[sdiy] uC with mul

[Jim Patchell]

     Actually, a 128K rom (27c010) is actaully about the smallest rom 
you can actually buy new these days, although, even at $5.00 each,  plus 
the two latches, you are right that this will probably add enough cost 
to justify a fancier micro...I sort of wish this option was availiable 
to me about 15 years ago when I needed a high speed multiplier in a 
project that used a 6502.  For that one, I built a multiplier using 
74F384 serial multiplier chips and ran it at like 40mhz.  The look up 
rom would have been easier, but parts that big were not availiable at 
the time, or if they were, would have been very expensive.

Theo wrote:

> That's a rather large ROM.
> Considering the added costs it might be cheaper to migrate to a
> MegaAVR with mult or move to a ARM part right away.
> 
> Theo
> 
> 
> From: Jim Patchell <patchell at silcom.com>
> 
>>      Here is a thought for doing an 8x8 multiply...use a look up table.
>> If you use a 128Kx8 ROM, you can put a look up table in it to make an
>> 8x8=16bit multiplier.  All you need are two 8 bit registers, and then
>> use one of the address lines to access the MSB and LSB parts of the
> 
> results.
> 
>>    Anyway, just a thought.
>> 
>>    -Jim
>> 
>> Scott Gravenhorst wrote:
>> 
>>> I did quite a bit of assembly programming with a MC6800
>>> MPU (not 68K !) which has no mult instruction at all.
>>> There are cases where you need to multiply a variable
>>> number by a constant.  In these cases, a short sequence
>>> of shifts and adds does the trick and can be faster than
>>> using a generic multiply subroutine.  So if you are
>>> stuck with a no-mult mcu, or if you have 8x8 and need
>>> 8x16 or 16x16, this technique can be useful as well.
>>> Much depends on how fast the builtin 8x8 is.
>>> 
>>>>> -----Original Message-----
>>>>> From: ext John Loffink [mailto:jloffink at austin.rr.com]
>>>>> Sent: 30. December 2001 17:08
>>>>> To: Byron G. Jacquot; synth-diy at dropmix.xs4all.nl
>>>>> Subject: RE: [sdiy] uC with mul
>>>>> 
>>>>> 
>>>>> The multiply cycle on the 68HC16 is 8 cycles.  Other micros
>>>>> do have the
>>>>> multiply instruction, usually multi cycle instructions.
>>>>> These might be
>>>>> good enough depending on your application, particularly for low
>>>>> frequency stuff like note generation, envelopes, LFOs,
>>>>> control sources,
>>>>> etc., but for anything related to analog synthesis it's
>>>>> generally better
>>>>> to have 12-16 bits of resolution to avoid stair-stepping.
>>>>> 
>>>>> John Loffink
>>>>> jloffink at austin.rr.com
>>>>> 
>>>>> -----Original Message-----
>>>>> From: owner-synth-diy at dropmix.xs4all.nl
>>>>> [mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Byron
>>>>> G. Jacquot
>>>>> Sent: Saturday, December 29, 2001 10:29 PM
>>>>> To: synth-diy at dropmix.xs4all.nl
>>>>> Subject: RE: [sdiy] uC with mul
>>>>> 
>>>>>> Motorola M68HC16 microcontroller series has a 16 bit
>>>>>> multiplier/accumulator.
>>>>> 
>>>>> The "little brother" 68HC11 could also do 8*8 multiply, and
>>>>> (IIRC) 16/8
>>>>> division.  They were both kinda slow.  I think a multiply took 12
>>>>> cycles,
>>>>> and the divide something amazing like 42.
>>>>> 
>>>>> Byron Jacquot
>>>>> 
>>> =========================================================
>>> - Government: The other religion.
>>> - The media's credibility should always be questioned.
>>> - Lambs who lie down with lions are lunch.
>>> 
>>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>>> -- Linux Rex         | RedWebMail by RedStarWare
>>> -- FatMan: home1.GTE.NET/res0658s/TekLab_FatMan_WebSite/index.html
>>> 
>>> 
>>> 
>>> 
>>>