Characteristics and electrical parameters of DDR4

DDR4 What are the characteristics ？

DDR Series have DDR,DDR2、DDR3、DDR4、DDR2L、DDR3L、DDR4L、 And the latest DDR5. Although technology is improving , But there are many similarities between each generation . Let's observe DDR Features of mainstream series products , It is not difficult to find rules .

Obviously , The voltage is getting lower and lower , The frequency is getting higher and higher , The data bus has developed from single ended to differential . We found that this is similar to the development history of the bus .

The root of science and technology is still the most common wisdom of mankind , Don't believe it ？ Let's take an inappropriate plum , chestnuts , Example .
If there are many plums on this side of the river , There are many chestnuts across the river , If only manpower , Carry the chestnuts across the river and the plums here back and forth as soon as possible , What methods do you have ？ You can easily come up with several solutions , Find more people to move at the same time ; Find a brother who swims fast ; Move where the river is narrow ;2 A group of people ,A Throw plums ,B Throw chestnuts ….

You think you are , Smart predecessors are just like you , This kind of thinking is applied to DDR On the design of .
At first, I found many people to move together , therefore DDR At first, parallel routing design ; Later, I looked for someone who could swim fast , therefore DDR The frequency of is increasing ; Swimming is fast but tired , We have to find a narrow place to swim , therefore DDR The working level is getting lower and lower （ The current transmission speed is certain , So the lower the voltage , The less time it takes ）. It's not a thing to swim back and forth all the time , Can two people form a group , You are responsible for moving 0, I'm in charge of moving 1, therefore DDR The bus is gradually evolving from parallel to differential .

That's it DDR The law of technological development , It is also the development law of many buses . Grasp the law , Let's look at features ,DDR4 level 1.2V,DQS The signal is differential , Frequency at 1600Mbps~3200Mbps.

DDR4 What are the key signals ？

DDR4 What are the key signals ？ A picture can be seen . Yes CK,ADDR,DQS,DQ,DQM The signal , Outside the circuit , You can also see enabling , Reset ,ODT,ZQ.

So what are the functions of these signals ？ Let's make a brief introduction next .

1. CK,Address,DQS,DQ,DM What are the functions of signals ？

CK yes DDR Data communication clock signal of , When CK and Address When the signal works together , Can be done CMD( command ) Operation and Address( Address ) Signal transmission . Because the address signal and command signal are in DDR The operation process belongs to the control signal , relative DDR Data transmission of , It belongs to a niche scene , therefore DDR Address signal and CMD The signal is still single ended , The working frequency is also lower than that of data transmission . So it's not hard for us to find out ,ADDR The signal is still single ended , And most of the matching resistors are still external .

Because data transmission is DDR Main business of , So in DDR Data transfer , Additional measures are needed to ensure data transmission , therefore DDR4 Added new roles ,DQS,DQ,DM The signal .

Data selection pulse （DQS） yes DDR The important function of , Its function is mainly used to accurately distinguish each transmission cycle in a clock cycle , And facilitate the receiver to accurately receive data . Every chip has a DQS The signal line , It's two-way , When writing, it is used to transfer CPU It's from DQS The signal , When reading , By DDR Generate DQS towards CPU send out . It can be said that , It is the synchronization signal of data . You can even say that ,CK and DQS coordinate , As DQ Reference clock of data line , This is also why when wiring ,DQS and DQ Reasons to consider together .

In the process of continuous data transmission , As the clock changes ,CPU You may read DDR Middle position 1, Location 2, Location 3, Location 4 The data of , Suppose in this continuous transmission process ,CPU Don't want position 2 The data of , Don't want to interrupt data transmission , So what can I do ？DQM That comes in handy , At transfer to location 2 yes ,DQM Can make , You can shield the position 2 The data of .

2. What is? ODT？

The chip terminal is terminated with matching resistance On-Die Termination Referred to as ODT.ODT It's from DDR2 Later products began to appear , And with the upgrading of products ,ODT Present more values . At present ,ODT Mainly used for data （data,DQ）、 Data gating （DQS/DQS＃） And data mask （Data Mask,DM） Three types of signal lines .ODT Its function is to replace the conventional circuit matching design , The discrete termination resistors originally outside the chip are integrated into the chip , The simplified circuit structure is shown in the figure below .

