When designing a multi-slice DDR, there is usually a topology for the DDRBelow, we will take a closer look at the differences between the various topologies and their application scenarios.
First of all, let's introduce that when there is only one DDR, it is usually a point-to-point connection, and the advantages of point-to-point wiring are simple structure, impedance and timing are easy to control, and it is suitable for high-speed bidirectional transmission.
Point-to-point topology.
Daisy chain (fly by) topology.
The daisy chain topology is connected to each driver in turn from the CPU, in terms of controlling the higher harmonic interference, the wiring effect of the daisy chain is the best, in the actual wiring, we need to pay attention to make the branch of the chrysanthemum chain as short as possible, usually the branch length should be less than one-tenth of the signal rising edge time,The daisy-chain topology has the characteristics of impedance characteristics and is easy to control, which is usually suitable for the transmission of high-speed signalsWhen processing multiple pieces of DDR, we should choose the daisy chain topology as much as possible, but it should be noted that when the daisy chain topology is used to transmit signals, there is a delay between the loads, and the signals cannot reach the load side at the same time, so on this basis, when using this topology, we need to pay attention to whether the main control chip supports read-write balance, and if it does not support read-write balance, it is not possible to use the daisy chain topology.
T-point topology:The t-point topology is also called the star topology and the tree topology.
Point t is close to the drive end:
It can effectively avoid the asynchronous problem of the clock, and it is easy to control the length of the load, which is used in places with strong driving abilityThe shorter the main line, the better。The disadvantage is that it is difficult to route and each branch requires a termination resistor, and the resistance value of the termination resistor should match the impedance of the wire.
Point t is close to the load side:
Each branch can be balanced, suitable for use when the load is more, at this time the transmission line branch is by the load end, only one termination resistor is needed when terminating, this connection mode is the most common when carrying out the design of the T point, it should be noted that the length of the main line and the branch line of the signal, the branch line should meet the length of the main line less than or equal to 1 3, and it is important to pay attention to this requirement when designing the T point.
There are various layouts of DDR, such as DDR front and back paste, staggered paste, single-sided layout, etc., but the topologies used are all the ones we introduced above.
When we design DDR3, DDR4, and DDR5, we need to pay attention to the choice of topology as much as possible to trend towards the daisy chain topologyCompared with the T-shaped topology, the daisy-chain topology can effectively inhibit the reflection of the branchIn order to solve this problem, DDR3 introduces time compensation technology, and realizes the synchronization of signals through internal adjustment, according to the results of ** and practical application, the daisy chain topology is more suitable for transmitting high-speed signals, and can better meet the integrity requirements of the signal.
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