[En-Nut-Discussion] Ethernet HW questions

Harald Kipp harald.kipp at egnite.de
Tue Aug 3 10:08:41 CEST 2004

Hi Henrik,

your post was held for approval, because the sender's
email wasn't subscribed at
No problem, but I do not check the requests every day.

At 11:52 02.08.2004 +1000, you wrote:

>A) Power-on reset generation
>I like to remove the DS1811 and replace it with a simple RC
>combination like many other AVR designs do. Isn't the DS1811 anyway
>redundant because of the ATmega's internal brown-out detection?
>B) RTL8019AS reset pin
>I like to connect the reset to an AVR port to save the need for an
>inverter gate.

Right, the ATmega128 doesn't require a reset controller,
but the RTL8019AS does. Using a port bit is the most
simple solution and is even supported by the standard
Nut/OS driver.

>C) RTL8091AS interrupt pin
>This pin can be connected directly to the ATmega128 to save an
>inverter gate. I assume the inverter was only necessary for the older
>ATmega103 chip which didn't have programable interrupt levels.

Yes, but the gate is still on Ethernut 1.3F (and will be on Rev-G)
for software compatibility. A few of egnite's long term customers
still use ATmega103 firmware and don't want to change anything.

>D) RTL8091AS half-duplex mode
>I also like to connect pin 77/EEDO/BD7 to ground to force the RT into
>half-duplex mode. This was suggested by RealTek's support. In this
>case LED_BNC has to be used to indicate the Link status as the LED
>configuration cannot be changed by the cfg registers. I have seen the
>LED_BNC solution in some other reference designs supplied by RealTek
>and also tested it using the PacketWhacker.
>When grounding pin 77/EEDO/BD7, the JP pin 65 can then be left open
>so the RealTek operates in "RT jumperless mode".

Never tested this. Ethernut 1.3G will use A13 and A14 to mimic
the EEPROM. This enables the software to select the duplex
mode and costs one serial resistor at BD7 only. The software
is a bit tricky and becomes simpler, if you use real port
bits instead of address lines.

>E) Realtek 270R pulldown resistors on pin 48,49
>Aren't these rsistors obsolete because AUI is not used?

Should be. But, if you got PCB space left, I'd still
add the pads. You may later decide about stuffing them
or not.

>F) /RD, /WR, ALE signals
>These signals are floating in reset or if the ports are not
>configured. Shouldn't they have pull-up resistors?

Do you see any harm during reset? As long as the memory
interface is enabled, they are configured as outputs.

>I won't need analogues on the board. What's the best way to deal with
>these supply signals? Connect them to VCC and GND or leave them
>simply open?

Under no circumstances I'd leave them open. Some internal
parts  may draw power from somewhere else. If you do not
use analog I/O, you can connect them to VCC/GND.

>H) RTL8091 address signals
>I like to use only the A0 to A4 and A8 (connected to ATmegas A15 as
>chip select) address signals on the RealTek. A9 is pulled high all
>other address signals low. This results in the RealTek occupying the
>whole upper 32K address space.
>Doing this I need less tracks to route and I save the inverter gate
>for A15.

Yes, if you do not use any RAM or memory mapped I/O you
can simplify the board's routing by not using the Realteks
internal address decoder.



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