Saturday, May 7, 2011

【 Weak current College 】 common logic level standard 】

Now commonly used level standard CMOS, TTL, LVTTL, LVCMOS, ECL, PECL, LVPECL, RS232, RS485, etc, there are some speed comparisons
High of GTL, LVDS, CML, PGTL, HSTL, SSTL, etc. The following briefly about their own power supply, voltage standards and usage considerations.

TTL: Transistor-TransistorLogic transistor structure.
Because 2.4V and 5V is still great free between, on the improvement of the noise and no good, also be increasing system power consumption, also affect the speed. So later on the part of the "cut off". Also is LVTTL behind.
LVTTL and 3.3V, 2.5V and lower voltage LVTTL (LowVoltageTTL).


Lower LVTTL infrequently spoke first does not. Most used in the processor and other high-speed chip, chip manuals when viewing the OK.

TTL use note: TTL level General overshoot is more serious, you may begin-end series 22 euro or 33 European resistance; TTL input feet dangling as internal considered high level. If you want to drop the following pull-down resistor 1k application. TTL output cannot be driven CMOS input.

Relative TTL with larger noise, input impedance is much larger than TTL input impedance. Corresponding 3.3VLVTTL, LVCMOS, can direct the 3 .3V LVTTL of mutual driven.



CMOS use note: CMOS structure, internal parasites have a thyristor structure, when you enter or input pin is above a certain value VCC (such as some chip is 0.7V), the current is large enough, it might cause latch effect, result in the burning of the chip.

ECL: EmitterCoupledLogic emitter coupled logic circuits (differential structure)
Speed, driving ability, low noise, very easy to reach several hundred M. But power, need to assume power. For simplicity, power, PECL (ECL structure, use the positive voltage supply) and LVPECL.



ECL, PECL, LVPECL use note: different level cannot directly driven. Intermediate available AC-coupled, the resistor networks or ASIC conversion. All three were shot with the output structure, you must have a resistance to a DC bias voltage. (As used on clock LVPECL: DC match with 130 euro pull-up and pull-down with 82 euro; exchange matching with 82 euro pull-up and pull-down with 130 euro. But after two ways are 1.95V DC level. )

The preceding level standard Swing are relatively large, in order to reduce electromagnetic radiation, while increasing the switching speed introduction of LVDS level standards.

Difference on input and output, internal has a constant current source 4mA 3.5-, in difference line changes direction to indicate 0 and 1. By external resistor 100 euro match (and in differential line near the receiving end) is converted to ± 350mV difference level.
LVDS use note: you can meet above the PCB 600M requires higher differential line, strictly equal length does not exceed the difference best 10mil (0.25mm). 100 European resistance from the receiving end distance must not exceed 500mil, best control in 300mil or less.
Following levels may not be used for many, just do itDescribes.

CML: internal do match a circuit, does not need to be matched. Transistor structure, differential line, the speed can reach 3G above. Only point-to-point transmission.

GTL: similar to the CMOS structure, entered as comparator structure, comparator a terminating reference level, the other terminating input signal. 1.2V power supply.


HSTL is primarily used for storage of a QDR level standards: General V? CCIO = 1.8V and V?? CCIO=1.5V。 And GTL similar above, entered as input to the comparator structure, comparator a terminating reference level (VCCIO/2), the other terminating input signal. The reference level requires relatively high (1% accuracy).
SSTL is mainly used for DDR memory. And HSTL basically the same. V?? CCIO = 2.5V, entered as input to the comparator structure, comparator a terminating reference level 1.25V, another terminating input signal. The reference level requires relatively high (1% accuracy).
HSTL and SSTL is mostly used in 300M following.

RS232 and RS485 basic and we compare ripe, only simple mention:
RS232 with ± 12-15V power supply, serial after our computer for the RS232 standard. + 12V,-12V 0 said that 1. You can use MAX3232 etc ASIC conversion, you can also use two transistor plus some peripheral circuits for RP and voltage matches.
RS485 is a relative difference structure, RS232 high anti-interference ability. Transmission distance can be reached at km.

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