Freescale Semiconductor MPC830x PowerQUICC II Pro Processor Evaluation Kit MPC8309-KIT MPC8309-KIT Fiche De Données

Freescale Semiconductor

75

 

to minimize inductance. Suggested bulk capacitors—100 to 330 µF (AVX TPS tantalum or Sanyo 
OSCON).

For all buses, the driver is a push-pull single-ended driver type (open drain for I

2

C).

To measure Z

0

 for the single-ended drivers, an external resistor is connected from the chip pad to OV

DD

 

or GND. Then, the value of each resistor is varied until the pad voltage is OV

DD

/2 (see 

). The 

output impedance is the average of two components, the resistances of the pull-up and pull-down devices. 
When data is held high, SW1 is closed (SW2 is open) and R

P

 is trimmed until the voltage at the pad equals 

OV

DD

/2. R

P

 then becomes the resistance of the pull-up devices. R

P

 and R

N

 are designed to be close to each 

other in value. Then, Z

The value of this resistance and the strength of the driver’s current source can be found by making two 
measurements. First, the output voltage is measured while driving logic 1 without an external differential 
termination resistor. The measured voltage is V

1

= R

source

× I

source

. Second, the output voltage is measured 

while driving logic 1 with an external precision differential termination resistor of value R

term

. The 

measured voltage is V

2

= (1/(1/R

1

+ 1/R

2

)) 

× I

source

. Solving for the output impedance gives 

R

source

= R

term

× (V

1

/V

2

– 1). The drive current is then I

source

= V

1

/R

source

.

OV

DD

OGND

R

P

R

N

Pad

Data

SW1

SW2