September 2004, Issue 170

Multilab
Build a Z8 Encore!-Based Multipurpose Test Instrument

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DESIGNING THE ARB

A block diagram for a basic arbitrary function generator is shown in Figure 3. The address counter accesses the waveform memory sequentially, and the data in the waveform memory array is fed to an 8-bit DAC. The DAC0801LCN is a current-mode DAC. An op-amp converts this into the required output voltage waveform.

(Click here to enlarge)

Figure 3—As you study the different sections of the arbitrary function generator, keep in mind that the green blocks are handled by the Z8 Encore!.

To generate a waveform of a desired frequency, the address counter must be incremented at this frequency multiplied by the depth of the waveform memory array. The depth of the array represents the number of data points used to make up one complete cycle of the output waveform. So, the greater the depth, the more accurately one can simulate a particular waveform. For simple waveforms such as square waves and triangle waves, this isn’t too critical. But, it’s obvious that to generate complex waveforms, the depth of the memory array must be commensurate with the complexity of the waveform.

To achieve adequate frequency resolution, the clock, which increments the address counter, must be tuneable in reasonably fine steps. So, for example, achieving a 1-Hz output waveform resolution with a waveform depth of 256 data points requires the clock be tuneable in 256-Hz amounts.

There are basically two ways to approach this. You can start with an extremely high-frequency quartz crystal oscillator (50 to 100 MHz) and use a variable modulus divider chip to produce a clock signal in the desired range. This approach can yield a reasonable resolution, but the resolution will vary throughout any given range. A better approach is to use a PLL (i.e., a VCO that’s phase-locked to a reference signal). The reference frequency is set equal to the desired frequency resolution multiplied by the waveform memory array depth. The VCO can be fine-tuned over an octave (or greater) frequency range, and Timer2 in the Z8 Encore! can divide this down to the correct frequency needed to clock the waveform memory at the correct rate.

I chose the latter option (see Figure 3). My goal was to use the Z8 Encore! for as much of the circuit as possible. All I really needed to add externally were a CMOS 74HC4046 VCO/phase detector chip and the DAC/output buffer. To generate a reference signal for the PLL, I could’ve used Timer1 in the Z8 Encore!. However, I allocated Timer1 to handle the timing for the DC voltmeter function to this project. Therefore, I used an MC14060 CMOS oscillator/divider chip and a 32,768-Hz watch crystal to provide the 512-Hz reference for the PLL.

The DMA function in the device is essential to make the Z8 Encore! function as an arb. The Z8 Encore! contains three DMA channels, one of which is dedicated to the ADC. The other two channels can be independently programmed to transfer data in or out of the Z8 Encore! RAM to any Z8 Encore! port that you select.

I connected the DAC to port B, so that is the destination register that I programmed into DMA0. Any of the Z8 Encore!’s timer outputs can be used to trigger this data transfer at the desired rate. As you can see in Figure 3, the VCO output is fed into the Z8 Encore!’s Timer2, which acts as a programmable divider, and triggers DMA0 at the desired rate.

The DMA controllers contain registers for RAM start and end addresses, so it is easy to program in the desired waveform depth by setting the RAM ending location accordingly. The Z8 Encore! contains 4 KB of RAM, which is more than enough to handle the waveform depths that I need.

The DMA controller can operate in Single Block or Repetitive mode. At this stage, my design is set up to produce repetitive waveforms, but changing this mode bit, and using an external trigger to initiate the generation of a one-shot wave output, can change this.

When the Z8 Encore!’s timers are clocked from an external source, as is the case with Timer2, the maximum clock rate is the system clock divided by four. I run the Z8 Encore! at 16 MHz, which is convenient for the pulse generator function, so the maximum VCO frequency shouldn’t exceed 4 MHz. I chose the VCO’s RC tuning components to produce frequencies in the 1- to 3-MHz range, which is safely under the 4-MHz limit. Timer2 divides the VCO frequency by N, so its output frequency range can go up to 1.5 MHz.

The Z8 Encore! datasheet does not specify how fast the DMA channels can operate. Because the DMA controller must steal the bus from the MCU to do the transfers, this timing will vary somewhat depending on which instructions the MCU is performing. With my code, I found that the DMA controller could be counted on to transfer data up to about a 2-MHz rate, so the 1.5-MHz maximum frequency output of Timer2 is OK.

To phase-lock the VCO at the necessary frequency in the 1- to 3-MHz range, its output must be divided down to the PLL’s 512-Hz reference frequency (provided by the MC14060 oscillator/divider chip). The Z8 Encore!’s Timer0 performs this task. I chose 512 Hz because it provides adequate output frequency resolution and isn’t so low as to be slow phase locking.

Table 1 shows the waveform depth and resolution for the various frequency ranges. The higher frequencies must use a smaller waveform depth because the maximum waveform memory clock rate is about 1.5 MHz, as outlined previously. With a fixed 512-Hz PLL reference frequency, as the waveform depth becomes smaller, the resolution also decreases but it is still reasonable.

The arb generator works in two modes. For common waveforms—such as sine, triangle, sawtooth, and square—the Z8 firmware calculates the waveforms, using the C library’s floating point routines, and loads them into RAM as needed. For any other waveform (i.e., arbitrary), the RAM is loaded with values that have been previously saved in a 24LC256 I2C flash memory chip.

For simplicity, I broke the 32-KB flash memory space into 1-KB segments, allowing one to store up to 32 arbitrary waveforms with a depth of 1,024 (or less). Generating and downloading these waveforms are the only functions performed without the Palm Pilot. Rather, they’re accomplished using a PC application, which also can act as the device’s front panel in lieu of the Palm Pilot.

There is nothing written in stone that says the output of the arb waveform memory must be limited to setting a DAC and thus producing an analog waveform. I added an eight-pin header to the board wired up to the eight data input lines of the DAC. By loading an arb waveform with the proper digital values, it’s possible to tap into these lines and get up to eight synchronized digital signals (of a repetitive nature). Along with the various analog waveforms like sine and triangle, the firmware also generates several stepper motor phase sequences that can be selected.

I was tempted to use the Z8 Encore! development board, with its on-board Z8F6403, for this project. This Z8 device, which contains four timers, would have eliminated the MC14060 circuitry that I used for the PLL reference clock. It would have also provided more versatility. However, I decided to save the development board for future development work and instead use one of the Z8F6401 (40-pin DIP) chips included in the kit.