Answers Database

Are 4000XL/XV I/O Thresholds Programable to TTL or CMOS Compatibility?


Record #2760

Product Family:  Hardware

Product Line:  4000XL

Problem Title:
Are 4000XL/XV I/O Thresholds Programable to TTL or CMOS Compatibility?


Problem Description:
Keywords: 4000XL, 4000XV, threshold, CMOS, TTL, I/O

Urgency: Standard

General Description:

The XC4000XL family of Xilinx FPGAs do not have programmable threshold
levels, but they are compatible with 3.3Volt-CMOS, 5Volt-CMOS, and TTL
Threshold Standards for both Inputs and Outputs.  This broad range of IO
compatibility allows the XC4000XL to operate in mixed voltage systems
(see also solution record 2505).


Solution 1:

The M1.3 version software does allow the user to choose the threshold levels
they wish to be compatible with (CMOS or TTL) for inputs and outputs.  The
setting (no matter the choice) will not have any effect on the implementation
or configuration of the device.

However currently with M1.4 choosing the incorrect option for CMOS level inputs

will generate bad speed numbers resulting in incorrect delays in TRACE and
back annotation. M1.5 will not have these options available to users.
See also (Xilinx Solution 3517)

The actual thresholds and switching characteristics of the device are derived
from CMOS technology, but designed to be compatible with both standards with
different operating specifications (see pp. 66-67 of the new xc4000XL
datasheet or at weblink below).

http://www.xilinx.com/partinfo/4kswch.pdf

INPUTS
======

The input thresholds for the XC4000XL are compatible for both CMOS and TTL.
This is because the VILmax for the XC4000XL is greater than the industry
standard VILmax specification, and the VIHmin for the XC4000XL is less than
the industry standard VIHmin specification.

For Voltage Input Logic Low (VIL) the threshold standards are:
TTL  = 0.8 volts max
CMOS = 1.0 volts max

The XC4000XL VIL is specified as 30% VCC.  If we assume a minimum VCC of 3.3
volts then VILmax = 1.0 Volts and both specifications are met.	The
specifications are conserved from the actual enough so that operating
conditions can work at the absolute limit of the specifications.

For Voltage Input Logic High (VIH) the threshold standards are:
TTL  = 2.0 volts min
CMOS = 70% VCC min

The XC4000XL datasheet specifies a VIH of 50% VCC.  For even at maximum VCC
(4.0 volts) the threshold is below the minimum for either threshold standard.
(Note: drivers should be above the minimum and loads should accept below the
minimum).

Therefore, no programmability is required.



Solution 2:

OUTPUTS
=======

The output characteristics of the 4000XL family are compatible to both TTL
and CMOS without the need for programmibility as well.	However, the
datasheet specifies two different ratings for each VOH and VOL for both TTL
and CMOS.  This should not be interpreted as a programmable option.  The
purpose for this specification style is to indicate the maximum current
ratings allowed to still be within a compatible range for a given threshold
standard.

For example,
VOH (TTL) is 2.4 volts @ 4mA
VOH (CMOS) is 90%VCC @ 500uA

The way that this should be interpreted is that to be CMOS compatible the IO
should not be sourcing more than 500uA, and to still be TTL compatible it
should not be sourcing more than 4mA.  (Note:  The databook actualy specifies
the current values as negative numbers.  This merely indicates that electron
flow is toward the source of higher potential.	It is merely a convention and
should not cause any concern.)



End of Record #2760