Digital TIPS and TIDBITS

TTL vs CMOS

The 74LS series TTL chips are very useful for most applications:

• Fast, ~10 ns gate delay
  
• Fairly low power consumption, ~1 mW/gate
  
• Widely established standard
  

The 74HC CMOS series eliminates many old objections to CMOS

• Slightly Faster than 74LS TTL ~< 10 ns
  
• much better than older 4000 and 74C chips
  
• Power consumption less than LS TTL at 1 MHz
  
• but power exceeds LS as frequency increases above 1 MHz
  
• More robust against abuse (static electricity etc)
  
• Logic thresholds not consistent with standard TTL
  
• Can use HCT chips which have TTL logic thresholds
  
• Can Run HC CMOS at 3.3V supply for level compatibility, 40% slower
  
• 4000 and 74C CMOS allow wide range for supply voltage: 3 V ¾ V_CC ¾ 13 V
  -->performance better at higher voltages, 9V recommemded
  

Interfacing TTL and CMOS

CMOS to TTL

• If V_CC 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 V_CC ~3.3 V, thresholds are still compatible with TTL
  
• Sometimes 4000 series or 74C chips are run at V_CC > 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 V_CC = 5 V
  
• Use a 74HCT buffer between them
  
• Use an open collector buffer with pullup to 5 V
  
• When CMOS uses V_CC = 3.3 V (Usually 74HC only)
  
• Direct connection from TTL to CMOS possible
  
• When CMOS uses V_CC > 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

• Make V_CC wire very large so that current surges don't cause much voltage drop
  
• make a large bus wire, don't daisy chain V_CC 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 V_CC and ground lines should be wide traces or #14 or #16 wire to minimize both inductance and resistance
  

Quirks with TTL

• Draws a lot of current when switching. Put despiking capacitors between VSIZE=2>_CC and gnd on every 2nd or 3rd chip to supply large current surges momentarily
  
• 0.01 µF to 0.1µF, ceramic
  
• 1 to 47 µF tantalum cap between whereV_CC and gnd comes on board.
  
• Noise immunity to low level is very bad with TTL
  -->always check for noise on gnd
  

Quirks with CMOS

• Input FET very easy to ruin
  
• Be very careful of static charge
  
• Discharge yourself before touching
  
• store and transport in conductive foam or pouch
  
• never put belly up
  
• Never plug in or unplug them with supply on
  
• Will "latch up" if an input is driven above supply V_CC
  
• V_CC shorts to ground, chip gets hot, pretty soon it's belly-up
  
• Unconnected inputs are indeterminate
  
• Connect all inputs of all gates on a chip, even if the gates aren't used.
  
• Failure to do this could make both complementary FETS conducting
  
• Draws a lot of current messing up other gates and chips
  
• If you forget to connect V_CC, or gnd, chip will still work as long as at least one pin is at V_CC or gnd.
  
• Unconnected inputs and unconnected supply lines can cause intermittent and unreproducible failures that drive you up a wall.
  

Driving External devices:

CMOS and TTL are designed to sink more current than source

• Put any device that draws current between V_CC and the output
  
• Use a current limiting resistor for LEDs (220‡ to 1k)
  
• For 5 V relays, always use diode to protect against inductive spike
  
• If you need more current (>10 mA) use gate output to drive a transistor
  
• If large transients are apt to come from a device, use optoisolator
  Open collector or drain outputs can be used for nonstandard voltages
  

Sending signals over a distance

• use an output buffer on sender and input Schmitt trigger on receiver
  
• Open collector or special line driver senders ensure clean levels
  
• Terminate line just before Schmitt trigger
  
• eliminates reflections
  
• 180‡ to 5V, 390 ‡ to gnd is standard (H&H Fig 9.32)
  
• Twisted pair line with differential input (H&H Fig. 9.34)
  
• Higher voltage signals using twisted pair (RS232)
  RS422 combines differential signal and higher signal voltages
  

Current sinking drivers

Coaxial cable

• good interference immunity
  
• data rates up to 100 kb/s over 1 mile (1.6 km)
  
• well standardized
  See H&H Fig 9.40,41,42 for examples
  

Fibre optic cable

• Standardized senders, receivers and cables available
  
• Cheap
  
• infrared LED sender ($1)
  
• phototransitor receiver ($1)
  
• 1mm plastic step-index cable (cheap)
  1Mb/s over 30 ft
  

Better system

• 200µm glass step index fibre
  
• detectors have builtin amps
  5Mb/s over 1 km
  

Current record (old?)