Design of manageable SAS hard disk backplane based on CPLD

Abstract: Based on CPLD as the core, a manageable SAS hard disk backplane is designed. While conveniently replacing the faulty hard disk, it can indicate the working status of the hard disk by controlling the LED light, and realize the monitoring of the hard disk status. The test results show that the backplane can complete the transmission of 6Ghps SAS signal, realize the hard disk status indication, and the high-speed SAS signal meets the system requirements.
Keywords: SAS hard disk; SGPIO; backplaneSAS

hard disk backplane is used in server products and plays the role of "value-added cable". The reason why it is called "value-added cable" is that on the one hand, it has a connection function for the hard disk, and on the other hand, compared with a single cable, the hard disk backplane can also provide some additional functions, such as hard disk working status indication, hard disk hot swap capture, etc. The hard disk backplane has been continuously improved from the early optical backplane to the recent electrical backplane. With the huge increase in the amount of data in the current user environment, the number and capacity of hard disks configured in a single server have increased significantly, so the demand for backplanes has also increased. The design and quality control of hard disk backplanes have become an important concern affecting the performance and stability of the server.

1 System Design
The hard disk backplane is an extension of the SAS line. While completing the extension, it displays the hard disk status information in real time. The hard disk backplane mainly consists of four parts, as shown in Figure 1, the interface unit, the CPLD control unit, the temperature detection unit and the power supply. The functions of each part are introduced below.

1.1 Interface unit
The interface unit includes Minisas interface, SAS interface, and SMBus (System Management Bus) interface. The Minisas interface transmits the signal obtained from the server-side HBA (Host Bus Adapter)/RAID (Redundant Array of Independent Disk) control card to the hard disk backplane through the Minisas cable. The SAS interface is used to connect the external hard disk, and the SMBus interface is used to transmit the temperature signal on the backplane to the server.
1.2 CPLD control unit
EPM240 is the core of the backplane management. It is a low-power and low-cost CPLD launched by Alteral. It belongs to the MAXⅡ family. It provides programmable solutions while reducing the cost and power consumption of the design. EPM240 parses the sideband signal of the Minisas interface into the corresponding hard disk status for output. The sideband signal complies with the SFF-8485 protocol, which defines the serial GPIO (SGPIO, Serial General Purpose Input/Output) bus used in conjunction with serial SCSI (SAS) and serial ATA (SATA). The SGPIO bus has a total of 4 signal lines for SAS/SATA HBA The communication between the HBA/RAID control card and the hard disk backplane, the control card obtains the status of the hard disk socket on the backplane and sends the hard disk LED status indication signal through SGPIO. Among the 4 lines, 3 are driven by HBA/RAID and 1 is driven by the backplane. The SGPIO bus and MiniSAS line are placed in one cable to facilitate the connection between the HBA/RAID card and the backplane. The definitions of the 4 SGPIO lines are as follows:
SClock: The clock line driven by HBA/RAID, the maximum clock frequency is 100kHz (typically 48kHz).
Sload: This line is synchronized with SClock and is mainly used to indicate the beginning of a new frame of data. When Slcad has at least 5 clock low levels, Sload follows to high when the SClock pulse rises to indicate the beginning of a new frame of SGPIO data. The 4-Bit Sload data specification corresponding to the subsequent 4 clock pulses is not clearly defined, and the definitions of different HBA/RAID card manufacturers are also different.
SDataOut: This line is used by the HBA/RAID to output the hard disk status signal.
SDataIn: This line provides the status of the backplane from the backplane to the HBA/RAID. The first high level indicates that the slot has a hard disk inserted, and the following two are usually low levels.
The CPLD analyzes the sideband signal and displays the working status of the hard disk by controlling the on and off of the LED light on the backplane. Each hard disk on the backplane has two LEDs, a single-color LED to indicate whether the hard disk is inserted and powered on, and a two-color LED to indicate the normal operation, failure, and hard disk location of the hard disk.
1.3 Temperature Monitoring Unit
The temperature monitoring unit collects the temperature information of the hard disk through the temperature sensors distributed on the hard disk backplane, mainly through reasonable distribution on the hard disk backplane to obtain the temperature parameters of the specified hard disk area. The temperature detection unit is very important for the backplane. It collects the temperature of the backplane in real time and transmits the temperature signal to the server motherboard. If the motherboard detects that the temperature on the backplane is too high, it will alarm to protect the backplane and the hard disk.
The temperature detection unit uses EMC1053, which is a temperature sensor of the SMBus interface. It can detect the temperature of up to three points, including one sensor internal temperature and two remote temperatures. The accuracy of the internal temperature of the sensor is ±3℃ (0℃～85℃), and the accuracy of the remote temperature is ±1℃ (40℃～80℃). EMC1053, as a slave device, transmits the detected temperature signal to the server through SMBus.

1.4 Power supply part
The +12V and +5V voltages of the backplane come directly from the server power supply. The operating voltage of the CPLD and temperature sensor chip requires 3.3V, so a voltage regulator chip AME1117 is added to convert the 5V voltage into 3.3V for the temperature detection unit and CPLD control unit. [page]

2 Test results
Figure 2 is a physical picture of the hard disk backplane. The size of the backplane is 140mmx80mm and the thickness is 2mm. It can support up to 8 2.5' SAS hard disks. The SAS hard disk backplane adopts a 6-layer PCB design, with independent power layer and ground layer, and isolates the signal layer, which not only enhances the anti-interference ability, but also ensures the integrity of the signal.

The transmission rate of SAS signal is 6Gb/s. The SAS backplane currently used adopts the surface routing method. Figure 3 is the eye diagram of 6G SAS signal. From the eye diagram results, the quality of SAS signal is satisfactory. Through the 7*24H reliability test of the backplane, the results show that the backplane can complete the reliable data transmission from the server to the hard disk.

3 Conclusion
A manageable SAS hard disk backplane based on CPLD is designed, which can display the working status of the hard disk while conveniently replacing the hard disk. The test results show that the hard disk backplane can reliably complete the data transmission between the server and the hard disk. At present, this backplane has been used in many products of Inspur.