dd-wrt forum post ---> https://forum.dd-wrt.com/phpBB2/viewtopic.php?t=317084

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#### Using USB-TTL, GND, RX, TX pins only ####

assuming you are using Windows system if not then you should be able to figure it all out :-)
Have TFTPD32 or TFTPD64 setup and ready to send the 'code.bin' file
Have putty.exe open ---
power on the WRT160NL

Hit any key to stop autoboot ---
When you get the prompt:
ar7100>
type "upgrade code.bin" and hit enter

be patient wait till it completely does its thing

SEE ---> 'serial-log.txt' this directory.

If have problems and you have not allowed the apps mentioned firewall access 
then you should disable all firewall on your computer.

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The 'code.bin' will install Linksys FW 1.0.0.4 build 2 Jun 19, 2013
pic is 'linksysFW.png' in this directory

If you cannot login to the Linksys firmware after it has 
successfully loaded then do a button reset

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to get back to dd-wrt firmware do a GUI install from Linksys firmware
use the 'linksys-to-ddwrt-firmware.bin' from this directory.
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you can then install the r34311_wrt160nl-firmware.bin if you want.
NOTE: if your WRT160NL has a '->Oops: flash id 0xc22017 .' you cannot install any later build.

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If you WRT160NL has '->Oops: flash id 0x1c2017 .' then you can use later builds.
I am currently using:
Linksys WRT160NL
DD-WRT v3.0-r37375 std (10/14/18)
Linux 3.10.108-d6 #65815 Sun Oct 14 00:09:57 UTC 2018 mips

but I only have the one unit with '->Oops: flash id 0x1c2017 .'
You can check this by running command:
dmesg | grep flash