MC7457RX1267LC Datasheet, PDF (56/71 Pages) Freescale Semiconductor, Inc – Microprocessor

System Design Information
Table 21. Impedance Characteristics
V_DD = 1.5 V, OV_DD = 1.8 V ± 5%, T_j = 5°–85°C
Impedance
Processor Bus
L3 Bus
Unit
Z₀ Typical
Maximum
33–42
31–51
34–42
Ω
32–44
Ω
9.6 Pull-Up/Pull-Down Resistor Requirements
The MPC7457 requires high-resistive (weak: 4.7-kΩ) 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 MPC7457 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 OV_DD or down to GND to ensure
proper device operation. For the MPC7447, 360 BGA, the pins that must be pulled up to OV_DD are:
LSSD_MODE and TEST[0:3]; the pins that must be pulled down to GND are: L1_TSTCLK and TEST[4].
For the MPC7457, 483 BGA, the pins that must be pulled up to OV_DD are: LSSD_MODE and TEST[0:5];
the pins that must be pulled down are: L1_TSTCLK and TEST[6]. The CKSTP_IN signal should likewise
be pulled up through a pull-up resistor (weak or stronger: 4.7–1 kΩ) to prevent erroneous assertions of this
signal.
In addition, the MPC7457 has one open-drain style output that requires a pull-up resistor (weak or
stronger: 4.7–1 kΩ) if it is used by the system. This pin is CKSTP_OUT.
If pull-down resistors are used to configure BVSEL or L3VSEL, the resistors should be less than 250 Ω
(see Table 16). Because PLL_CFG[0:4] must remain stable during normal operation, strong pull-up and
pull-down resistors (1 kΩ or less) are recommended to configure these signals in order to protect against
erroneous switching due to ground bounce, power supply noise or noise coupling.
During inactive periods on the bus, the address and transfer attributes may not be driven by any master and
may, therefore, float in the high-impedance state for relatively long periods of time. Because the MPC7457
must continually monitor these signals for snooping, this float condition may cause excessive power draw
by the input receivers on the MPC7457 or by other receivers in the system. These signals can be pulled up
through weak (10-kΩ) pull-up resistors by the system, address bus driven mode enabled (see the MPC7450
RISC Microprocessor Family Users’ Manual for more information about this mode), or they may be
otherwise driven by the system during inactive periods of the bus to avoid this additional power draw.
Preliminary studies have shown the additional power draw by the MPC7457 input receivers to be
negligible and, in any event, none of these measures are necessary for proper device operation. The
snooped address and transfer attribute inputs are: A[0:35], AP[0:4], TT[0:4], CI, WT, and GBL.
If extended addressing is not used, A[0:3] are unused and must be pulled low to GND through weak
pull-down resistors. If the MPC7457 is in 60x bus mode, DTI[0:3] must be pulled low to GND through
weak pull-down resistors.
The data bus input receivers are normally turned off when no read operation is in progress and, therefore,
do not require pull-up resistors on the bus. Other data bus receivers in the system, however, may require
pull-ups, or that those signals be otherwise driven by the system during inactive periods by the system. The
data bus signals are: D[0:63] and DP[0:7].
MPC7457 RISC Microprocessor Hardware Specifications, Rev. 8
56
Freescale Semiconductor