ODT Introduction of circuit , yes DDR Milestones of development ,ODT It has the following advantages .
Removed PCB Discrete terminating resistor elements on the board , Reduce the cost of hardware design .
Since there is no termination resistance , send PCB The design has more wiring space , convenient EDA wiring .
because ODT Adjust through the internal register , It can be turned on and off by software , Reduce the debugging workload .
Chip internal termination is more effective than board level termination , There are not too many parasitic effects .
Reduce the number of external components , The reliability is also optimized .
ODT The introduction of , To further improve DDRx The working frequency of the memory is paved .
Of course ,ODT Not all are advantages , Because of the use of ODT, The type and value of matching resistance have been determined and cannot be modified ; To some extent, it will also bring a certain increase in power consumption , Therefore, some consumer products are not always open ODT function .

3. What is? ZQ calibration ？

ZQ yes DDR3 Then add a new pin , and DDR3 It is through ZQ Conduct output resistance （Ron） and ODT Calibration of , This calibration process is in DRAM The initialization process will be completed .ZQ It is relatively simple in principle design ,ZQ The pin passes a low tolerance 240Ω（ The general tolerance is 1％） The resistance of is directly connected to the ground .

Except eye chart ,DDR4 Also look at those electrical parameters ？

DDR4 Whether the design is designed OK, You need to see DDR4 Whether the measured parameters of meet JEDEC standard . What electrical parameters are defined in the specification ？ What meaning does each electrical parameter represent ？ Next we will make a brief introduction .

1. Overshoot and undershoot .

If the impedance of the signal is discontinuous during transmission , There will be reflection , Reflection is shown on oscilloscope waveform , It's the signal up and down , stay DDR During the design process , We are not concerned with the overshoot amplitude of the signal at a certain point in time , But the integral of overshoot in the time dimension . Focus on a process , Not a point .

For each signal , The protocol specifications will be here 4 Define and standardize signal integrity in three dimensions .

2. High and low levels of the signal .

In high-speed signal transmission , Whether the signal can reach the minimum threshold of high level and the maximum threshold of low level specified in the Protocol , It also directly determines the stability of the product .
stay DDR Specification , Single ended address and control commands 、 Single ended data and data mask signals AC and DC Input level , Differential clock and data strobe signal AC and DC The input level will be clearly defined . During simulation and testing , We only need to pay attention to whether the measured value in a specific scenario reaches the value specified in the agreement .

3. Differential signal intersection voltage .

In order to meet the signal establishment and holding time , The single ended intersection of differential signal must be within a certain range .

The intersection is marked with V DD/2 As a reference voltage , The measurement is relative to V DD/2 The deviation of , The smaller the deviation, the better .

4. Timing requirements .

The main requirements of time order relationship are 3 Group ： Address 、 Between control and command signals and clock signals ; Between data strobe signal and clock signal ; data 、 Between data mask and data strobe . So in PCB When the design , It is also designed in groups according to signals , Address 、 Control and command signals are grouped with clock signals , data 、 Each byte of data mask signal and data strobe signal is a group , The signals of the same group will basically keep the length difference within a certain range during design , And the design method is basically the same （ As if the group wiring is on the same floor 、 Have the same number of vias ）, Only in this way can we meet the requirements of timing .

5. Slope derating .

There are two ways to measure the slope in the specification of the bus , They are the conventional measurement method and the tangent measurement method , By address 、 Take control and command signals for example , Its definition is shown in the figure below .

These two measurement methods are mainly determined by the quality of the waveform obtained , If the monotonicity of the waveform is better , Then measure in a conventional way ; If monotonicity is not good , There are some hooks or steps , Then use tangent method to measure .

The red box in the figure represents the conventional slope measurement method , The green arrow represents the tangent measurement method . stay DDR in , The establishment and holding time of the signal is not the only constant , It will change as the slope changes , To compensate for the effect of slope , In the simulation of the final “ Build and maintain time ” when , The measured timing parameters need to be added to the derating parameters . Derating parameters and signal slope and clock / The slope of data gating .

Reference to the original ：《DDR4 Practical teaching ( 3、 ... and )： Characteristics and electrical parameters 》