Interfacing between CMOS and TTL:

Interfacing CMOS and TTL are as follows:

CMOS to TTL

• If VCC is +5V, then one CMOS output can drive one LS TTL input
• CMOS logic levels are close to 0 V or 5 V, so no threshold incompatibility
• If CMOS is run at VCC ~3.3 V, thresholds are still compatible with TTL
• Sometimes 4000 series or 74C chips are run at VCC > 5 V for improved speed
• Need level-shifter chip to interface to TTL, for example 4049/50, 74C901/2

TTL to CMOS

• TTL output thresholds are inconsistent with 74HC, 74C and 40' CMOS inputs
• When CMOS is run with VCC = 5 V
• Use a 74HCT buffer between them
• Use an open collector buffer with pullup to 5 V
• When CMOS uses VCC = 3.3 V (Usually 74HC only)
• Direct connection from TTL to CMOS possible
• When CMOS uses VCC > 5 V (Usually 4000 or 74C series< use level shifter buffer chip 40109, LTC1045, 14504 or use open collector buffer with pullup to 5 V.

Common Precautions

• Noisy supply and ground lines can cause troubles that take hours to find.
• Make VCC wire very large so that current surges don't cause much voltage drop.
• Make a large bus wire, don't daisy chain VCC or ground lines.
• Be very careful with grounding. Typical precautions are:
  • Ground all devices at one single point.
  • Use a ground plane.
  • One side of pc board a solid conductor for gnd connections.
• Both VCC and ground lines should be wide traces or #14 or #16 wire to minimize both inductance and resistance.