26-12-2020, 02:44 PM
(This post was last modified: 26-12-2020, 02:50 PM by ppppenguin.)
The whole business of 5V, 3.3V, and sometimes lower voltage, logic can be a real PITA. All current FPGAs are 3.3V max and won't withstand 5V on their inputs. Some of the more sophisticated ones may be even lower voltage. Because they have clamp diodes to 3.3V and 0V you can do the interface from 5V with just a series resistor to limit the current. Allowing for the clamp diode about 1K will work to limit the current to about 1mA. But only at low speed as the stray C will get you.
I've looked at the data sheet for the display module and logic HI is 0.7*VCC, or 3.5V when running at 5V. In practice they will work down to not much above 2.5V but there's no guarantee. If you're careful you could run the display at 4.8V, the minimum allowed and push the 3.3V rail a touch high to get guaranteed logic levels. All a bit tricky and marginal. Another wheeze would be to power the display from about +4.5V and --0.5V. This would just about give you guaranteed logic levels but you'd need to create a -ve supply.
If it's just I2C that needs to be interfaced you can get 5V<>3V3 I2C converter chips. Another trick is to pull SCLand SDA up to 5V. Most 3V3 parts won't mind this due to limited current and clamp diodes. Logic HI will get to about 4V which is good enough. This will also work if you simulate open collector outputs on a FPGA and use external pullups to +5V. That's done by tristating the output for logic HI.
The digital pots are amazingly cheap! 11p+VAT each from RS in the UK.
I like the look of those drop-in switchmode 7805 regs. What you can't do is put a diode in the 0V leg to jack the voltage up a bit. This is a trick I've used with ordinary 7805 etc on occasion.
I've looked at the data sheet for the display module and logic HI is 0.7*VCC, or 3.5V when running at 5V. In practice they will work down to not much above 2.5V but there's no guarantee. If you're careful you could run the display at 4.8V, the minimum allowed and push the 3.3V rail a touch high to get guaranteed logic levels. All a bit tricky and marginal. Another wheeze would be to power the display from about +4.5V and --0.5V. This would just about give you guaranteed logic levels but you'd need to create a -ve supply.
If it's just I2C that needs to be interfaced you can get 5V<>3V3 I2C converter chips. Another trick is to pull SCLand SDA up to 5V. Most 3V3 parts won't mind this due to limited current and clamp diodes. Logic HI will get to about 4V which is good enough. This will also work if you simulate open collector outputs on a FPGA and use external pullups to +5V. That's done by tristating the output for logic HI.
The digital pots are amazingly cheap! 11p+VAT each from RS in the UK.
I like the look of those drop-in switchmode 7805 regs. What you can't do is put a diode in the 0V leg to jack the voltage up a bit. This is a trick I've used with ordinary 7805 etc on occasion.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
