JESD204B character 传输协议讲解(简单透彻)

Understanding control characters inJESD204BHere's a closer examination of the control characters that are employed in the JESD204 interface.By Jonathan HarrisProduct Applications EngineerAnalog Devices Inc.The shift to JESD204B as the digital interface of choice for high speed data converters is well underway. TheJESD204 interface was released in its original form, JESD204, in 2006 revised to JESD204A in 2008, and in August 20011 revised once more to the current JESD204B. The interface brings efficiency and offers several advantages over preceding technologies like LVDS. Designs employing JESD204B enjoy the benefits of a faster interface to keep pace with the faster sampling rates of converters. There is a reduction in package pin count which leads to smaller packages and less trace routes. The standard applies to both analogue-to-digital converters (A/D) as well as digital-to-analogue converters (D/A), and is primarily intended as a common interface to field programmable gate arrays (FPGAs) – for example the Xilinx Kintex or Vertex platforms – but it may also be used with ASICs.JESD204B differs from its predecessors in up-front complexity due to the new terms and parameters that it introduces. In this article, we'll take a closer examination of the control characters that are used in the JESD204 interface. Understanding the control characters helps provide a better understanding of how a link is synchronised and aligned. This helps designers to understand how to debug link issues that may arise when prototyping a design with the JESD204 interface. Each of the control characters performs a different function and helps maintain the alignment of data on the link as well as synchronisation and error monitoring. If an expected character is missed or an unexpected character is received, the receiver knows that an error exists.How it works is that the JESD204B words get mapped into valid 8b/10b encoded words and are set up in frames and multi-frames with specific rules. In the process, the 8b/10b encoding provides some benefits for the serial data link by using control characters that provide the ability to perform various lane alignment functions. There are five main control characters utilised in the 8b/10b encoding that allow for various functions in the JESD204B data stream. These characters are /K/, /F/, /A/, /R/, and /Q/ control characters.What a bunch of characters!The /K/ = /K28.5/ control character is used in the code group synchronisation process via the synchronisation interface (by asserting !SYNC). Once the receiver issues a synchronisation request, the transmitter begins emitting /K/ = /K28.5/ characters. The receiver synchronises and will wait for at least four consecutive /K/ = /K28.5/ characters. The receiver then deactivates its synchronisation request. This process is done according to the subclass of operation. For subclass 0 (no deterministic latency), the receiver deactivates the synchronisation request on any frame boundary after four consecutive /K/ = /K28.5/ characters have been received. For subclass 1 and 2 (deterministic latency with SYSREF or !SYNC, respectively), the receiver deactivates the synchronisation request on any local multi-frame clock boundary after four consecutive /K/ = /K28.5/ characters have been received.Figure 1: /K/ control character streaming.The /A/ = /K28.3/ control character is used for multi-frame alignment in the serial data stream. It is inserted at the end of a multi-frame by the transmitter under certain conditions. In this case, when the last octet of the current frame at the end of a multi-frame is equal to the last octet of the previous frame, an /A/ = /K28.3/ character replaces that octet. This happens even if the last octet of the previous frame was also a control character.The /R/ = /K28.0/ control character is used to indicate the beginning of a multi-frame. If the transmitter emits an initial lane alignment sequence, then the /R/ = /K28.0/ is the first non-/K28.5/ character that is transmitted. During an initial lane alignment sequence, the transmitter will always emit an /R/ = /K28.0/ character to indicate the beginning of a multi-frame and an /A/ = /K28.3/ character to indicate the end of a multi-frame.The /Q/ = /K28.4/ control character is used during the initial lane alignment sequence to indicate to the receiver that the configuration data is going to start. It is important to remember that this particular control character is only used during the initial lane alignment sequence and not during any other phase of data transmission.Figure 2: /R/, /A/, and /Q/ control characters.The /F/ = /K28.7/ control character is used for frame alignment in the serial data stream. This character is inserted at the end of a frame by the transmitter under certain conditions. When the last octet of the current frame (which is not the last octect of a multi-frame) is equal to the last octet of the previous frame, a /F/ = /K28.7/ character is used to replace that octet. However, if a case arises where the last octet in the previous frame was a /F/ = /K28.7/ character, the current octet is not replaced.Figure 3: /F/ and /A/ control character – frame and multi-frame alignment.All of these control characters, /K/, /F/, /A/, /R/ and /Q/, are given along with their 10bit binary representations in the table. There are two corresponding 10bit binary representations of each control character so that DC balance is provided in the data transmission and the interface is AC-coupled. This follows suit with the data which also has DC balance in the 8b/10b data words. This scheme aligns with the 8b/10b encoding described in IEEE 802.3. ConclusionIt is important to become familiar with the many aspects of the JESD204 interface as it becomes the interface of choice for A/D and D/A converters. It has more complexity than its preceding interface technologies. However, JESD204B does provide many benefits.The control characters used in JESD204 allow the link to be synchronised properly as well as monitored for alignment. The various control characters each performs a specific function in maintaining the link between the JESD204 transmitter and receiver. These control characters also provide a method of monitoring the JESD204B link for errors.Table: 8b/10b control characters.As you peel the onion back more and more on the JESD204B standard, the many benefits such built-in alignment, monitoring, and error detection become evident demonstrating the robustness of the standard. JESD204B is ready to carry the latest generations of data converters forward into faster sample rates and smaller packages.References1. The JESD204B Survival Guide2. JEDEC Standard JESD204B (July 2011). JEDEC Solid State Technology Association ()About the authorJonathan Harris is a product applications engineer in the high speed converter group at Analog Devices in Greensboro, NC. He has over 8 years of experience as an applications engineer supporting products in the RF industry. Jonathan received his MSEE from Auburn University and his BSEE from UNC-Charlotte. In his spare time he enjoys motorcycle riding, mobile audio, nitro R/C, college football and spending time with his family. Contact Jon with your JESD204B questions on ADI's EngineerZone at J.Harris.。