worry about doing this yourself. Next month， you'll read how this is implemented as a separate mode.

The optional serial terminal display output looks like what you see in Figure 6. At power-up， the title line is transmitted. This provides labels for the data. Then， whenever the potentiometers are scanned， the applied gains will be looked up and displayed on the terminal. Because it all fits on a single line， special software isn‘t needed for a stationary， updating display. （Carriage returns without linefeeds typically result in overwriting the same line repeatedly in terminal applications.）


In this article， I described how to build an eight-channel mixer with adjustment knobs， using a Cypress Semiconductor CY8C27443 PSoC as the only active component. Next month， I will explain how to further enhance this single-chip design with DSP effects， an intercom mode， a user interface， speech capability， and permanent setting memory.

Author‘s note： Many of the application notes referenced in this article were recently submitted， so they may or may not be available on Cypress's web site at this time. For your convenience， the original，unedited versions of the relevant application notes are available on my web site， along with their project and related files. Go to www.chrispaiano.com for more information. In addition， I have a few homemade PSoC-based electronics kits and devices for sale on my site. The project described in this article is available as either a kit or an assembled unit

Chris Paiano has written more than 30 application notes for the Cypress PSoC chipset over the years， including such novelties as PongSoC and the Video RTA. You may contact him at chris.paiano@gmail.com.

PROJECT FILES
To download code, go to ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2008/216.

RESOURCES
Cypress Semiconductor, "PSoC Mixed-Signal Array: Technical Reference Manual,"001-14463, www.psocdeveloper.com/uploads/media/PSoC_TRM_00.pdf.

D. Van Ess, "AN2041: Understanding Switched Capacitor Blocks," Cypress Microsystems, 2004, www.future-mag.com/0707/docs/analog_understanding_switched_capacitor_analog_blocks_an2041_12.pdf.

SOURCE
CY8C27443 PSoC
Cypress Semiconductor Corp.
www.cypress.com