6.9  Summary

Among the options available in I/O cells are: different drive strengths, TTL-compatibility, registered or direct inputs, registered or direct outputs, pull-up resistors, over-voltage protection, slew-rate control, and boundary-scan. Table 6.4 shows a list of features. Interfacing an ASIC with a system starts at the outputs where you check the voltage levels first, then the current levels. Table 6.5 is a look-up table for Tables 6.6 and 6.7 , which show the I/O resources present in each type of programmable ASIC (using the abbreviations of Table 6.4 ).

TABLE 6.4  I/O options for programmable ASICs.

Code 1

I/O Option

Function

IT/C

TTL/CMOS input

Programmable input buffer threshold

OT/C

TTL/CMOS output

Complementary or totem-pole output

nSNK

Sink capability

Maximum current sink ability (e.g., 12SNK is I 0 = 12 mA sink)

nSRC

Source capability

Maximum current source ability (e.g., 12SRC is I 0 = –12 mA source)

5/3

5V/3V

Separate I/O and core voltage supplies

OD

Open drain/collector

Programmable open-drain at the output buffer

TS

Three-state

Output buffer with three-state control

SR

Slew-rate control

Fast or slew-rate limited output buffer to reduce ground bounce

PD

Pull-down

Programmable pull-down device or resistor at the I/O pad

PU

Pull-up

Programmable pull-up device or resistor at the I/O pad

EP

Enable polarity

Driver control can be positive (three-state) or negative (enable).

RI

Registered input

Inputs may be registered in I/O cell.

RO

Registered output

Outputs may be registered in I/O cell.

RIO

Registered I/O

Both inputs and outputs may be registered in I/O cell.

ID

Input delay

Input delay to eliminate input hold time

JTAG

JTAG

Boundary-scan test

SCH

Schmitt trigger

Schmitt trigger or input hysteresis

HOT

Hot insertion

Inputs protected from hot insertion

PCI

PCI compliant

Output buffer characteristics comply with PCI specifications.

Important points that we covered in this chapter are the following:

  • Outputs can typically source or sink 5–10 mA continuously into a DC load, and 50–200 mA transiently into an AC load.
  • Input buffers can be CMOS (threshold at 0.5 V DD ) or TTL (1.4 V).
  • Input buffers normally have a small hysteresis (100–200 mV).
  • CMOS inputs must never be left floating.
  • Clamp diodes to GND and VDD are present on every pin.
  • Inputs and outputs can be registered or direct.
  • I/O registers can be in the I/O cell or in the core.
  • Metastability is a problem when working with asynchronous inputs.

1. These codes are used in Tables 6.6 and 6.7 .


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SolidCAM: Program your CNCs directly inside your existing CAD system.



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