Information gathering

So the beginning of designing anything is gathering some technical information on what you need to make it work. The main thing is to figure out what ICs to get for the job.Here a lot of things need to be considered from voltages they work on to communication protocols they use to make sure everything can talk to each other.

Luckily the ethernet is already taken care of on the board, so we dont have to worry about that.Next big thing is the IDE hard drive. As you know the IDE interface has a whole pile of pins, but luckily a lot are unused (gnd,obsolete,safety…). The most important group of pins are the 16 data lines that form the 16bit parallel bus along those are the flow control lines for it (read,write and such) and also a few address lines.Then there are a few extra pins like a reset,busy,interrupt etc. All this together gets to about 24 I/O pins and the board happens to have 24 I/O per core on the headers so its exactly enough pins to interface it. Okey so we have enough I/O but also the levels must work. Our I/O is 3.3V and 5V tolerant, but the IDE bus happens to use the 5V levels. Okey that sounds like a gazillion level shifters but the IDE bus works by the TTL standard so it should see a 1 when the voltage is above 2V so that means our 3.3V can generate a high level on the bus.I will still look in to that but from what i seen now there should be no problem just sticking the IDE bus directly on the I/O pins. Also the IDE Hdd needs a 12V supply along its 5V supply so a 12V power source will be used that will regulate down to 5V

While we are at mass storage lets look at the SD card now then.The card is much friendlyer to interface as it first of all only needs a low power 3.3V supply to work. Then the next beauty of the SD card is that its interfaced over the good’ol simple SPI bus on 3.3V levels, also by the SD spec it works up to 25Mhz so thats quite a impressive data rate.

Also on the topic of memory is the FRAM chip.It is a new memory technology called magnetic RAM that is quite revolutionary. It is the speed of RAM but it keeps its memory like flash, also there is unlimited write cycles.I happen to have one of these 1Mbit chips so im going to use it as memory to store various big stuff. Also one nice bonus is that the SPI bus on it works at up to 40Mhz and its a 3.3V chip so it will work with my I/O.

Moving on there is the matter of getting analog audio in and out.For that i plan to use audio DACs and ADCs that work with the standard I2S bus.That means the communication cosists of 3 clocks (master,bit,L/R) and 1 data line.The I/O should be esaly capable of making this interface run.Also a nice thing is that the digital I/O on these is usually 3.3V.

One interesting thing is the bluetooth module. Its a very cool chip from National Semi that contains a 8051 MCU (bluetooth stack) along with a 2,4Ghz radio and a antenna all inside the chip! By default the module is a virtual com port on bluetooth and the standard USART signals on the pins. It can also be told over the USART interface to do other stuff like connect to others or act as a bluetooth headset and such. Really cool chip that only needs a RX and TX line to work. Also confidentiality its all 3.3V too.

Only thing left is the LCD that I’m not sure if to really put in to it yet.However this is where the 3.3V ends as the display needs 5V to work however on the displays i have the I/O dose take 3.3V as a logical high so it is possible to interface without level shifters.

So thats about it in the end there are no 5V level shifters needed (I was scared i will need a truck load) and the IC count is pretty low in the end(around 4)  Only thing is that i don’t know if i will be able to fit it all on to a credit card sized PCB as i make my PCBs at home but i will see that when i get to PCB design.Main worry are the connectors as there is little room for them (especially the huge IDE connector)

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One Comment on “Information gathering”

  1. Jason Says:

    A great start! Good to see you are getting more familiar with the XMOS products!


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