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From: D Yuniskis on 12 Apr 2010 11:04
azraiyl wrote:
>> Are you expecting 1MB/s as your total throughput?
>> Or, for each "link"? (i.e., as if star configured)
>
> Optimally for each "link".

This could be a potential problem for USB (1.1), then
(store-and-forward USB*2* hub??)

>> Do the other processors ever have to talk to each other
>> directly? Or, is everything Master-slave with the
>> Master always the "Main Board"?
>
> Most often only Master/Slave but there should be a possibility for
> asynchronous replies.

But, you can support asynchronous replies within a M-S protocol.
I.e., if said protocol causes each slave to be polled regularly
then the slave could queue a deferred reply for "next polling
time".

>> Why can't you use the SPI/I2C channels? Can you
>> rethink how you are encoding your data to use lower
>> data rates?
>
> Unfortunately the are all used on the processor. The main processor is
> currently an OMAP-L138. SPI, UART, I2C, McBSP: Everything is already used.
> Ethernet is not available (because of pinmultiplexing with McBSP).
>
>> You could use USB host controller on the Main (easy
>> to find MCU's with slave controllers built in).
>
> That's an idea. The processor has an USB port, I only need a USB hub.
> Software overhead on the "slave" processors is maybe a bit high.

Cheat. You "know" what you are talking to. Use that to
trim overhead (certain types of error handling) from the
protocol.

Decouple the processes to a greater extent. I.e., *don't*
expect synchronous messaging. Throw a message to a slave
and "move on"; check back later for the reply -- even if
the actual "virtual protocol" remains synchronous.

>> You could use *SCSI* controllers ("SCSI-1" will give you
>> 5MB/s easily). Not sure if you can find any legacy
>> products (e.g., 5380) for this :-( This also gives you
>> some structure to the data stream (UARTS don't -- you
>> have to impose structure on the "character stream")
>
> Noted.
>
>> Likewise, you can use ethernet and probably strip out the
>> magnetics with some passive trickery (since it is a real
>> short haul and you know *exactly* how many nodes you have).
>
> Ethernet is not availble for the current main processor (see above).
>
>> Personally, I'd explore the USB route, first. You might be able
>> to spec different processors (if it is early enough in the
>> design stage) for your "slaves" and then just take the
>> hit for an external host controller on the master (there
>> are some MCUs with this ability but nowhere near as many
>> as peripherals).
>
> I'm early in the design phase and therefore open to anything.

How sensitive is the data? Can you afford to drop any?
Can you afford to delay any? Can you afford out-of-order
packet handling? etc.

I.e., are you just looking for a NUMA approach to shared
memory? Or, something more "abstract"?
From: Mark Borgerson on 12 Apr 2010 14:28
In article <l9CdnSXxQYlrmV7WnZ2dnUVZ_uadnZ2d(a)giganews.com>,
azraiyl(a)n_o_s_p_a_m.gmail.com says...
> I've a 'Main' Board that needs to communicate with 4 other processors on
> other boards with at least 1 MiByte/s. A prefer to use the Adress/Data-Bus
> on the 'Main' Board. The 'other' processors support SPI, I2C, UART and have
> an A/D-Bus aswell.
> My first idea was to take a quad UART an put it on the 'Main' board.
> Problem is that they are all expensive. Any other idea?
>
>
1MByte/S is a bit fast for standard async comms. I use the UART ports
on MSP430 chips to communicate between ADC boards and a storage system
at 115KBaud.

You might try SPI comms. However, clocking at 8MHZ may stress some
processors in the slave mode.


Mark Borgerson


From: Vladimir Vassilevsky on 12 Apr 2010 14:46


azraiyl wrote:

> I've a 'Main' Board that needs to communicate with 4 other processors on
> other boards with at least 1 MiByte/s. A prefer to use the Adress/Data-Bus
> on the 'Main' Board. The 'other' processors support SPI, I2C, UART and have
> an A/D-Bus aswell.
> My first idea was to take a quad UART an put it on the 'Main' board.
> Problem is that they are all expensive. Any other idea?

For that kind of transfer rates, you will need hardware support for
buffering regardless of the transport used. You can certainly stretch
UART or SPI or I2C to that speed, however that could result in major
inconveniences. The standard solution could be Ethernet or PCIe or some
sort of parallel bus with DMA access.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
From: Mark Borgerson on 12 Apr 2010 19:12
In article <CJ-dnZCbCpDi917WnZ2dnUVZ_j6dnZ2d(a)giganews.com>,
nospam(a)nowhere.com says...
>
>
> azraiyl wrote:
>
> > I've a 'Main' Board that needs to communicate with 4 other processors on
> > other boards with at least 1 MiByte/s. A prefer to use the Adress/Data-Bus
> > on the 'Main' Board. The 'other' processors support SPI, I2C, UART and have
> > an A/D-Bus aswell.
> > My first idea was to take a quad UART an put it on the 'Main' board.
> > Problem is that they are all expensive. Any other idea?
>
> For that kind of transfer rates, you will need hardware support for
> buffering regardless of the transport used. You can certainly stretch
> UART or SPI or I2C to that speed, however that could result in major
> inconveniences. The standard solution could be Ethernet or PCIe or some
> sort of parallel bus with DMA access.
>
On the MSP430, I use the UART ports with DMA receive. Since packet
lengths are fixed, the DMA setup is fairly simple. The main board
prompts for a packet, sets up the DMA for a receive that puts
the packet into a queue. The end-of-packet interrupt increments
the qeueue pointer and count. If you bumped this up to 1MB/s
the DMA might start taking an appreciable part of the CPU
bandwidth on a 16MHz CPU. At that data rate, I'd probably
switch to an ARM or Cortex at 30 to 60Mhz so I'd have time
to do something useful with the data.

SPI is a bit trickier, since it requires a transmit DMA channel
to generate the clocks, in addition to the receive DMA channel.

Mark Borgerson


From: David Brown on 13 Apr 2010 03:53
On 12/04/2010 20:28, Mark Borgerson wrote:
> In article<l9CdnSXxQYlrmV7WnZ2dnUVZ_uadnZ2d(a)giganews.com>,
> azraiyl(a)n_o_s_p_a_m.gmail.com says...
>> I've a 'Main' Board that needs to communicate with 4 other processors on
>> other boards with at least 1 MiByte/s. A prefer to use the Adress/Data-Bus
>> on the 'Main' Board. The 'other' processors support SPI, I2C, UART and have
>> an A/D-Bus aswell.
>> My first idea was to take a quad UART an put it on the 'Main' board.
>> Problem is that they are all expensive. Any other idea?
>>
>>
> 1MByte/S is a bit fast for standard async comms. I use the UART ports
> on MSP430 chips to communicate between ADC boards and a storage system
> at 115KBaud.
>

It's uncommon to have speeds about perhaps 921.6 kbaud with RS-232, but
there are high-speed RS-485 drivers that work happily with 30+ MBaud.
But it's not going to be easy to get a microcontroller's UART to run at
the 10 MBaud you need for 1 MB/s - it would have to be external uarts.

> You might try SPI comms. However, clocking at 8MHZ may stress some
> processors in the slave mode.
>
>
> Mark Borgerson
>
>

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