Texas Instruments THUNDER TNETX3270 User Manual

Page 38

TNETX3270
ThunderSWITCH



24/3 ETHERNET



SWITCH

WITH 24 10-MBIT/S PORTS AND 3 10-/100-MBIT/S PORTS

SPWS043B – NOVEMBER 1997 – REVISED APRIL 1999

POST OFFICE BOX 655303

•

DALLAS, TEXAS 75265

SDRAM-type and quantity indication

Before beginning operation (by writing to the start bit of Syscontrol), it is necessary to indicate to the SDRAM
interface whether

8 or

16 SDRAMs are being used. This is done by setting the bit in the RAMsize register

(by 8 = 0 for

16, by 8 = 1 for

8) during the load from EEPROM or via a DIO write. It also is necessary to inform

the SDRAM interface of the quantity of external SDRAM available. This is done by writing to the RAMsize
register, while at the same time setting the

8 or

16 SDRAMs.

initialization

SDRAMs require controlled initialization. Specifically, SDRAMs require up to 200

s of inactivity after power has

been supplied, during which they are supplied only with an active CLK. The system designer must ensure that
this inactivity period is observed while TNETX3270 is held in hardware or software reset.

Table 17 shows the state of the SDRAM interface terminals during hardware or software reset.

Table 17. SDRAM Interface Terminal State During Hardware or Software Reset

TNETX3270

TERMINAL

STATE

DURING RESET

DA13–DA00

Driven high

DRAS

Driven high

DCAS

Driven high

DCLK

Active

DD31–DD00

High impedance

Any other SDRAM requirements during this period that need to be observed, such as the state of chip selects
and clock-enable and data-mask controls, also are the responsibility of the system designer. This SDRAM
interface does not drive the DD bus during hardware or software reset, or following either reset, until the SDRAM
initialization process has been completed.

refresh

After the initialization process, the SDRAM interface then performs 4096 REFR commands at least every 64 ms.
SDRAM data is, however, lost during any subsequent resets, as the SDRAM interface does not issue any REFR
commands during any hardware or software reset.