Help converting ftdi ft232rl uart to serial down to 1.8v - Page 1
Forum section: electronics / general

author: bigjoncoop
author_rank: Regular Contributor
timestamp: August 24, 2023, 01:54:11 am
content: Hey guys I have a bunch of ftdi uart to serial boards and they have on board options for 5 volt and 3.3 volt.I'm trying to flash a device but it has a maximum of 1.8 volts signal. Will there be a simple way of converting the Bible or 3.3 volt signal from the ftdi board down to 1.8 volts without using a dedicated level shifter?The ftdi adapters I have are using the ftdi ft232rl chip. Which I believe are capable of 1.8 volt signals.*** actually now that I think about it, I know I can drop the 3.3 volt signal coming from the adapter with a resistor divider down to 1.8 volts and since the ftdi chip on these boards is capable of 1.8 volt signals it might just work. I just dropped the 3.3 volt TX from the board down to 1.8 volts and see if it just works.I understand I could buy a new uart to serial adapter or purchase a level shifter but I was hoping to do this without having to buy more stuff like always LOL it seems like no matter how much crap you have It's never enough. Thanks for your help

author: Nominal Animal
author_rank: Super Contributor
timestamp: August 24, 2023, 02:17:01 am
content: As you can see in thedatasheet, pin 4 = VCCIO is the UART (and CBUS) logic level supply on FT232RL (28-lead SSOP package of FT232R). Basically, you need to derive a 1.8V supply (capable of providing at least 50mA) from the USB 5V, and supply that to pin 4. Note: I suspect a simple 16R:9Rvoltage divider (5×9/(16+9)=1.8) willnotwork; the current draw will make the VCCIO vary (drop) too much when using a simple voltage divider. And SBCs and FPGAs really don't like exceeding that 1.8V by much, so you cannot compensate by letting the unloaded VCCIO voltage be much higher than the 1.8V. (On my SBCs, there is less than 0.1V of headroom.)If the end device uses USB power, and generates a 1.8V supply from it, you can connect that to the FT232R pin 4. The datasheet mentions that any 1.8V to 5.25V level works, but that 1.8V to 3.3V "should originate from the same supply as VCC", which would be USB 5V: the 3.3V linear regulator is built-in to the FT232RL, output on pin 17. It can only provide the aforementioned 50mA or so.If your adapter is one that has a three-pin jumper, where connecting the center pin to one side selects 5V, and center pin to the other side 3.3V, just omit the jumper and provide the 1.8V to that center pin.Me, I prefer to keep TI TXU0202 (UART), TXU0204 (UART+RTSCTS), and TXU0304 (SPI) in stock. Plus some 74LVC1T45 in SOT-23 for dead-bugging (in air, no board) in a pinch.

author: Ian.M
author_rank: Super Contributor
timestamp: August 24, 2023, 02:39:18 am
content: The phrase VCCIO "should originate from the same supply as VCC" means it should come up when VCC comes up, and go away when VCC does. This means you shouldn't use external power to feed VCCIO unless you switch it, controlled by VCC. One way to do that is to connect a low Vgs theshold N-MOSFET to the VCCIO selection header, Source to the center pin, Gate to the 5V pin and Drain to your target 1.8V supply. When USB power is present the gate will be approximately 3V positive of the 1.8V rail, turning the MOSFET on to power VCCIO. However, if you want this to work for any target voltage this simple arrangement wont work as the 5V supply wont always be far enough above the target voltage to provide enough gate drive, and you'll need a P-MOSFET, switched by a N-MOSFET, controlled by Vcc.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 24, 2023, 03:38:49 am
content: Yep.Also if you're considering a FTDI USB-UART chip, IMHO focus on the newer series (like 230X/231X) rather than the oldish 232RL. They are overall in more stock, are much more modern, have never been cloned (that I know of) and are usually cheaper, unless of course you happen to buy non-genuine FT232RL parts from a fishy reseller. Just my 2 cents.They also have a VCCIO input pin that allows I/O voltages from 1.62V to 3.63V according to the DS.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: August 24, 2023, 04:46:43 am
content: A generic solution to use LVC2T45, made for this purpose.

author: Ian.M
author_rank: Super Contributor
timestamp: August 24, 2023, 06:01:00 am
content: Yes, 74LVC2T45 is a great level shifter if you've got the luxury of laying out your own board. It has the further advantage its rated for partial power down applications - if one side is powered down the remaining powered side is guaranteed tristate - so there are no problems if either USB power or target power is lost. Also see 74LVC1T45 when you only need to level shift a single signal.

author: magic
author_rank: Super Contributor
timestamp: August 24, 2023, 06:26:37 am
content: Quote from: bigjoncoop on August 24, 2023, 01:54:11 amSurprisingly, yes, this should work.The answer is in the datasheet section "DC characteristics", table 5.4 "UART and CBUS I/O Pin Characteristics (VCCIO = +3.3V, Standard Drive Level)".It specifies that input pins switch between low and high level at a fixed threshold between 1V and 1.5V with 20~30mV hysteresis, so your 1.8V signal should drive them just fine.Most of the time though, CMOS chip inputs switch at approximately 50% of supply voltage (1.65V for 3.3V) and then operation would be very marginal, because the exact value of "approximately" may be as much as ±10% of VCC so your 1.8V signal could always register as low.Note that this may or may not work with fake FT232 chips which are omnipresent on auction sites, including ready PCBs sold there.I don't know if those clones replicate input behavior correctly, it's also possible that some of them do but others don't.