I''m having trouble with a project i've undertaken recently where i am trying to interface a Anybus module to a PIC18f452. Everything was going fine until i found that I don't have enough I/O pins to address the Anybus module's 12bit address. Therefore I wanted to use a PCF8575 I/O expander chip which uses I2C interface. But I'm already using the pins SCL and SDA pins for the DAC which I use (its interfaced using the SPI interface) hence I've got a problem. I'm already using PORTD an PORTE for the Parallel Slave Port (PSP) to get data to the Anybus and PORTA is used for the Analog to Digital converter and I only have PORTB (8bits) left. I've tried to implement a latching system where i address the 12bits with 8bits by de-multiplexing but its very complex. I'm using the PIC18f452 chip as stated before with MICRO CHIP MPLAB C18 compiler.
The simple answer is to make a bit bang interface using two currently unused pins. If your PIC is the master, set one pin as an output for clock (SCL) and you'll have to fiddle with the data line to make it an input except when transmitting.
About all I could suggest is for you to take a couple of unused i/o pins and bit-bang the I2C. Either that or switch to an I2C DAC. I have some PIC assembler code that bit-bangs I2C if you'd like it, but you should be able to find a C example somewhere.
Hint: toggling the TRIS bit is the easy way to simulate an open collector output. You preset things by defaulting the i/o pin to input mode, using an external pull-up, and preloading the PORTx bit with a zero. Then to write a zero to the bus, you simply toggle the TRIS pin to output mode. This will cause the pin to pull the resistor down to Vdd causing other devices on the bus to interpret it as a 0. To write a
1 to the bus, you toggle the TRIS bit to input mode by setting it. The pin then reverts to high impedance allowing the external pull-up resistor to pull the bus line high. This will naturally be interpreted as a 1 by other devices on the I2C bus.
"aerona" schreef in bericht news: snipped-for-privacy@g43g2000cwa.googlegroups.com...
The I2C and the SPI interfaces differ too much to use the same pins (unless you want to run into a maze of both hard- and software problems). It simply does not pay off.
Guess you already understand that either a DAC with an I2C interface or a busexpander with an SPI interface will solve your problem.
You can use two I/O pins to emulate an I2C master as PeteS already mentioned. As you have the PCF8575 already you don't need extra hardware. Drawback: It's a lot of software and s-l-o-w.
Best solution I can provide is using a busregister. You split up the 12-bits address in two 6-bits parts. Six out off the eight available output pins should be used for the address bus. You write one half of the address on that bus and then use one of the remaining output pins to clock it into an old LS363 similar CMOS buslatch. When done, you put the other half of the address on the bus and then use the remaining output pin to signal that the address is valid. Due to the properties of the PIC processors, you need to do five full port output commands. Consider you want to latch the high part of the address first.
- output high address part with latch enable and address valid off
- output high address part with latch disable and address valid off
- output low address part with latch disabled and address valid off
- output low address part with latch disabled and address valid on
- output low address part with litch disabled and address valid off So you only need one LS363 like chip instead of the PCF8575. The first being smaller and much cheaper then the last. You can also use a LS364 or similar. These are edge-triggered D-flipflops so you need to give an extra output command before the other five:
- output high address part with latch disable and address valid off to make sure the addresspart is stable when it is clocked into the flipflops.
Thanks alot for all the replies... I've looked at all the ideas mentioned and at first bit-banging the I2C on to 2 other pins seemed good. However I am not sure if the data rates would be satisfactory... Therefore I am also looking at using an I2C DAC with the PCF8575 as mentioned by you guys... I've come across the DAC6573 by TI which seems good alternative to the MAX522. If there is anyone who has a C code example on how to implement 2, I2C devices on the same bus line it would be very helpful. As for the latch method I don't really get it still... I mean wouldn't I have to design logic circuits to every 12 bit address I want to address from the 8 bits? As far as my understanding goes it's not possible to get the full range of addresses in the 12bit from the 8 right? If it is possible to give some places for further reference it would be great. I thank you once again for all the help and I am very grateful to all of you... Regards, Aerona.
"Pooh Bear" schreef in bericht news: snipped-for-privacy@hotmail.com...
As so often it depends. I've no idea how often that that Anybus needs to be accessed - I even don't know what it looks like - but if you have to access all of the 2^12 addresses you'll be busy bitbanging for quite some time. I simply don't know whether or not that time is available. Besides, you'll have to implement and debug that I2C interface. It's not very difficult and examples are available but it *is* work that takes its time. The address latch on the other hand is easy to implement requires hardly extra software and can be considered proven technology. The old 6800 series, the 6502, the
8088 and several others used this way of address demultiplexing already.
"Pooh Bear" schreef in bericht news: snipped-for-privacy@hotmail.com...
Yes, guess he should.
Sure. Apparently you have experience in this kind of software. But although I ever build one myself - so I have it on the shelf - an old hardware guy like me will choose for the addressbuslatch option. The only disadvantage I see is the need for all eight output pins. The I2C emulater requires only two.
That's what I deny in this particular case. It just requires one LS363 instead of the PCF8575.
As described earlier, first latch the upper part of the twelve bit address into the LS363, then put the lower part on the address bus which has been directly connected to the Anybus. This way you can offer all of the 2^12 addresses to the Anybus module.
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