PCI1251B Datasheet, PDF (23/126 Pages) Texas Instruments – PC CARD CONTROLLER

PCI1251B GFN/GJG
PC CARD CONTROLLER
SCPS043A – OCTOBER 1998
power supply sequencing
The PCI1251B contains 3.3-V I/O buffers with 5-V tolerance requiring a core power supply and clamping
voltage. The core power supply is always 3.3 V. The clamping voltage can be either 3.3 V or 5 V, depending
on the interface. The following power-up and power-down sequences are recommended.
The power-up sequence is:
1. Apply 3.3-V power to the core.
2. Assert PRST to the device to disable the outputs during power up. Output drivers must be powered up in
the high-impedance state to prevent high current levels through the clamping diodes to the 5-V supply.
3. Apply the clamping voltage.
The power-down sequence is:
1. Use PRST to switch outputs to a high-impedance state.
2. Remove the clamping voltage.
3. Remove the 3.3-V power from the core.
I/O characteristics
Figure 1 shows a 3-state bidirectional buffer. The recommended operating conditions table, on page 115,
provides the electrical characteristics of the inputs and outputs. The PCI1251B meets the ac specifications of
the 1997 PC Card Standard and PCI Local Bus Specification Rev. 2.2.
Tied for Open Drain
OE
VCCP
Pad
Figure 1. 3-State Bidirectional Buffer
NOTE:
Unused pins (input or I/O) must be held high or low to prevent them from floating.
clamping voltages
The clamping voltages are set to match whatever external environment the PCI1251B will be working with: 3.3 V
or 5 V. The I/O sites can be pulled through a clamping diode to a voltage that protects the core from external
signals. The core power supply is always 3.3 V and is independent of the clamping voltages. For example, PCI
signaling can be either 3.3 V or 5 V, and the PCI1251B must reliably accommodate both voltage levels. This
is accomplished by using a 3.3-V I/O buffer that is 5-V tolerant, with the applicable clamping voltage applied.
If a system designer desires a 5-V PCI bus, then V_CCP can be connected to a 5-V power supply.
The PCI1251B requires five separate clamping voltages because it supports a wide range of features. The five
voltages are listed and defined in the recommended operating conditions table, on page 115.
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