74LVC16245 bus transceiver. As a logic level shifter ? - Page 1
Forum section: electronics / projects

author: carl0s
author_rank: Unknown
timestamp: July 25, 2018, 05:50:02 pm
content: Hi.One of my projects involves using an RGB to MIPI bridge (SSD2828) chip. Before I actually have a go (I'm getting there.. with all the LCD connecter prototype/FPC board adapters etc.), I came across an existing PCB and Schematic layout.It looks good. However this person seems to be using a 74LVC16245 bus transceiver across the RGB input (actually every input signal from the source microcontroller board). I've put that part of the schematic below.Is this just a simple/clever way ofshifting fromallowing for 5v to 3.3v transistion?I was also a bit confused, because I don't know how these bus tranceivers work or how they should be wired up, but it sort of looked like 1A1 should go to 1B1 etc, but this person has inverted all that (they have RED7 going to R0, and RED6 going to R1, etc.). In hindsight, I am presuming this is because whatever ARM source board he had in mind was LSB and the SSD2828 wants MSB on the RGB pins, or vice-versa. Or is there something else I should know ?Other designs that use the SSD2828 don't use this bus transceiver, but then those aren't being designed to be multi-MCU/logic-level safe.What do you think? Does this transceiver literally just pass everything from A side through to B side, and reduce the voltage to VDDIO ? For as long as 1DIR is high or whatever? (which looks to be permanently). Actually it can't be permanently because the SPI will need two-way comms. So does it just links both sides (I now notice that 2DIR is also high...), and deal with the logic level differences automatically? I thought logic level converters were more complicated than a single chip solution like this. I know they have functional diagrams in the datasheet showing lots of diodes and some other things but I'm not at a stage where I can really understand that too clearly.Here's the relevant part of the schematic:

author: carl0s
author_rank: Unknown
timestamp: July 25, 2018, 06:01:23 pm
content: ooh just found thishttps://www.adafruit.com/product/735

author: Sceptre
author_rank: Regular Contributor
timestamp: July 28, 2018, 05:05:52 am
content: Quote from: carl0s on July 25, 2018, 05:50:02 pmYes. From Page 1 of the datasheet: Operates From 1.65 V to 3.6 V, Inputs Accept Voltages to 5.5 V.http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=SN74LVC16245A&fileType=pdfQuoteYes, given that DIR is tied high and OE low.QuoteActually, all of the SPI signals on the transceiver are SSD2828 inputs; the output pin would be SDO, connected to the signal (probably) SPI_MISO (below the bottom edge of your picture). SPI_MISO doesn't pass through the transceivers.QuoteNot for unidirectional signals. If you're flipping the direction of the transceiver AND level-shifting, then you need a part with separate supplies for A and B, e.g.:http://www.ti.com/product/SN74ALVC164245http://www.ti.com/product/SN74AVC16T245

author: carl0s
author_rank: Unknown
timestamp: July 28, 2018, 02:53:28 pm
content: Thanks very much! and yes, I see now on the schematic, MISO goes direct. All the rest go into the transceiver with 1 appended added to their netname/label. (SPI_MOSI1, SPI_CS1, etc.).I still struggle with these datasheets though. It's not immediately obvious to me from their description that it's simply a level shifter.