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Tekmos Talks Newsletters

A newsletter for the semiconductor industry

Tekmos Talks 2016

 
Tekmos Talks

A Newsletter for the Semiconductor Industry
December 2016 
 

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Welcome to Tekmos Talks December 2016. This month we will share part II on Block Ram, talk about Tekmos' MCT Handler, and share Inside Tekmos Fall Camp Out.

From the Desk of the President, Lynn Reed

 

20141210 Lynn 111 
        Lynn Reed, President

 

Dual Port RAM Design - The Bit Design Goals

It can be asked why design a dual port memory bit? Is this not a case of re-inventing the wheel? Not necessarily. Most memories are designed with speed being the main design goal. You achieve speed by limiting that range of voltages and temperatures that the memory will operate over. In our case, our main goals are reliability and robustness. We serve military, medical, and industrial customers whose applications frequently push the voltage and temperature limits. We do not want the memory to be the weak link in the design.

RAM design begins with the bit. It is amazingly complex for a design that has only three types of transistors. The bit consists of a pair of cross coupled inverters, and a pair of N-channel transmission gates that are connected to the bit lines. In the case of a dual port memory, there are two pairs of transmission gates.

The first requirement for the bit is that all transistors be as small as possible. This bit is replicated millions of times, so the bit area must be minimized. The second requirement is that the bit must be writable. The transmission gate transistor must be strong enough to pull the inverter output low enough so that the bit toggles. A third requirement is that the bit must be readable. That is that the transmission gate must be weak enough so that the bit line capacitance does not cause the bit to toggle when being read. Or more specifically, approach a meta stable value that would allow noise to easily toggle the bit. A fourth requirement specifically for the dual-port bit is that any operation on one port on a specific bit cannot interfere with the operation of the other port on that same bit. And finally, the bit must work over all combinations of voltage, temperature and process variations. This particular technology uses a 1.8-volt supply. Still, we would like for the memory to work with supplies of 1.5 and 1.2 volts, and over temperature ranges from -65ºC to +150ºC. And we will try to get it to work up to 200ºC.

For convenience, we will refer to the inverter transistors as "P" and "N", and the transmission gate N-channel transistor as "T".

Writing the Bit

The worst case for writing a bit occurs when we are trying to toggle the bit while the other port is reading it. The "T" that is setting the bit must overcome both the "P" pulling up, and the other "T" that is attached to the precharged bit line. For simulation purposes, the bit line can be assumed to be tied to Add. In reality, the bit line is a large capacitor that is precharged to Vdd. Assuming that it is directly connected to Vdd is a worst-case assumption, and if the bit works with that assumption, then there will never be any problems with using the bit design in larger memories.

Reading the Bit

The danger in reading the bit is that the "T "devices will pull the low node in the memory up to a point where it can switch state in the presence of noise. The worst case occurs when both ports are reading the same memory bit. You have two "T" s pulling up and one "N" pulling down. Also, the stronger the "N" device, the lower the switching threshold, which makes the situation worse. Increasing the strength of the "P" raises the threshold, but makes it harder to write to the memory. There are no free lunches in nature.

Running Spice

We run spice to determine the optimum sized for the "P", "N", and "T" transistors. There are an almost infinite number of choices in transistor sizes, and there is not enough time to check them all. So, the best approach is to start with a previous solution, and then vary individual transistors until an optimum is reached. Because individual processes and process nodes vary from each other, each process will have its own optimum device size. The layout also introduces limitations on device sizes. To achieve a given W/L ratio, one can vary either the width or the length. Normally, the length is the process minimum, but sometimes it is better to modify the length, and allow the width to be set by layout concerns.

The Layout

Once the transistor sizes are known, the layout can be competed. The best bit layout was determined a long time ago, so it is a matter of adjusting the sizes and spacings to match your particular process. As an option, you can introduce a little bit of asymmetry into the layout diffusions so that the bit will tend to power up in a known state. This is not required, but is a nice feature.

Along with the bit, we also design a dummy bit, whose output is always a zero. This will be used in the self-timing logic. We also have to design a set of substrate contacts that will be inserted every 8th bit. This saves space as contrasted to including them in every bit.

Future articles will discuss the word line driver, the sense amps, and the RAM timing controls.

MCT Handler

 

MCT Handler

handlerTekmos sells many micro controllers and Flash memories that are in PLCC packages. These mature packages are not being supported by modern handlers, and so we are fortunate to have acquired both a number of MCT handlers, and production kits for packages sizes ranging from 20-pin PLCC up to 84-pin PLCC packages.

While old, the handler is capable of both hot and cold operation, and can test parts at a 1800 uph rate. The handler has a clever mechanism that reverses the part orientation during test. Parts that are fed into the handler with pin 1 first, and leave the handler with pin 1 last. This allows a tube of continuity rejects to be easily retested.

We also use this handler in our programming operations. We pre-program about half of our micro controllers, and to do so, we use the combination of our production tester and the MCT handler.

The MCT handlers are just another capital investment Tekmos has made in order to continue to support mature products.

Inside Tekmos

 

Tekmos Fall Camping Adventure

campTekmos Team Members returned to Oxford Ranch in the beautiful Texas Hill Country the first weekend of November. Lynn and Phil arrived first to set up the Tekmos pavilion with a help from You Tube as it was the first time for the two of them to set it up. Mission accomplished. Jon, Kelsey, Allan and Tyler arrived later and set up their tents and sat around the fire visiting until turning in. During the night rustling of a band of wild boars were heard nearby digging in the dirt for food. No one seemed too alarmed, not even furry companions.

dogThe following morning Jon, Kelsey, Allan and Tyler made their way to Enchanted Rock for a hike on the trails and a crawl through the caves. Enchanted Rock is such a popular destination that it closes its gates several times a day due to overcrowding. It is a massive 425' pink granite dome with 11 miles of hiking. Humans have been camping there for over 12,000 years and more than 400 archaeological sights have been discovered there.

caveOther Tekmos Team Members, contractors and family members arrived in the afternoon for a few vigorous games of badminton and steaks grilled over a fire that evening.

Thank You for Reading Tekmos Talks

 

Thank you for reading Tekmos Talks and helping us celebrate 20 plus years.

Sincerely,

Lynn Reed, President

 

 

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