Building a DSP board, Part Four: Hooking up RAM & ROM
This is the fourth in a series on how I went about building a dual Motorola DSP56000 sampling board.
As I had no real high-speed (well, high for me) digital design experience, I wanted to keep this part as simple as possible. I chose to use static RAMs since the 56000 doesn't have a built in RAM refresh. The only problem with this choice is that they are *expensive*, especially the fast ones. Thankfully, Integrated Device Technologies came to my aid and gave me four 64k x 16bit 70ns SRAMS. Unfortunately, these are not quite fast enough to go with no wait states (<60 ns would do it).
However, that is no big deal. Wait states are metered out in terms of clock cycles, not instruction cycles. Therefore, I only needed half an instruction cycle of wait time. Besides, I mainly would use this part for storing data for delay effects. The 256 words of fast RAM (no wait states) maps into the first 256 address spaces, with the rest taken by the external SRAM. The chip is smart enough to know not to wait on the internal RAM. So, if I needed to do any fast calculations and memory stores, I would use the first 256 words. Delay storage gets set above that 256 words. Not only that, but in my case, the first 256 words are 24 bits and the rest of the 64k is 16 bits (BTW, the address bus is 16 bits, the data register is 24 bits... kinda unusual to to the later greater than the former!). 64 k is enough to store 1 and 2/3rds seconds of audio at my 39kHz sampling rate (and remember, since I am using X and Y memory, it's in stereo!). This is more than enough memory for most audio applications.
[ side note: The separate X and Y memories make this chip ideal for stereo applications. It makes it very easy to write routines - you simply duplicate the routine and make it work with the other address space instead. ]
Okay, where were we? Oh yeah... I think I've covered most of the SRAM details. One last thing. Since we are not using the bottom 8 bits of the external memory, we tie them to ground through a 47k ohm resistor. This is important! If we don't ground these bits, they will probably be read as high and we will have to clear them in software every time we load from external RAM. We have to use the resistor, as these are outputs, too, and if we just ground them, we'll short those pins if they are high. Also, the ROM gets connected to these lower bits.
Making a program ROM for this chip is very easy. You can use off the shelf EPROMS (like the 2716) of only 8 bits wide. There are two mode bits that you wire up on the chip that determine its bootup mode (there is a bootstrap ROM in the chip). We select mode 1 (bootstrap mode) which boots a program that will load program data through the lower 8 bits of the data bus. But first, we must tie the high bit of the data bus through a 47k ohm resistor to +5V. The bootstrap program checks the high data bit at program memory $C000 to check whether or not to load from the host port or from the data port. The host port loading will make program loading easy for those of you with PCs.
The program then loads the program from the ROM. The 56000 boots in slow mode (15 wait states on all types of external memory), so we don't have to worry about speed on the 2716 (I think I used 250 nsec). Addresses $0000-$0002 hold the first word in LSB, MidSB, MSB order (Intel bassackwards format). Remember that it is NOT MSB, MidSB, LSB!!! It took me a couple hours to figure this one out, as I am used to the "correct" way of MSB, MidSB, LSB that Motorola uses in all of their products. Internally, the 56000 keeps things in MSB LSB order; it is just the external bootstrap ROM that has to be backwards (not sure why they did this... suspect that it has to do with some standard for PROM burning...).
Now we have to do the external chip select decoding. This is the *only* glue logic needed! This is also the only part that I think Motorola screwed up on (although I'm sure they have their reasons). There are three chip select outputs. _PS_ (this is my notation for PS with a bar over it) is low for external program memory, _DS_ is low for external data memory, and X/_Y_ determines which external data memory is referenced. Here's a chart:
__ __ _
PS DS X/Y External Memory reference
-----------------------------------------
1 1 1 No activity
1 0 1 X data memory on data bus
1 0 0 Y data memory on data bus
0 1 1 Program memory on data bus
0 1 0 External exception fetch (development mode)
0 0 X Reserved
1 1 0 Reserved
I thought it would be difficult to decode the X, Y and P lines, but there is a simple solution: just use a three to eight decoder. I chose the 74138 because it uses negative logic (the SRAM lines are negative logic). I just hooked up the enable lines to the decoder outputs that went low when the proper input pattern was selected. The delay time that this chip adds is another thing you must account for if you want to use no wait state SRAMS.
One thing I forgot to mention is that the bootstrap ROM is mapped from $C000 to $C5FF in the external program ROM. I just ignore the upper bits of the address bus so that my EPROM is automatically selected. Mode 1 maps the external program memory over the internal program memory for reading, but uses the internal program memory for writing. After the internal bootstrap program has loaded the external program into the internal program RAM, it switches to mode 2 (maps internal program RAM for reading and writing) and starts executing at $0000.
I forgot to mention that these IDT SRAMS are SIPS (single in-line packages). I need two of them for (one for X mem, one for Y mem).
They each have 40 pins. Hookup is quite simple.
First, I put 5 16-pin dips in a row like this:
______ ______ ______ ______ ______
/ \/ \/ \/ \/ \ <-- each one of this is a 16 pin dip
oooooooooooooooooooooooooooooooooooooooo
oooooooooooooooooooooooooooooooooooooooo
This gives me two rows of sockets for the SIPS. Basically, I just connect each of the adjacent pins like this:
X
|
oooooooooooooooooooooooooooooooooooooooo
|||||||||||||||||||| |||||||||||||||||||
oooooooooooooooooooooooooooooooooooooooo
|
Y
The X and Y are the individual chips selects. The rest of the pins are power, control signals, address lines, and data lines, which are the same for both chips.
This arangement takes very little board space and is quite easy to hook up.
Next: Power supply considerations
