Re: Need DSP recommendation

From: Rahul Agarwal (ragarwal_at_fit.edu)
Date: 01/09/04 

Date: Thu, 08 Jan 2004 22:54:52 -0500

On Thu, 08 Jan 2004 09:52:14 +0200, Paul Keinanen wrote:

> With 50 kHz sample rate at 10 bits/sample, the net bit rate would be
> 500 kbit/s and adding bit stuffing, self clocking (Manchester coding),
> headers etc, the bit banger would have to operate around 1 MHz clock
> rate. Thus, for each sample, the bit banger would have to run 20
> times. With the 40 MHz processor, 40 clock cycles would be needed for
> banging out one bit, but of course also reading the A/D will need some
> cycles, so the number of cycles for bit banging would be less.

Yes all of that is correct. 40 clock cycles on pic means 10 instruction
cycles. The way I have the program written it is impossible to have it
sending out a bit with only 10 instructions. Keep in mind it is not just
a bit shift, I have to encode the sample every sample period and have
flags to check to see whether I am transmitting header or data etc.

>
>> Assume plenty rf bandwidth.
>
> Even if you would have available a noise free bandwidth of at least 1.5
> MHz, a line of sight path would be required. However, with reflections
> from various objects and especially from the ground, there are going to
> be frequency selective multipath dips within the signal spectrum
> corrupting the signal. Thus, spread spectrum or similar systems would be
> required for such large data rates. You should check for WLAN and
> similar devices that operate in the 2.45 GHz ISM band.

Pardon my ignorance as I have very little knowledge of rf. However the rf
engineer at rfm.com said that we could get 1 mbps throughput from the
TR1100 transceiver module over short distances. We are transmitting a
maximum of 50 yards. It is not possible to use a standard WLAN card with
TCP/IP due to the extremely high delays (~milliseconds) and protocol
overheads. However, would it help to have a rf trasmitter/receiver in 2.4
Ghz frequency? Is there anything available out there?

>>What is the cheapest solution to this? I cannot use a SPI module since
>>the rf module doesn't like them.
>
> If you are trying to use some home grown transceiver system, you should
> first test it with some external 1 MHz clock (e.g. signal generator) and
> observe the waveform at the receiving end with an oscilloscope or other
> device that can be used to calculate errors when the transmitter and/or
> receiver are moving around in the final application area, _not_ in the
> lab. After this, you can decide, is the RF module usable for this
> application.

Well, the final application is also in the lab. This is a reasearch
project at a university. I do plan to see the signal delay and accuracy
using A/D->micro->transmitter->receiver->micro->D/A and comparing the
original and obtained analog signal.

>
> Unless the RF module provide these services, I would suggest that some
> external USART chip to be used that can be put into HDLC mode and
> capable of generating Manchester coding. The interface between the
> processor and USART should be with parallel ports so that only a few
> instructions are needed to transfer a complete _sample_ to the USART
> compared that multiple instructions are needed to generate a
> single_bit_in the bit banging case.

This would be ideal. Do you know of any USART chip that can do this? A
12-bit encoding will be even better as that will save 50% overhead
compared to Manchester.

> Paul

Thanks a lot Paul.
Rahul

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