74hc vs 74ls
eugenehhk,
"LS" stands for low power Shottky. It is a technology that uses bipolar transistors. The first stage of the gate has the collector "clamped" to the base via a Schottky diode. The diode prevents the first stage from saturating, thus ensuring that the turn-off time of this stage will be minimal.
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"HC" stands for hign speed CMOS. It uses complementary MOS transistors, along with low value resistors to achieve high speed switching.
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"HC" is the faster of the two technologies.
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Compatibility depends on the application. An HC device can adrive a TTL device, but not vice-versa. The reason is that the input switching point of a TTL device is typically around 1.6 Volts. The CMNOS output typically swing s almost rail to rail. However, the guaranteed "1" state output voltage of a TTL device is typically around 2.4 volts (For a 5V supply). The input switching point of a CMOS device is typically around 1/2 the supply voltage (2.5V for a 5 V supplY). As a result, you can not get reliable performance by driving a CMOS input directly from a TTL output. You can partially mitigate this problem by providing a pull-up resistor from the TTL output to the supply.
Regards,
Kral