• Mixed signal design
• “Defensive” electronics
• PCBs and Power
• Testing

Mixed signal design of circuit card involved three steps as below:

Motors

• Big, variable current
• Varying impedance
• Inductive
– Back EMF
– Spikes & Transients
– Dissipation
• RF Interference
• Stall protection
• Thermal

Logic

• Low current (~mA)
• Extreme supply sensitivity
• Low drive capability
• RF Interference
– Ground Bounce
– Fast edges
– Stray inductance
• Static sensitivity
• Pin density

Analogue

• Small signals
– μA
– mV
• Low power drive capability-Don’t use a battery as a reference voltage!
• Supply sensitivity
• Sensitive to interference-Screen / Placement
• Split supply (?)

Motors are nasty:

Defensive Electronics (Motors) (1)

Inductance + Intermittent contact…..

If contact breaks
Current drops to zero instantly
Voltage is infinite!

Something will break down (Air) Generates big voltage spikes / RF (Interference) • Control voltage , not current • Provide "escape route" for spikes • Provide local power filtering (to protect driver)

Defensive Electronics (Motors) 2

Defensive Electronics (Logic) (1)
• Design the regulator / supply carefully
– Read the data sheet
– Use recommended values e.g. capacitors
– Use a decent heatsink
• Think about fault conditions / turning
On/Off
– Short circuit to ground
– Short circuit to battery supply
– Reverse power connection (Murphy!)...
• Think about "Airbags"
– Fuse? Crowbar? Undervolt alarm?