i2C LCD Controller PCF8574 Address

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Jay Dee
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i2C LCD Controller PCF8574 Address

Postby Jay Dee » Sat Jan 23, 2016 1:45 pm

Hi Guys,
The i2C to LCD controllers available via eBay, they are great for controlling LCD displays via just 2 wires + power but little usefull info is given.
A few tips that I've found when playing with them.

The boards main i2C chip, PCF8574 actually comes in two flavours, PCF8574T and PCF8574AT and they have different fixed addresses.
On my board it has three links to set the hardware address. Un-connected, these pins are pulled high (check, some other boards pull them low!)
You can of course set any address via the links but most people either have all links open or closed for simplicity.

The address is 7 bits long, the lowest bit defines if its a read of write request, Flow code handles this so you only need to enter the required address in Decimal.

PCF8574A (Address pins Pulled High) 0111 111 R/W = 0x3F = 63 dec
PCF8574A (Address pins Pulled Low) 0111 000 R/W = 0x3F = 56 dec

PCF8574 (Address pins Pulled High) 0100 111 R/W = 0x27 = 39 dec
PCF8574 (Address pins Pulled Low) 0100 000 R/W = 0x20 = 32 dec

Ensure you add an I2C master component and initialise it first.
Then Add the i2C LCD component and start that.
Then its the same as all the other LCD devices..

Use 4K to 10K pull ups on SCL and SDA lines
If the lines are long and noisy place a low value resistor in-line near the receiving device (LCD) this helps damp spikes and ringing.
Once I found the damn alternate addresses everything work just great! J.
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Re: i2C LCD Controller PCF8574 Address

Postby medelec35 » Sat Jan 23, 2016 3:52 pm

Hi John,
Thanks for this useful information.
I have moved topic to Tips and Tricks section where it should be easier to find.

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Re: i2C LCD Controller PCF8574 Address

Postby siliconchip » Sun Dec 31, 2017 7:28 pm

jay dee
thanks for the information on the I2C lcd controller absolutely brilliant thanks for posting