PowerPC 405GP Embedded Processor Data Sheet
that the use of these pins for strapping is not considered multiplexing since the strapping function is not
programmable.
Pull-Up and Pull-Down Resistors
Pull-up and pull-down resistors are used for strapping during reset and to retain unused or undriven inputs in
an appropriate state. The recommended pull-up value of 3kΩ to +3.3V (10kΩ to +5V can be used on 5V
tolerant I/Os) and pull-down value of 1kΩ to GND, applies only to individually terminated signals. To prevent
possible damage to the device, I/Os capable of becoming outputs must never be tied together and terminated
through a common resistor.
If your system-level test methodology permits, input-only signals can be connected together and terminated
through either a common resistor or directly to +3.3V or GND. When a resistor is used, its value must ensure
that the grouped I/Os reach a valid logic zero or logic one state when accounting for the total input current into
the PPC405GP.
Unused I/Os
Termination of some pins may be necessary when they are unused. Although the PPC405GP requires only
the pull-up and pull-down terminations as specified in the “Signal Functional Description” on page 34, good
design practice is to terminate all unused inputs or to configure I/Os such that they always drive. If unused,
the peripheral, SDRAM, and PCI buses should be configured and terminated as follows:
• Peripheral interface—PerAddr0:31, PerData0:31, and all of the control signals are driven by default.
Terminate PerReady high and PerError low.
• SDRAM—Program SDRAM0_CFG[EMDULR]=1 and SDRAM0_CFG[DCE]=1. This causes the
PPC405GP to actively drive all of the SDRAM address, data, and control signals.
• PCI—The PCI pull-up requirements given in the Signal Functional Description apply only when the PCI
interface is being used. When the PCI bridge is unused, configure the PCI controller to park on the bus
and actively drive PCIAD31:0, PCIC3:0[BE3:0], and the remaining PCI control signals by doing the
following:
- Strap the PPC405GP to disable the internal PCI arbiter and to operate the PCI interface in
synchronous mode.
- Individually connect PCISErr, PCIPErr, PCITRDY, and PCIStop through 3kΩ resistors to +3.3V.
- Terminate PCIReq1:5 to +3.3V.
- Terminate PCIReq0[Gnt] to GND.
External Bus Control Signals
All peripheral bus control signals (PerCS0:7, PerR/W, PerWBE0:3, PerOE, PerWE, PerBLast, HoldAck,
ExtAck) are set to the high-impedance state when ExtReset=0. In addition, as detailed in the PowerPC
405GP Embedded Processor User’s Manual, the peripheral bus controller can be programmed via
EBC0_CFG to float some of these control signals between transactions and/or when an external master
owns the peripheral bus. As a result, a pull-up resistor should be added to those control signals where an
undriven state may affect any devices receiving that particular signal.
The following table lists all of the I/O signals provided by the PPC405GP. Please refer to “Signals Listed
Alphabetically” on page 16 for the pin number to which each signal is assigned.
Page 33 of 60
6/20/03
IBM [ IBM ]
