MPC7457 datasheet(58/71 Pages) FREESCALE

58 / 71 page

MPC7457 RISC Microprocessor Hardware Specifications, Rev. 8

System Design Information

Freescale Semiconductor

The QACK signal shown in Figure 26 is usually connected to the PCI bridge chip in a system and is an

input to the MPC7457 informing it that it can go into the quiescent state. Under normal operation this

occurs during a low-power mode selection. In order for COP to work, the MPC7457 must see this signal

asserted (pulled down). While shown on the COP header, not all emulator products drive this signal. If the

product does not, a pull-down resistor can be populated to assert this signal. Additionally, some emulator

products implement open-drain type outputs and can only drive QACK asserted; for these tools, a pull-up

resistor can be implemented to ensure this signal is deasserted when it is not being driven by the tool. Note

that the pull-up and pull-down resistors on the QACK signal are mutually exclusive and it is never

necessary to populate both in a system. To preserve correct power-down operation, QACK should be

merged via logic so that it also can be driven by the PCI bridge.

MPC7457 Datasheet(PDF) 58 Page - Freescale Semiconductor, Inc