Ask The Applications Engineer—19: Interfacing To Serial Converters-I

Q. I need data converters to fit in a tight space, and I suspect that a serial interface will help. What do I need to know to choose and use one?

A. Let's start by looking at how a serial interface works and then compare it to a parallel interface. In doing this we will dispel some myths about serial data converters.

Figure 1

The figure shows an AD7890 8-Channel multiplexed 12-bit serial A/D converter (ADC) connected to the serial port of an ADSP-2105 digital signal processor (DSP). Also shown is the timing sequence that the DSP uses to communicate with the ADC. The 12 bits that constitute the conversion result are transmitted as a serial data stream over a single line. The data stream also includes three additional bits that identify the input channel that the AD7890's multiplexer is currently selecting. To distinguish the bits of the serial data stream from one another, a clock signal (SCLK) must be provided, usually by the DSP; However, sometimes the ADC supplies this clock as an output. The DSP usually (but not always) supplies an additional framing pulse that is active either for one cycle at the beginning of the communication or, as shown (TFS/RFS), for the duration of the transmission.

In this example, the DSP's serial port is used to program an internal 5-bit register in the ADC. The register's bits control such functions as selecting the channel to be converted, putting the device in power-down mode, and starting a conversion. It should be evident that the serial interface, in this case, must be bi-directional.

A parallel ADC, on the other hand, connects directly (or possibly through buffers) to the data bus of the processor it is interfaced with. The figure shows the AD7892 interfaced to an ADSP-2101. When a conversion is complete, the AD7892 interrupts the DSP, which responds by doing a single read of the ADC's decoded memory address.

Figure 2

The key difference between serial and parallel data converters lies in the number of interface lines required. From a space saving point of view, serial converters offer a clear advantage because of reduced device pin-count. This makes it possible to package a 12-bit serial ADC or DAC in an 8-pin DIP or SO package. More significantly, board space is saved because serial interface connections require fewer PCB tracks.

Q. My digital-to-analog converters have to be physically remote from the central processor and from one another. What is the best way to approach this?

A. Initially, you must decide whether to use serial or parallel DACs. With parallel DACs, you could map each one into a memory mapped I/O location, as shown in the figure. You would then program each DAC by simply doing a Write command to the appropriate I/O location. However, this configuration has a significant disadvantage. It requires a parallel data bus, along with some control signals, to all of the remote locations. Clearly, a serial interface, that can have as few as two wires, is much more economical.

Figure 3

Serial converters cannot in general be mapped into a processor's memory. But a number of serial DACs could be connected to the serial I/O port of the processor. Then, other ports on the processor could be used to generate Chip Select signals to enable the DACs individually. The Chip Select signals will require a line from each device to the interface. But there may be a limit to the number of lines on the processor that can be configured to transmit Chip Select signals.

One way of getting around this problem is to use serial DACs that can be daisy-chained together. The figure shows how to connect multiple DACs to a single I/O port. Each DAC has a Serial Data Out (SDO) pin that connects to the Serial Data In (SDI) pin of the next DAC in the chain. LDAC and SCLK are fed in parallel to all the DACs in the chain. Because the data clocked into SDI eventually appears at SDO (N clock cycles later), a single I/O port can address multiple DACs. However, the port must output a long data stream (N bits per DAC times the number of devices in the chain). The great advantage of this configuration is that device decoding is not needed. All devices are effectively at the same I/O location. The main drawback of daisy chaining is accessibility (or latency). To change the state of even a single DAC, the processor must still output a complete data stream from the I/O port.

Figure 4

Q. If serial data converters save so much space and wire, why aren't they used in every space-sensitive application?

A. A major disadvantage of serial interfacing is the tradeoff of speed for space. For example, to program a parallel DAC, just place the data on the data bus and clock it into the DAC with a single pulse. However, when writing to a serial DAC, the bits must be clocked in sequentially (N clock pulses for an N-bit converter) and followed by a Load pulse. The processor's I/O port spends a relatively large amount of time communicating with a serial converter. Consequently, serial converters with throughput rates above 500 ksps are uncommon.

