Comments on: Parts: 8bit IO Expander (PCF8574)

By: rf

rf — Thu, 29 Oct 2009 15:29:12 +0000

As Adam Schabtach on Dec 28th, 2008 already mentioned, the chip can drive only ~20mA for a LED in pull-down mode. So the schematic is wrong.

please fix it, or (unskilled) readers of hack a day will produce more electronic waste …

2nd hint: if you use Arduino, the pull-ups on SCL/SDA are not nessescary, because the Wire lib activates the internal pullups of the atmega.

greetz

By: Code

Code — Mon, 06 Jul 2009 12:35:27 +0000

Cleaning up my room, Found me Arduino back.
Also already orderd a Bus-Pirate.

And found a bag of samples (PCF8574) So i’m busy this Vacation :P

By: ramzi

ramzi — Mon, 27 Apr 2009 21:57:15 +0000

I have a project and want to help. I need to have an assembly which connects 4 channels PCF8574 output port 8-bit ( ‘output x8), connected to an I2C bus operated by the 16F877.

Contact me if you enjoy it on my email directly. Ramzi.bk @ hotmail.com

thank you in advance.

By: heretic

heretic — Mon, 29 Dec 2008 16:17:50 +0000

@ njay:

when i wrote “low-speed”, what i was thinking was “slower than the microcontrollers gpio pins”.
It’s still very fast, yes.

By: error404

error404 — Mon, 29 Dec 2008 06:17:08 +0000

I use the MCP23008 (8 bit) and MCP23016 (16 bit) from Microchip. They’re available in SPI variants as well (I find I2C simpler to use, and it uses quite a bit less uC IO too) as MCP23Sxx. The 16 bit version is around the same price as this TI chip, it comes in DIP, SOIC or SSOP (or QFN if you’re insane) and it can source/sink 25mA on every pin. Pretty much nicer in every way ;)

By: jetblack

jetblack — Mon, 29 Dec 2008 02:45:45 +0000

thanks for the write up , good work keep them coming

By: Njay

Njay — Mon, 29 Dec 2008 00:14:24 +0000

4094 is also a good option, I’ve use a pair of them in a project. “low-speed”? What’s low-speed for you, heretic? In my project I had it running at ~302.5KBit/s, including the routine’s RET instruction and non-optimized code&algorithm.

i2c looks “pretty”, but I actually don’t like it much. The problem is that, even if you have a hardware peripheral for that, you need a lot of code to handle it correctly, when compared to SPI or UART.

By: Adam Schabtach

Adam Schabtach — Sun, 28 Dec 2008 17:29:02 +0000

There’s a hidden gotcha with this part which I just encountered (unfortunately after having PC boards fabricated). The outputs can’t actually source much current. The statement on the data sheet that says “latched outputs with high-current drive capability for directly driving leds” actually refers to inverse-logic current sinking, not sourcing. If you look at the current ratings on the Absolute Maximum Ratings table you’ll see that they seem to be “backwards” if you’re thinking in terms of a positive-logic current-sourcing configuration.

The schematic for this article shows a current-sourcing configuration. Personally I can’t get this chip to drive even a single LED as shown in this schematic, but it works fine in a current-sink configuration.

See http://www.picaxeforum.co.uk/showthread.php?p=81787 for further discussion.

By: heretic

heretic — Sun, 28 Dec 2008 09:34:16 +0000

If you need lots of low-speed digital inputs/outputs, you might consider using chained shift registers, eg. 4094’s. They’re cheap, widely available and easy to interface with.

Using I²C on AVRs is quite simple using this library:
http://homepage.hispeed.ch/peterfleury/avr-software.html

I really like it, since it enables you to use the TWI or a software implementation of the protocol via the same interface. Didn’t try the TWI-based stuff, but the software-only part worked great. Just include the header file, add the assembler file to your makefile, adjust the delay routine if necessary and you’re done.

By: jproach

jproach — Sun, 28 Dec 2008 07:26:52 +0000

Software i2c on an avr/pic is quite simple IMO. Here is the first avr example I found: http://google.com/codesearch/p?hl=en#u1hJZLnlbRM/lcd_i2c/i2c.c&q=i2c%20avr

Hardware is definitely more complex, but also offers a lot more features (see atmel appnote AVR315). Also see “two-wire serial interface” in most avr’s datasheets to understand the hardware behind it.

Anyway, as for the micro vs. a custom IC: think of ease of design (writing/testing firmware), and ease of manufacturing when used commercially.

By: andrew

andrew — Sun, 28 Dec 2008 03:56:39 +0000

*woops, i meant @bau

By: andrew

andrew — Sun, 28 Dec 2008 03:55:45 +0000

@joel:
i agree. i use avr’s and stay far away from i2c. i’m currently working on a pretty big project with and i’ve made sure to use all spi ic’s in the design. just reading about i2c makes my head spin. i can see it being *really* nice for those ucontrollers with hardware support for it, but the avr is not one of them.

By: Njay

Njay — Sun, 28 Dec 2008 02:46:16 +0000

I have used (actually, built) this one, ED1021:

http://embeddeddreams.com/site/products-page/edchips/ed1021—io-expander-with-uart-interface/

Additionally to the general purpose I/O pins, it can read an analog signal from almost any pin and it has a calibrated internal reference, which means you can read voltages from 0 to around 2.5V with precision of a few mV. It can also connect almost directly (2 resistors required) to most (if not all) typical PC UART ports.

Feel free to give me improvements/changes suggestions :)!

By: jimmys

jimmys — Sat, 27 Dec 2008 21:59:41 +0000

brianbek-
thanks but if you’re trying to help someone not familiar with these chips then you might want to give a little more info.

UDN2981A
8 channel source driver. 80v / 500ma max.

ULN2003A
7 channel darlington array. 50v / 500ma.

TLC2543CN
11 channel, 12bit adc

By: Brianbek

Brianbek — Sat, 27 Dec 2008 21:31:18 +0000

Have used this chip in the past, worked well ;) two others that I keep a good stock of are

UDN2981A
ULN2003A

great for quick projects that pop up.

A bit pricy, but this came in handy a few times

TLC2543CN

Hope someone else finds these useful