MPC8308 datasheet(84/88 Pages) FREESCALE

84 / 88 page

MPC8308 PowerQUICC II Pro Processor Hardware Specification, Rev. 1

Freescale Semiconductor

System Design Information

Figure 55. Driver Impedance Measurement

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 V1 = Rsource × Isource. Second, the output voltage is measured

while driving logic 1 with an external precision differential termination resistor of value Rterm. The

measured voltage is V2 =(1/(1/R1 +1/R2)) × Isource. Solving for the output impedance gives Rsource =

Rterm × (V1/V2 – 1). The drive current is then Isource =V1/Rsource.

Table 60 summarizes the signal impedance targets. The driver impedance are targeted at minimum VDD,

nominal NVDD, 105°C.

23.6

Configuration Pin Muxing

The device provides the user with power-on configuration options that can be set through the use of

external pull-up or pull-down resistors of 4.7 K

Ω on certain output pins (see customer-visible

configuration pins). These pins are generally used as output-only pins in normal operation.

While PORESET is asserted, these pins are treated as inputs. The value presented on these pins while

PORESET is asserted is latched when PORESET deasserts, at which time the input receiver is disabled

and the I/O circuit takes on its normal function. Careful board layout with stubless connections to these

pull-up/pull-down resistors coupled with the large value of the pull-up/pull-down resistor should minimize

the disruption of signal quality or speed for output pins thus configured.

Table 60. Impedance Characteristics

Impedance

Local Bus, Ethernet, DUART, Control,

Configuration, Power Management

DDR DRAM

Symbol

Unit

42 Target

20 Target

42 Target

20 Target

Note: Nominal supply voltages. See Table 2, Tj = 105°C.

NVDD

VSS

Pad

Data

SW1

SW2

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