On 02/12/2011 02:33 AM, Oliver Seitz wrote:
Yes, but they really select chips, not drives. They select different PATA
register sets on the drive.
I see.
USART ("RS232") can be used, of course. As it is only internal communication,
you're free to choose any baud rate. You can configure a 16F877@20MHz to communicate at 5
megabaud, I think. You can not however translate this to RS232 levels using a MAX232, but
you don't have to. Just connect the TX line of one PIC to the RX line of another.
SPI does not need start and stop bits, so the usable data rate can be slightly
higher, but you need more pins. And, SPI supports chip-select inputs or
daisy-chain setup, so multi-device-communication is easier with SPI.
Interesting... 5Mbaud over RS232 pins?
I didn't think RS232 could allow for such speeds... given the hazards of
crosstalk, etc.
The daisy-chain setup is quite useful in a serial bus... what's the
theoretical limit with PIC SPI?
128 devices per bus? or is it higher than that?
How do you count four? PATA only supports Master and Slave, that makes two
drives per controller.
You can multiplex the PATA connectors using bus drivers, like 74HC245. If
you're only using one drive at a time, that could be a cheap and simple
solution. You can easily expand one controller to 20 hard drives this way.
Given the number of IOs on a 40 pin PIC like the two I have... getting 4
drives per PIC isn't exactly a magic trick.
The data bus remains the same for the two buses (hence 4 drives)... as
well as some of the control lines.
The remaining lines that differentiate drives would have their own IO
pins still remaining within the IO limit of a 40 pin PIC.
The original schematic (here:
http://justanotherlanguage.org/sites/default/files/pata_hard_disk_schematic.jpg
)
only used 16 data pins, and 5 control lines! To add a second "bus"...
all you'd need is another 5 control pins
(if you need all 5 separate), which is very easy given the left over
real-estate of the original schematic.
You certainly don't need a bus driver to handle a measly 4 PATA drives.
Either way... using bus drivers or not... you can only access 1 drive at
a time.
More than sufficient for a media player project.
Lyos Gemini Norezel
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