82551QM Datasheet, PDF (50/120 Pages) Intel Corporation – Fast Ethernet Multifunction PCI/CardBus Controller

82551QM — Networking Silicon
7.3.1
Programming Details
For designs that use both Flash and modem devices, the 82551QM supports the coexistence of
BootROM accesses (for Preboot eXtension Environment [PXE] code) and modem:
1. Set the EEPROM’s MDM bit.
2. Clear the BD bit in the EEPROM.
This enables both the modem and boot ROM. This allows the Boot Enable bit in the Expansion
BAR to select which external device (modem or Flash) is active on the local bus through the use of
the MDMCS# pin. After initialization, the 82551QM enables the Flash on the local bus (in other
words, the Boot Enable bit in the BAR equals 1b) and the modem is disabled. Following the
execution of the boot code from the Flash device, the enable bit is cleared, and the modem is
enabled. The clearing of the Boot Enable bit causes the MDMCS# pin to be asserted, enabling the
modem and Function 1 (Modem) Configuration space to be available.
7.3.2
Support Circuitry
An example of support circuitry is shown in Figure 14. When MDMCS# is low, the modem is
enabled; MDMCS# is high, the Flash device is enabled.
Figure 14. Support Circuitry Example
FLCS#
MDMCS#
(Modem_Mode#)
MODEM_CS#
FLSH_CS#
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Datasheet