Connecting 2.8V LCD to 3.3V MCU - Page 1
Forum section: electronics / beginners

author: Kaisha
author_rank: Contributor
timestamp: August 06, 2021, 10:15:41 am
content: I'm looking at connecting a 2.8V LCD (https://www.buydisplay.com/download/manual/ER-TFT032A3-3_Datasheet.pdf) to an ESP32 (https://www.espressif.com/sites/default/files/documentation/esp32_datasheet_en.pdf). The voltages are different, but still overlap, so I'm wondering what the proper approach is. Should I:1) Run both the ESP32 and LCD at 3.3v.2) Run both the ESP32 and LCD at 3.0v (I don't think I can pull the ESP32 lower than 3.0v without messing with the flash).3) Run the ESP32 at 3.3v, the LCD at 2.8V, and just connect them directly.4) Use logic level shifters to interface the two.Any other thoughts/suggestions?

author: Ian.M
author_rank: Super Contributor
timestamp: August 06, 2021, 10:29:26 am
content: Plot the acceptable Vcc ranges fornormal operation*of all chips in the system side by side as a bar graph. Look at the voltage range common to all (if any). If you want to avoid level shifters you need to find a regulator voltage within the common range with enough margin either side of it for the total regulator tolerance (including that of any external feedback divider and any change with Vin and with load current). If you can't do that because there isn't enough (or any) common range, use level shifters to deal with the chip with the furthest outlying operating voltage range.Rinse and repeat till all constraints are satisfied with adequate margin.* Including special operations like FLASH bulk erase! A cautionary tale: Circa 2010, the low voltage LF variant of many Microchip 8 bit PICs could run or be programmed at as low as 2.0V, but required 4.5V min to bulk erase the FLASH. If you put them on the same Vcc/Vcc bus as chips with under 4.5V abs. max Vcc, short of cutting tracks to isolate the PIC Vdd, your target board would be effectively OTP, as any attempt to erase the PIC for reprogramming would blow the other chips!

author: Kaisha
author_rank: Contributor
timestamp: August 06, 2021, 11:15:21 am
content: There is overlap between the ESP32 and the LCD. The LCD's upper limit of it's recommended range is 3.3V. But I'm kinda a beginner so I'm not sure if pushing the upper limit is safe. But running them both at 3.3V is by far the easiest.The ESP datasheet says it can go as low as 3.0V, but again that's a lower limit, so I'm not sure how 'safe' it is (they mention flash issues, but then later state that 2.3V should be used as the lower limit... so I'm kinda confused as to what they mean).

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2021, 11:30:22 am
content: Relax, the absolute maximum voltage rating for the LCD is 4.3V, so it will reliably work from 3.3V, which is the maximum recommended operating voltage.The backlight is just LEDs, so will require a constant current, rather than constant voltage supply. Do you have a higher voltage than 3.3V available? If not, then you should use a 1 Ohm resistor, just to be sure the current is limited.

author: Kaisha
author_rank: Contributor
timestamp: August 06, 2021, 11:48:51 am
content: QuoteFair enough, I just don't have enough experience to know how strict these limits are. Would it make sense to just run the whole thing at, say, 3.15V or something (split the difference)? Or am I still over thinking thisQuoteI was planning on using a boost converter for the backlight, something like: TPS61040. Can I run it straight off a 3.3V? Wouldn't I need something like a 41 Ohm resistor?

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2021, 12:24:48 pm
content: Quote from: Kaisha on August 06, 2021, 11:48:51 amThe electrical characteristics are specified over a power supply voltage range of 2.5V to 3.3V. The absolute maximum rating is 0.3V to 4.6V, which it should survive for short periods, without releasing magic smoke, but there's no guarantee it'll work properly.In short, run it off 3.3V, it'll be fine.QuoteI wouldn't recommend powering the backlight directly off 3.3V, because it's very close to the forward voltage of the LEDs. The TPS61040 is a decent enough solution. The circuit on the data sheet, figure 21 will work quite well, with a different value of RSto set the correct forward current.

author: Kaisha
author_rank: Contributor
timestamp: August 06, 2021, 12:29:57 pm
content: I appreciate the help