MPC7450EC datasheet(40/52 Pages) MOTOROLA

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MPC7450 RISC Microprocessor Hardware Speciﬁcations

MOTOROLA

System Design Information

1.9.6

Output Buffer DC Impedance

The MPC7450 processor bus and L3 I/O drivers are characterized over process, voltage, and temperature.

To measure Z0, an external resistor is connected from the chip pad to OVDD or GND. Then, the value of

each resistor is varied until the pad voltage is OVDD/2 (see Figure 22).

The output impedance is the average of two components, the resistances of the pull-up and pull-down

devices. When data is held low, SW2 is closed (SW1 is open), and RN is trimmed until the voltage at the

pad equals OVDD/2. RN then becomes the resistance of the pull-down devices. When data is held high, SW1

is closed (SW2 is open), and RP is trimmed until the voltage at the pad equals OVDD/2. RP then becomes

the resistance of the pull-up devices. RP and RN are designed to be close to each other in value. Then, Z0 =

(RP + RN)/2.

Figure 22. Driver Impedance Measurement

Table 18 summarizes the signal impedance results. The impedance increases with junction temperature and

is relatively unaffected by bus voltage.

1.9.7

Pull-Up/Pull-Down Resistor Requirements

The MPC7450 requires high-resistive (weak: 4.7 k

Ω) pull-up resistors on several control pins of the bus

interface to maintain the control signals in the negated state after they have been actively negated and

released by the MPC7450 or other bus masters. These pins are: TS, ARTRY, SHDO, and SHD1.

Some pins designated as being for factory test must be pulled up to OVDD or down to GND to ensure proper

device operation. For the MPC7450, 483 BGA, the pins that must be pulled up to OVDD are: LSSD_MODE

and TEST[0:5]; the pins that must be pulled down are: L1_TSTCLK and TEST[6].

Table 18. Impedance Characteristics

VDD = 1.5 V, OVDD = 1.8 V ± 5%, Tj = 5°–85°C

Impedance

Processor Bus

L3 Bus

Unit

Typical

33–42

34–42

Ω

Maximum

31–51

32–44

Ω

OVDD

OGND

Pad

Data

SW1

SW2

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

IVE

D B

Y F

INC

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