Hi,
I want to implement PCF8574T SMD in my dev board design.
I searched a typical circuit and found one in TI datasheet.
My questions:
Decopling caps are recommended for each chip to filter digital noise from the power supply. 0.1uF across the power to the chip as close to the chip as possible.
They use high value resistors for pullup. Each PCF8574 GPIO output can only source* up to 300uA so, I suppose, the pullups need to be high resistance. Since we don't know what those subsystems consist of there is no telling why they need pullups. I don't know what the switch at the bottom is doing.
What do you need the I2C expander for? The MCP23008 8 bit I2C expander will source or sink 25mA from each GPIO output, has internal pullups that can be enabled and has pin change interrupts. May be a better solution.
*IOL LOW-level output current VOL = 1 V; VDD = 5 V Iol(max) = 25 mA sink
IOH HIGH-level output current VOH = VSS Ioh(max) = 300 uA source
he says he want to implement it, rather than using it in a application.
What does that mean?
There are symbols on the diagram that appear to indicate those are being used as inputs, while P0 uses a similar symbol indicating output. P2 and P3 are confusingly marked near the devices they are driving instead of at the PCF8574, and are also outputs.
I want to use it to provide me 8 switches for different SPI modules.
Because I'm doing a dev board with Arduino nano.
And I'm using different SPI modules and each one need CS pin, so I thought of using an 8-bit expander to control CS for each module.
Oh yes I remember using the IO expander module with the PCF8574 which isn't good for pulling up things, it's better for pulling down.
I think also if the pins of the PCF8574 used as inputs or outputs there has to be a pullup resistor to have active low signaling.
I got this from the datasheet explaining the diagram:
A. Device address is configured as 0100000 for this example.
B. P0, P2, and P3 are configured as outputs.
C. P1, P4, and P5 are configured as inputs.
D. P6 and P7 are not used and must be configured as outputs.
Also this for wiring:
But there are two capacitors instead of one. Why ?
Does the MCP23008 need any aux components beside the filtering capacitor ?
I found it in Aliexpress for $10/10pcs. The PCF8574 is a bit cheaper for $3.20/10pcs.
The PCF8574 would require pullup resistors + filtering capacitor.
Except in some special cases of battery operation, specified in the PCF8574 data sheet, where minimal power consumption is desirable, you do not need pull up resistors on output pins.
It is the case that the MCP23008 has switchable built-in pull up resistors .
I have a broken board for the MCP23017, I can desolder it and use it in my design.
I would try to desolder the pullup resistors and the cap also.
If a PCF8574 is suitable for your application then an MCP23008 or MCP23017 will probably also be suitable. External pull up resistors are then not required but a decoupling capacitor is required. However, the MCP230XX chips are a bit more difficult to use that a PCF8574, although a library may help.
For a given I2C clock rate, the MCP parts will always be slower at transferring data since they require sending register information prior to sending the port value.
That said, the MCP parts support a higher clock rate that can more than make up for that difference.
--- bill
Yes you're right. I tried to test the MCP23017 before, I was able to read/write to either port A or B.
Yes there are a lot of addressing. But since I have it and also it has 16 I/O pins. But it still under consideration. I might use the PCF8574.
I'm thinking of using it with SPI.
I'm reading the datasheet to know the spi pins for the mcp23017.
But I'm a bit confused between mcp23017 and mcp23S17.
Now, what should I do with the mcp23017 spi pins ? For example, does the mcp23017 require a CS pin or it doesn't matter ? also what about if I want to read data with MISO ?
For SPI, you will need the MCP23S17, not the MCP23017 and you can ignore information that is specific to the MCP23017
If you only have one device on the SPI bus, you can hardwire the CS to GND; but if you ever want to expand with another SPI device (no matter which one), you have a problem.
What about it?
They assume the input signals are open drain. Those need a pull-up resistor to work properly. But according to the Datasheet the PCF has internal pull-up current source which is much better than a mere pull-up resistor. Why TI decided to include weaker external pull-ups is mystery for me. I came up with two possible explanations:
I think they don't use pull-ups for outputs for reason 1. - they didn't realize the pins are "pseudobidirectional" and OUTPUT HIGH is the same as INPUT_PULLUP (and it is unable to be input without pull-up).
Also note input to P5 makes no sense. The button connects the pin to another pull-up while it should ground the pin. As drawn pressing the button does nothing. Clearly TI didn't invest much effort in this schematic.
To your other questions: the MCP needs decoupling capacitors as well. All digital chips need some.
PCF has pin change interrupt capability too. In MCP it may be configured which pins trigger interrupt while PCF interrupts on any change in any pin.
There are two reasons for multiple filtering caps:
I think 1. is what they had in mind because this simple IC hardly needs so much filtering.
For SPI, you will need the MCP23S17, not the MCP23017 and you can ignore information that is specific to the MCP23017
If you only have one device on the SPI bus, you can hardwire the CS to GND; but if you ever want to expand with another SPI device (no matter which one), you have a problem.
Yes, I'm thinking of using different SPI modules and I designed this circuit.
What you think ? Is it good ? I used 8 pins for on board spi modules and ported the other 8 pins for any other use.
What about it?
I was thinking that MCP23017 can use SPI but it doesn't. So in case of I2C the read/write would be on SDA apparently.
If you are building a complex SPI-based system, why make it even more complex by using i2c bus also? It should be easier and neater to stick to one bus system. Could you use a humble 74hc595 instead of pcf/MCP chips?
Or, if you have 3 Arduino pins available, a 74hc138 ?
