74LSxx versus. 74HCxx

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     74LS屬於TTL類型的整合電路，而74HC屬於CMOS整合電路。    LS、HC 二者高電平低電平定義不同，HC高電平規定為0.7倍電源電壓，低電平規定為0.3倍電源電壓。LS規定高電平為2.0V，低電平為0.8V。 帶負載特性不同。HC上拉下拉能力相同，LS上拉弱而下拉強。 輸入特性不同。HC輸入電阻很高，輸入開路時電平不定。LS輸入內部有上拉，輸入開路時為高電平。    （1）74LS系列是“低功耗肖特基TTL”，統稱74LS系列。其改進型為“先進低功耗肖特基TTL”，既74ALS系列，它 的效能比74LS更好。    （2）74HC系列，它具有CMOS的低功耗和相當於74LS高速度的效能，屬於一種高速低功耗產品。    （3）上述兩者的工作頻率都在30mHz以下，74ALS略高，可達50mHz。    （4）但它們的工作電壓卻大不相同：74LS系列為5V，74HC系列為2~6V。    （5）扇出能力：74LS系列為20，而74HC系列在直流時則高達1000以上，但在交流時很低，由工作頻率決定。

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LinkTo：http://forums.parallax.com/showthread.php/79113-74LS595-verses-74HC595

Question：

I have been using 74LS595 as LED Display drivers at 15ma per 595 output.· These 595's work with the SX28.· I previously noticed in the Jameco Catalog a vast price difference in the HC595 $.62 and the LS595 $4.55 .· The pinouts are the same so I ordered a few to see if they work, and they did.· However I lack the expertise to determine from the data sheets what the difference is and if there are any drawbacks to using the HC verses the LS for projects.

Answer：

Short answer to your question; it is fine to use the HC (high Speed CMOS) series logic with the SX.· This is one of the few times in life you actually do get more for less.· A quick perusal of the data sheet shows that the devices are roughly equivilent, but the HC version has a much higher allowable clock frequency (100MHz vs 20MHz), this is good as far as the super fast SX is concerned.As others have stated, the LS series is based on TTL (total transistor logic) which means that·thay are made with·bipolar transisitors (BJT).· BJT's were·one of the first transistors used in ICs.The HC series use CMOS (complementary metal oxide semiconductor) devices·that are a later generation transistor that use, as the name denotes, metal over an oxide as the gate in the transistor (nowdays it's polysilicon over an oxide).· The advantage to these devices·is that the input impedance is much higher than the BJT.· This translates into much lower current and power usage.As a simple rule (which has exceptions), CMOS is used in logic devices where the low power·needs are good, and BJTs are used in higher power devices (switching, power management apps., etc).· There are also BiCMOS devices that put both CMOS and BJTs on the same die and use the best atributes·of·each device.The major reason for the price difference is as Paul said, supply and demand.· Well actually, supply.· The HC series has about 8 bazillion made per year, while there are only a piddling of the LS series produced per year.· So use the HC series and laugh at the fact that you are getting more for less.