If multiple “triggers” occur for the UART due to many things happening at the same time, this will be invoked through multiple hardware interrupts. As explained earlier, multiple serial communication devices can share the same hardware interrupt. The first two bits Bit 0 and Bit 1 control how many data bits are sent for each data “word” that is transmitted via serial protocol. Also, you can attempt to communicate with older equipment in this fashion where a standard API library might not allow a specific baud rate that should be compatible. Bits 3, 4, and 5 control how each serial word responds to parity information. Finally, when you can’t seem to solve the problems of trying to prevent overrun errors from showing up, you might want to think about reducing the baud rate for the serial transmission. Finally we are moving away from wires and voltages and hard-core electrical engineering applications, although we still need to know quite a bit regarding computer chip architectures at this level.

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There are several causes for this, including that you have the timing between the two computer mismatched.

16550 UART

When it was built, there was only one chip on the motherboard. This comes from other scientific areas like rocket science where delta-vee means a change in velocity. If you are using “no parity” in the setup of the UART, this bit should always be a logical “0”. Similarly numbered devices, with varying levels of compatibility with the original National Semiconductor part, are made by other manufacturers. This is not a mistake but something you need to keep in mind when you are writing an hart service routine.

If you know your computer has a UART, have fun taking advantage of this increased functionality. Bits 6 and 7 describe the trigger threshold value. Keep in mind that these are chip families, not simply the chip part number itself.

UART – Wikipedia

You may even have the number of data bits off, so when errors like this are encountered, check the serial data protocol very closely to make sure that all of the settings for the UART data bit length, parity, and stop bit count are what should be expected. From Wikipedia, the free encyclopedia. The exact details varied based on chip design and other factors too detailed for the current discussion, but the general theory is fairly straightforward.


Just like thethe has evolved quite a bit as well, e. There are 2 pending changes awaiting review. Other operating systems like Linux or MS-Windows use the approach of having a “driver” that hooks into these interrupt handlers or service routines, and then the application software deals with the drivers rather than dealing directly with the equipment.

In a CPU like the or a Pentium, these are the memory areas that are used to directly perform mathematical operations like adding two numbers together. Another thing to keep in mind is that the RS standard only specifies that at least one data bit cycle will be kept a logical “1” at the end of uqrt serial data word in other words, a complete character from start bit, data bits, parity 1650, and stop bits.

How you deal with the device is based on how complex it is and what you are going to be doing.

Serial UART information

It can be, but lets take it one simple little piece at a time. Views Read Latest draft Edit View history.

Compilers often hide these details, because setting up these interrupt routines can be a uatr tricky. When your software is performing an interrupt handler, there is no automated method for the CPU to signal to the chip that you have finished, so a specific “register” in the PIC needs to be set to let the next interrupt handler be able to access the computer system.

On earlier chips you should treat these bits as “Reserved”, and only put a “0” into them.

This seldom, if ever, needs to be tested by an end user, but might be useful for some initial testing of some software that uses the UART. Just as it is possible to identify many of the components on a computer system through just software routines, it is also possible to detect which version or variant of the UART that is found on your computer as well. There are other legacy issues that show up, but fortunately for the chip and serial communications in general this isn’t a concern, unless you happen to have a serial driver that “took advantage” of this aliasing situation.


This is also one of the areas where later versions of the chip have a significant impact, as the later models incorporate some internal buffering of the data within the chip before it gets transmitted as serial data.

The modem accessed in this manner can either be an external modem, or an internal modem that uses a UART as an interface to the computer. The A F version was a must-have to use modems with a data transmit rate of baud.

This is a relatively “new” register that was not a part of the original UART implementation. When you get down to actually using this in your software, the assembly language instruction to send or receive data to port 9 looks something like this:. When you are working with equipment at this level, the electrical engineers who designed the equipment refer to registers that change the configuration of the equipment. We will not cover that topic here.

The Transmit and Receive buffers are related, and often even use the very same memory. If you write some data to this register and it comes back changed, you know that the UART in your computer is one of these two chip models. If you are working with the computer at this level, the goal is to change as little as possible so you don’t cause damage to any other software you are using.