Is LVTTL or LVCMOS can be used for PCI Signaling? BTW, I'm using Atera MAX-II EPM1270T144C5...
- kai -
Is LVTTL or LVCMOS can be used for PCI Signaling? BTW, I'm using Atera MAX-II EPM1270T144C5...
- kai -
Depends on the
- Physical size of the PCI bus
- Number of loads
- PCI bus speed (33 or 66 MHz)
But in some situations LVTTL can be used with no problem.
KJ
You're quite right. The original CMOS (4xxx) and TTL (74xxx) chips had different outputs in terms of voltage, drain and source current. These differences don't seem to apply to Xilinx FPGA's, and that is a little bit confusing.
If people like the LVTTL standard, we don't want to disappoint them. Its Vohmin spec of 2.4 V is derived from the original 5-V Vcc bipolar totem-pole outputs which usually have two diode driops below Vcc. And at 4.5 V and -55 degrees that can, worst-case, result in a 2.4 V output High voltage. Dictated T.I. fourty years ago.
Today we don't use 5 V, and we don't use bipolar circuitry anymore, but this old number just hangs around and confuses designers who might hardly know what TTL really stands for.
Except for the special cases of LVDs and CML, each CMOS output today has a bunch of n-channel transistors that pull to GRND and a bunch of p-channel transistors that pull to Vcc. The standards then differ in the number of transistors in the bunch being activated. (And the "c" in Vcc means "collector" , another bipolar anachronism. Even a young industry like ours carries a lot of historical baggage...) Peter Alfke
John Smith wrote:
Peter Alfke schrieb:
which one? ;-)
Kolja
Both. The rule was (is): Vc describes the actual collector voltage, which usually is a variable. Vcc describes the source of that coltage, the power supply voltage.
See also Vgg and Vdd,same reas> Peter Alfke schrieb:
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