The interrupt flags are cleared when the processor branches to the interrupt service routine ISR. Memory Microcontroller requires a program which is a collection of instructions. This program tells microcontroller to do specific tasks. These programs require a memory on which these can be saved and read by Microcontroller to perform specific operations of a particular task.

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Friday, July 19, Programming the Vintage Intel MCS Microcontrollers I have had a box in my parts collection for a few years that contains a variety of interesting vintage components. The most prominent are the Intel L microcontrollers. These microcontrollers are from the Intel MCS commonly known as the line. They have 64 bytes of RAM and access to bytes of external program memory.

Thankfully they are not the one-time-programmable OTP variety so I am free to put them up to any task. These processors have a copyright date of which puts them at roughly twice my age.

Obviously whoever owned these components prior to me was building some interesting embedded systems. Vintage ICs They are in relatively good shape. I would say that these are socket pulls. They have some signs of prior use adhesive on the quartz windows. These may have also been "development" units. These components coupled with my new EPROM tools were enough for me to bring this vintage processor online. I managed to get the classic blinking LED working as shown on the top trace of my oscilloscope.

Intel L in Action :] Continue reading to see how I did it! I decided that I could just tie the upper address bits to ground and it would work fine. Once I knew that I could successfully load code into program memory I had to find a compiler or assembler read toolchain of sorts. I decided to become friendly with the hardware and stick with assembly. I have used Windows 7 as my development host for this project since the programming tools are running on that platform.

My test program 3 bytes and the output of the assembler. Once I had the program assembled and ready to load, I had to look at the datasheet for more hardware information. This document served as an instruction set manual and helped me determine how to connect the program memory to the processor. This was done to save costs and keep pin count down. Unfortunately this complicates the connection to an external PROM.

I studied this waveform for a couple of hours to determine the best way to interface with the external program memory. I devised that I would need some form of external latch to store the address lines after the bus returned to a floating state. I could use the ALE line to signal when the address was stable and retain that value. Instruction Fetch Waveform I looked through my discrete logic collection and came across a bag of 74LS devices.

I had labelled these "8-Bit 3-State Transparent Latch" at some point in the past. I looked up the datasheet and decided that this device would be suitable. All of this design work was done mentally. I have drawn it up quickly for your benefit. The processor will assert the address that it wishes to read from on the bus. When the address is stable on the outputs, the ALE line will go low. I use this signal to latch the address lines into the 74LS which is otherwise transparent.

The processor then puts the bus into a high impedance floating state. When it is ready to read the byte of data from the program memory it will assert PSEN by lowering it. The processor will raise PSEN when it is finished putting the bus back into in a floating state. This sort of handshaking action will ensure that there is never contention on the bus. A keen reader will spot the error in my drawing. CS should be OE. I validated that the processor was working by observing the state of the address lines.

Since my program is only 3 bytes long only the first two bits of the address lines should change. My hypothesis was correct. The rest of the address lines were idle in a low state. I wrote a slightly more complex program to toggle the state of the output lines and managed to observe a blinking LED!

This blog outlines the process that I took to get scaled up on this very simple vintage processor. Next steps would be to experiment with interrupts. I would like to also toy with the timer that this processor has. Perhaps my favorite discovery throughout this process was the fact that this setup uses mA at 5V. These days we are concerned about getting into the uA range for low power devices.

I am interested in building a clock using Nixie tubes. I may end up using this processor as the brains behind the operation. Posted by.



The will also accept commands to turn indicator LEDs on or off. There are eight registers in each bank. Retrieved 10 February — via Synthfool. The families differ mostly in instruction sets and architecture.


Intel MCS-48



8051 Microcontroller Architecture and Block Diagram with Applications




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