Using 2.5Gbps SerDes with Built-In Bit-Error Rate Test Circuitry Makes Measuring Link Quality Easy

Introduction

Any application that involves sending information at high data rates over long distances requires tests to ensure good link quality, and the MAX9259/MAX9260 gigabit multimedia serial link (GMSL) serializer/deserializer (SerDes) are no exception. This chipset sends audio/video data at payload rates of up to 2.5Gbps over a single twisted-pair cable up to 15m in length. One of the most straightforward tests of link quality is the bit-error rate (BER) test. Here, the transmitter sends a known data pattern across the serial link and the receiver checks the incoming data for any bit errors. A second test, the eye diagram, gives a graphical presentation of the link quality.

Both of these tests require a suitable pattern to be generated and transmitted over the serial link. While various test patterns are available, one of the common choices is to use is a long pseudorandom bit sequence (PRBS).

The MAX9259/MAX9260 chipset features an internal PRBS generator and BER checker to simplify testing, requiring only a parallel clock and a UART port to program the devices. The MAX9259 sends a 2³⁰ - 1 (in 32-bit mode) or a 2²² - 1 (in 24-bit mode) PRBS data pattern across the serial link. The MAX9260 checks the received data and records the number of bit errors. This application note describes how to set up the MAX9259/MAX9260 evaluation (EV) kits for the internal PRBS test with bit-error analysis. It also compares test results for the default and optimized preemphasis/equalizer settings using a 15m cable.

Note that data sheets for the chipset and the EV kits, as well as the latest software, can be obtained through Maxim's website.

Required Equipment

• MAX9259 and MAX9260 EV kits
• Agilent® 33250A arbitrary waveform generator or similar device for clock generation
• USB cable
• Shielded twisted-pair cable with Rosenberger connectors

(Optional to analyze the eye diagram)

• Agilent DSO81004B or similar high-bandwidth digital oscilloscope
• Tektronix® P6248 differential probes or similar probes

Built-In BER Test

Generally, getting a PRBS output from a serializer would require inputting data on all inputs to be serialized. For the MAX9259, this would require inputting data on the video, audio, and UART inputs. This can easily become very complicated due to the large differences in bit rates of the three interfaces. The MAX9259 includes an internal PRBS generator, which can be controlled through a single UART command. Set PRBSEN = 1 on the MAX9259 to turn on the PRBS generator. The serial data rate will be 30x PCLK (24-bit mode) or 40x PCLK (32-bit mode) based on the bus-width selection (BWS).

Table 1 below shows the results of BER test performed with different PCLK frequencies, a 15m STP cable with Rosenberger connector, 1.1dB or 10.5dB preemphasis, and a 9.4dB equalization level.

Activating the PRBS Mode

To begin the testing procedure, follow these guidelines to power up the EV kits and start the software.

1. Verify that the EV kit boards, software, and drivers are properly installed and operating by running through the Quick Start procedures as discussed in the MAX9259EVKIT/MAX9260EVKIT data sheets.
2. Verify that all the jumpers are set in the following positions, as shown in Table 2.

3. Connect the STP cable from MAX9259 EV kit connector J1 to MAX9260 EV kit connector J1.
4. Connect the parallel data source or arbitrary waveform generator output to header H1-62, PCLK_IN. Set the parallel data source frequency between 12.5MHz to 78MHz and enable the output.
5. Connect the USB cable from the PC to the MAX9259 EV kit.
6. Verify that MAX9259 EV kit LED120 lights up, indicating that the microcontroller is powered and enabled.
7. Verify that MAX9260 EV kit LED120 lights up, indicating that the microcontroller is powered and enabled.
8. Verify that MAX9260 EV kit LED2 lights up, indicating that the link has been successfully established. If LED2 is off or LED1 is on, double-check that the PCLK_IN signal is clocking data.
9. Start the MAX9259/MAX9260 EV kit program by opening its icon in the Start | Programs menu. The EV kit software configuration window appears as shown in Figure 2.
10. Press the Connect button and the configuration window disappears.



11. The Read All button reads the entire MAX9259 and MAX9260 device registers. The Write All button writes all MAX9259 and MAX9260 device registers with the values shown Figure 3 and Figure 4.





12. From the MAX9260 tab sheet (Figure 4), read the 8-bit error count register (0x0E) to clear the errors before the PRBS test.
13. Set PRBSEN = 1 (0x04 D5) first in the MAX9259 and then the MAX9260 to start the PRBS test.
14. Run the PRBS self test for the desired test time, and then set PRBSEN = 0 (0x04 D5), first in the MAX9260 tab and then the MAX9259 tab to exit the PRBS self test.
15. Read 8-bit error count register (0x0E) in the MAX9260 tab once. The total number of bit errors will report in the window in "PRBS error counter PRBSERR." The error counter register is an 8-bit register, so the maximum number of errors that can be recorded is 255.

Eye Diagram

The BER test is a straightforward method to measure the link quality. Another approach to evaluate the system performance is analyzing the eye diagram opening. This provides a graphical presentation of the link quality, as well as insight into the nature of channel imperfections. Figure 5 and Figure 6 are eye diagrams of a serial link captured with a high-bandwidth digital oscilloscope (in infinite persistence mode) at the deserializer side of a 15m STP cable. The preemphasis settings of 1.1dB (Figure 5) and 10.5dB (Figure 6) were selected to provide the visible differences in the eye diagrams. The user can determine impedance mismatches, reflections, timing variation, and even frequency attenuation issues from the analysis of eye diagrams.