Processor options
Requirements
Processing power
The current implementation of buffer manager software, written in
assembler and running on a ~15 MIP processor, just about manages the
designed 100kHz event rate. The equivalent program in C ran at about
half the speed, therefore to allow us to use C instead of assembler we
need ~30 MIPs. However this is a minimal implementation without the
monitoring and other requirements specified in the URD. In order to
comfortably implement all functionality at the
required rate, without resorting to coding in assembler, we should
double this number again to give us a processor power requirement of
~60MIPs.
Power consumption
For a PMC module total power consumption is limited to 7.5W.
Chips consuming a large amount of power require heat-sinks which are a
problem in terms of the physical size requirements.
Physical size
A PMC card is not very big!!
DMA
After some discussion we have decided that DMA capability is not
required in the processor. The only place where DMA is desirable is
in the PCI bridge.
The contenders
Quick comparison
| Processor | processing power (MIPS) | power consumption
| physical size
|
| C40 (60MHz) | 30 | 1.75W | 1.86"
|
| C30 (60MHz) | 30 | ? | 0.9"
|
| SHARC (40MHz) | 40 | 1.75W | 1.428"
|
| Am29040 (50MHz) | 68.8 * | 1.65W | 1.54"
|
| i960 (RP) (33MHz) | 31 * | 3W | 35mm
|
| Pentium (100MHz) | ? | 10.1W | 1.95"
|
| Pentium (610=) (75MHz) | ? | 7.6W | 24mm
|
| PowerPC 602 (66MHz) | 40 ** | 1.2W | ?
|
| PowerPC 603 (80MHz) | 75 ** | 3.0W | ? ***
|
| PowerPC 603e (100MHz) | 120 ** | 3.5W | ?
|
| IDT79R3051 (40MHz) | 35 | 2.5W | 1.195"
|
* Quoted as 'VAX MIPS'
** SPECint92
*** 240 CQFP / 256 BGA
Notes
- C30, C40, SHARC and i960 all have some DMA channels.
- Pentium & Powerpc 603e look promising for speed but are they
too large / power hungry?
- Cxx, SHARC & Am29040 all have in circuit emulator development
/ debugging environments.
- PowerPC requires PowerPC based development system (RIO2?).
- The i960RP looks interesting because of the ease of interfacing
to PCI (it has 2 PCI busses in addition to its local bus).
Other considerations
Smaller chips may require a heat sink where a larger chip could dissipate
enough heat without.
Chips packaged with a BGA may be more difficult to debug (hardware wise)
because it is not obvious how you would attach logic analyser
probes.
Processors such as the Cxx would be much simpler to integrate since we
already have experience with the C40.
Processors such as the PowerPC would require a new debugging
environment.
High clock speeds would make the design much more difficult in terms
of routing of signals on the PCB.
Conclusions
- Pentium consumes far too much electrical power.
- Current models of C3x, C4x, SHARC & i960 do not appear to
have quite enough raw processing power, but:
- C30 should not be ruled out yet because writing in assembler
rather than C it could cope and it has the following advantages:
- Same family as C40 for which we have experience,
development tools, hardware debugger.
- Small size
- Low power
- i960RP should not be ruled out because integral PCI bus
should make the design much simpler:
- Fewer components as no separate bridge chip.
- Less logic, no multi-master bus impementation.
Our choice
We have decided to use the i960RP as this seems to be the easiest
from the hardware point of view. If we find that the processing
power is really not enough for the job the newly announced i960RD (a
clock doubled version of the i960RP) should give us the extra we need.
GJC
(gjc@hep.ucl.ac.uk)