Q. My 8-bit processor doesn't have a serial port. Is there a way to interface a serial 12-bit ADC like the AD7893 to the processor's parallel bus?

A. It can of course be done using an external shift register, which is loaded serially (and asynchronously), then clocked into the processor's parallel port. However, if the sense of the question is "without external logic", the serial ADC can be interfaced as if it were a 1-bit parallel ADC. Connect the converter's SDATA pin to one of the processor's data bus lines (it is connected to D0 in the diagram). Using some decode logic, the converter can be mapped into one of the processor's memory locations so that the result of the conversion can be read with 12 successive Read commands. Then additional software commands integrate the LSBs of the 12 bytes that were read into a single 12-bit parallel word.

Figure 5

This technique, which is sometimes referred to as "bit banging", is very inefficient from a software perspective. But it may be acceptable in applications in which the processor runs much faster than the converter.

Q. In the last example, a gated version of the processor's write signal was used to start conversions on the AD7893. Are there problems with that approach?

A. I am glad you spotted that. In this example, a conversion can be initiated by doing a dummy write to the AD7893's mapped memory location. No data is exchanged, but the processor provides the write pulse needed to begin the conversion. From a hardware perspective, this configuration is very simple because it avoids the need to generate a conversion signal. However, the technique is not recommended in ac data-acquisition applications, in which signals must be sampled periodically. Even if the processor is programmed to do periodic writes to the ADC, phase jitter on the Write pulse will seriously degrade the attainable signal-to-noise ratio (SNR). The gating process may make the Write signal jitter even worse. A sampling clock phase jitter level of as little as 1 ns, for example, would degrade the SNR of and ideal 100-kHz sine wave to about 60dB (less than 10 effective bits of resolution). There is also an additional danger that overshoot and noise on the sampling signal will further degrade the integrity of the analog to digital conversion.

Q.When should I choose a converter with an asynchronous serial interface? 

A. An asynchronous link allows devices to exchange unclocked data with each other. The devices must initially be programmed to use the identical data formats. This involves setting particular data rate (usually expressed in baud, or bits per second). A convention, that defines how to initiate and end transmissions, is also necessary. We do this using identifiable data sequences called start and stop bits. The transmission may also include parity bits that facilitate error detection.

Figure 6

The figure shows how the AD1B60 Digitizing Signal Conditioner interfaces to a PC's asynchronous COM Port. This is a 3-wire bidirectional interface (the ground lines have been omitted for clarity). Notice that the receive and transmit lines exchange roles at the other end of the line.

An asynchronous data link is useful in applications in which devices communicate only sporadically. Since start and stop bits are included in every transmission, a device can initiate communication at any time by simply outputting its data. The number of connections between devices is reduced because clocking and control signals are no longer necessary.

Q. The data sheet of an ADC I am considering recommends using non-continuous clock on the serial interface. Why?

A. The specification probably requires that the clock be kept inactive while the conversion is in progress. Some ADCs require this because a continuous data clock can feed through to the analog section of the device and adversely affect the integrity of the conversion. A continuous clock signal can be discontinued during conversion if the I/O port has a framing pulse; it is used as a gating signal that enables the serial clock to the converter only during data transfer.

Q. What makes a device SPI or MICROWIRE compatible?

A. SPI (Serial Peripheral Interface) and MICROWIRE are serial interface standards developed by Motorola and National Semiconductor, respectively. Most synchronous serial converters can be easily interfaced to these ports; but in some cases additional "glue" logic may be necessary.

Q. O.K. I decided to put prejudice aside and use a serial ADC in my current design. I have just wired it up as the data sheet specifies. When my micro reads the conversion result, the ADC always seems to output FFFHEX. What's happening?

A. Perhaps you are having a communications problem. We need to look at the connections between the ADC and the processor-and at how the timing and control signals have been set up. We also need to look at the Interrupt structure. The next installment will return to this issue, discussing the problems encountered when designing serial interfaces.



Eamon Nash

Eamon Nash is an Applications Engineering Manager in Analog Devices' RF Products Group. He has worked at Analog Devices for over 20 years, first as a Field Applications Engineer, covering mixed signal and DSP products, then as an Applications Engineer specializing in discrete RF components for wireless applications.