May 20th, 2015 ~ by admin

TI TMS7000: The SCAT Microcontroller

TI TMX70P81 - Early 8K Prototype. Never released

TI TMX70P81 – Early 8K Piggyback Prototype. Never released

The 1980’s brought many 8-bit microcontrollers to the market, such famous designs as the Intel MCS-51, the Zilog Z8, and the Motorola MC680x.  There were many others as well, including TI’s entry into the market.  After the race into the market with one of the first microcomputers, the 4-bit TMS1000, and the top of the line TMS9900 16-bit processor, TI saw the need to fill in the middle, the 8-bit market.  TI didn’t want to make the 7000 series just another 8-bit MCU either, they wanted something different, not so different as to be eccentric, but something to set them apart.  They did so with an innovation they called SCAT.

TMS7000 SCAT Layout. Notice the 'strips' that form the different sections of the MCU (click to enlarge)

TMS7020 (2K EPROM + 128 bytes RAM) SCAT Layout. Notice the ‘strips’ that form the different sections of the MCU (click to enlarge)

SCAT, Strip Chip Architecture Topology, was TI’s die layout design for the TMS7000.  Instead of generating each of the blocks for the chip (SLU, ROM, RAM, etc) making them as small as possible, and then using random logic to tie them all together, they laid them out in strips on the die.  The ROM in a strip, the RAM in a strip, and the ALU etc in another.  This allowed the sections to be wired up with a minimum of random logic, resulting in a smaller die, that was also easier to test.  More importantly it allowed the TMS7000 to be easily expanded.  Adding more ROM, or RAM didn’t require redoing the entire layout, it was just added to its respective ‘strip’.

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March 6th, 2011 ~ by admin

CPU of the Day: NS87P50R-6: Piggyback CPUs

National Semiconductor NS87P50D-11

National Semiconductor NS87P50R-6

In the 1980’s most high-volume microcontrollers were OTP (one-time-programmable) or were factory programmed (Mask ROM).  This made developing code for them a bit tricky.  Some companies made lower volume version with an onboard EPROM, such as the Intel 8751.  Other designs this was not practical so another solution had to be found.

The most common solution became the ‘piggyback’ package.  The CPU would reside on a ceramic (pictured on the left) or organic (on the right) package that had a socket on top of it for an EPROM.  This provided an easy way to develop code for the processor, and EPROMs could be stopped out and erased at will.  Obviously these ‘piggyback’ parts were not intended for production use, their cost would be much to high for that.  They were made in relatively small quantities solely for engineering and prototype work.

This National Semiconductor NS87P50R-6 is a 6MHz MCU.  It includes a 24-pin socket on top that supports up to a 32k EPROM (2758, 2716 or 2732).  The other group of 4 pins on top are yet another feature.  It would be cost prohibitive to make a separate development device for each member of the MCU family so the 87P50 can be told to emulate several.  It can emulate a 8048, 8049, or if all jumpers are removed, the 8050. (The only difference in these is the RAM size, 64bytes, 128bytes, or 256bytes for the 8050).  The NS87P50R-6 is in an organic package, the die is actually placed directly on a circuit board, and covered in a black epoxy.  This is rather less expensive then the NS87P50D-11 ceramic and gold version, though is not as tolerant to heat.

If you have ever taken apart a cheap consumer electronic device, you will likely find a black ‘blob’ on the circuit board.  Thats a die, and usually the microcontroller of that device.  ID’ing it is next to impossible without acid and a microscope however.

National Semiconductor was not the only company to use this type of design.  Zilog and Synertek used it for the Z8 series, Hitachi for the HD6301, Mostek for the 3870 and most all other companies that made a MCU int he 1980’s.