Lowest power RAM

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Hi,

What kind of RAM can be used with the lowest power available, with an
MCU that might run either at 15MHz or 1MHz, possibly both? The chip is
LPC2210, which allows 8,16,32-bit wide data busses and can do SRAM,
DRAM or EDO. I'm not worried about the cost or speed. The ram will also
be underclocked to match the MCU, and also to lower power requirements.
I'm hoping for under 1mA @ 3.3v or less, for 1M x 8. What kind or make
of memory should I use?

Ive seen The cellularram chips, but like most other DRAM the difference
between active and standby is big, and I dont know how to calculate the
average usage @ 1MHz. Surely there'd be hardly any activity on the ram
for the most part at that clock... can I safely say the usage will be
under 1mA even with a memory intensive app running on the MCU @ 1MHz?

Can I have some sort of SRAM that can give me lower power usage?

Since I'm underclocking, the signals to the ram chip will be running
much slower. But the fraction of time the activity pin is high will
remain the same (thus the power usage on sram). Can I somehow (with
capacitor and high speed latch tricks or otherwise) reduce the active
high state on the sram such that a 133MHz sram read at 1MHz will only
see the active high at 1/133rd times the normal?

Lastly will using the 8-bit wide data path for ram chips significantly
lower power consumption compared to the 32-bit wide data path?


Re: Lowest power RAM

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The LPC2210 memory controller doesn't support DRAM or EDO, only SRAM (and
burst ROM).  Low power SRAM, made by Samsung and others, looks like the best
fit.

The K6X8016T3B family (512K x 16, VCC=3.3V) for example have a standby
current of 15uA (CS inactive), and max 3mA when reading at 1 Mword/sec.  If
you can keep it inactive most of the time, you'll get well below 1mA.


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If you need to do four times as many cycles, 8-bit data means more power.

Karl Olsen



Re: Lowest power RAM

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Thanks guys. It helped. I'll use 32-bit wide data paths, and will look
for 256Kx32  to use, else 2x 256-x16 chips to keep chipselect low.

Will using PSRAM or an external dram controller help lower power usage
further or is it not worth it?


Re: Lowest power RAM

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DRAM or SDRAM use much more power than SRAM.  Check out the Samsung
datasheets.  http://www.samsungsemi.com /

Karl Olsen





Re: Lowest power RAM

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Have you done any analysis of the distribution of RAM access for 8, 16
and 32 bit variables (both read and write) ?

If the RAM access is predominantly 32 bit, then 32 bit wide paths
would make sense.

However, if you have a lot of 8 or 16 bit access, the situation can be
quite different. In 32 bit memory mode, does the processor have four
separate chip selects for each of the bytes in the 32 bit word ? In
this case the non-selected bytes would not consume any extra power
during read.

How is 8 or 16 bit writes implemented in the 32 bit system ? Are there
four separate chip select or write enables for each byte ?  If not, a
8 or 16 bit write would require a 32 bit read, modification of 8 or 16
bits by the memory controller and write back of the whole 32 bit
memory word.

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At least some old DRAM systems required quite a lot of average power
(mainly due to frequent refreshes) when the system was required to
operate at high temperatures. Check for the requirements on current
products at extreme temperatures.

Paul


Re: Lowest power RAM
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First, a SRAM that is not chipselected only draws standby current.
Second, 4 8 bit chips beside each other also make a 32 bit datapath.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Re: Lowest power RAM
I'm sorry. By 'active high' I meant chipselected. Is there a way to
pull a chipselect low for a larger fraction of time, on an SRAM chip if
the MCU is underclocked?

"Second, 4 8 bit chips beside each other also make a 32 bit datapath"

I know. However my MCU allows 8-bit data paths as well.. for say 1Mx8.
The question was, will using fewer traces make a significant dent in
the power usage?


Re: Lowest power RAM

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The controller does whatever is necessary. You just
have to make sure that unused RAM is not unnecessarily
chipselected.

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It just makes 4 times as many access. You won't gain
much by using an 8bit wide bus in terms of power.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

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