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Tektronix TDS744 CRT to LCD color converter FPGA module DIY

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TerrySt:
It looks like the Spartan 6 supports LVDS_33 and LVDS_25.  But the Spartan 7 supports LVDS_18 and LVDS_25, but not LVDS_33.  So the Spartan 7 dev boards that have 3.3V applied to VCCO (all of them that I found) cannot support LVDS.

Terry

ltarjanyi75:
I have checked the Spartan 3 and 6 datasheet. LVDS_33 and LVDS_25 use the same DC charasteristics of user I/Os (e.g. same output common mode voltage). The difference is the necessary output driver supply voltage level, VCCO (2.5V for LVDS_25 and 3.3 for LVDS_33).

So I think Terry, you could also use directly the LVDS I/Os of your Spartan 7 devboard (I assume it works the same way, though I have not checked the Spartan 7 documentation).

ltarjanyi75:

--- Quote from: TerrySt on December 11, 2023, 08:15:45 pm ---It looks like the Spartan 6 supports LVDS_33 and LVDS_25.  But the Spartan 7 supports LVDS_18 and LVDS_25, but not LVDS_33.  So the Spartan 7 dev boards that have 3.3V applied to VCCO (all of them that I found) cannot support LVDS.

--- End quote ---

I see, then it is different for Spartan 7...

Tantratron:
Just found that article regarding the 1.2V center offset, the topic of LVDS_25 LVDS_33 and other
https://www.diodes.com/assets/App-Note-Files/AN041-P.pdf

Note that sentence from page 1... It has an offset voltage of 1.2V above ground. Because its operating voltage is centered around 1.2V with respect to the driver􏰀s ground, LVDS does not depend on a specific power supply such as 5V or 3.3V making it easy for LVDS to migrate to new low supply voltage technology. Since it is centered around 1.2V, it is also less susceptible to noise since noise often occurs at Vcc or Ground.

TerrySt:

--- Quote from: Tantratron on December 12, 2023, 08:23:28 am ---Just found that article regarding the 1.2V center offset, the topic of LVDS_25 LVDS_33 and other
https://www.diodes.com/assets/App-Note-Files/AN041-P.pdf

Note that sentence from page 1... It has an offset voltage of 1.2V above ground. Because its operating voltage is centered around 1.2V with respect to the driver􏰀s ground, LVDS does not depend on a specific power supply such as 5V or 3.3V making it easy for LVDS to migrate to new low supply voltage technology. Since it is centered around 1.2V, it is also less susceptible to noise since noise often occurs at Vcc or Ground.

--- End quote ---

But unfortunately the Spartan 7 will not allow me to use the LVDS outputs unless the bank voltage (VCCO) is set to 2.5V.
From the I/O User Guide:  https://docs.xilinx.com/v/u/en-US/ug471_7Series_SelectIO   Page 91.

The LVDS I/O standard is only available in the HP I/O banks. It requires a VCCO to be
powered at 1.8V for outputs and for inputs when the optional internal differential
termination is implemented (DIFF_TERM = TRUE).
The LVDS_25 I/O standard is only available in the HR I/O banks. It requires a VCCO to be
powered at 2.5V for outputs and for inputs

The Spartan 3 and Spartan 6 allow LVDS on Banks powered by 3.3V, but the Spartan 7 does not.
The Spartan 3 would be a better choice for this project, but I have decided to use the Spartan 7 mainly because the new version of the tools (Vivado) does not support the older chips.  I am doing this mainly as a learning exercise, so want to use the latest tools.  Also, I want to try my hand at working with BGA parts (layout and assembly).  I don't know if I'll be successful, but this is a hobby for me now (I'm retired).  If I was doing this to make a product for the marketplace, I would probably use the Spartan 3N, which is non-volatile, so doesn't need the serial EEPROM for configuration.

Terry

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