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Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
One thing to keep in mind when using normal photodiodes for comunication is that they are extreemly slow, I usually wouldn't use them at >100khz... Also, I tend to aviod the pic18fxxx3 series, as microchips C compiler doesn't support any of the chips with 12 bit a/d (denoted by the 3 at the end). So if you ever get sick of assembler and decide to go for C (there is enogh power in them 18f25xx series to do a lot of stuff ) you are stuck. In my GPS tracker I am using a pic18f2550 and an external a/d chip, (the MCP3421is an 18bit serial adc that is in a tiny 6pin package, and costs under $2). Also, if you go that route you can connect the dac directly to the internal i2c hardware of the chip (remembering to set the BRGH register nice and low, 10khz should be more than enough), which also frees up the RX pin for the hardware EAUSART (usefull if you ever want to give it a async interface (ie, rs232) to a computer or something).
BTW, when using SPI, keep in mind that you are suppose to be always transfering data in both directions whevever there is a clock (even if it is just garbage data, like all 0's). And IIRC, the slowest I2C divider is 1/64, which will give you a resonably slow clock. (250khz) If thst is still to fast, is is pretty easy to 'bit bang' a SPI-ish serial interface to transfer data between the chips. You would just disable the internal hardware module and have code that looks for the line to go low, then cycles the clock, sends a bit, cycle the clock, etc.
Registered Member #120
Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83
Wait i'm sorry, but in a master slave setup the clock is only in one direction, right? Also since I just discovered that i can change the frequency I will run my serial interface at 50kHz, I don't think i will have a problem. I just wanted to confirm the direction of data. I got everything else working, just not the serial part. Other then that I have to code a 12 bit to 4 bcd digits for output. I just want to solve this little issue before i order my boards.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The clock always goes from master to slave, the data goes in both directions at the same time. Check the datasheet to find out what the largest divider is.
SPI ports can be used for many things, I just made a 15 MHz frequency counter with one and 10 lines of code.
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a PIC based capacitor charger
