Eight Misunderstandings in PCB Design (2)

Misunderstanding 6:
The memory has so many control signals. My board only needs to use the OE and WE signals. The chip selection should be grounded so that the data comes out much faster during the read operation.

Summary: The power consumption of most memories when the chip selection is valid (regardless of OE and WE) will be more than 100 times larger than when the chip selection is invalid. Therefore, CS should be used to control the chip as much as possible, and as long as other requirements are met. It is possible to shorten the width of the chip select pulse.

Misunderstanding 7:
The power consumption of these small chips is very low, so don’t care about it.

Summary: It is difficult to determine the power consumption of the internal chip that is not too complicated. It is mainly determined by the current on the pin. An ABT16244 consumes less than 1 mA without load, but its indicator is each pin. It can drive a load of 60 mA (such as matching resistance of tens of ohms), that is, the maximum power consumption of a full load can reach 60*16=960mA, of course, only the power supply current is so large, and the heat falls on the load.

Misunderstanding 8:
Why do these signals have overshoot? As long as they are matched well, it can be eliminated.

Summary: Except for a few specific signals (such as 100BASE-T, CML), they all have overshot. As long as they are not very large, they do not necessarily need to be matched, and even if they are matched, they do not necessarily match the best. For example, the output impedance of TTL is less than 50 ohms and some even 20 ohms. If such a large matching resistance is used, the current will be very large, the power consumption will be unacceptable, and the signal amplitude will be too small to be used. Besides, the output impedance of a general signal when outputting a high level and outputting a low level is not the same, and there is no way to achieve a complete match. Therefore, the matching of TTL, LVDS, 422 and other signals can be acceptable as long as the overshoot is achieved.

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