Rules of Serial
The asynchronous serial protocol has a number of built-in rules - mechanisms that help ensure robust and error-free data transfers. These mechanisms, which we get for eschewing the external clock signal, are:
- Data bits,
- Synchronization bits,
- Parity bits,
- and Baud rate.
Through the variety of these signaling mechanisms, you’ll find that there’s no one way to send data serially. The protocol is highly configurable. The critical part is making sure that both devices on a serial bus are configured to use the exact same protocols.
The baud rate specifies how fast data is sent over a serial line. It’s usually expressed in units of bits-per-second (bps). If you invert the baud rate, you can find out just how long it takes to transmit a single bit. This value determines how long the transmitter holds a serial line high/low or at what period the receiving device samples its line.
Baud rates can be just about any value within reason. The only requirement is that both devices operate at the same rate. One of the more common baud rates, especially for simple stuff where speed isn’t critical, is 9600 bps. Other “standard” baud are 1200, 2400, 4800, 19200, 38400, 57600, and 115200.
The higher a baud rate goes, the faster data is sent/received, but there are limits to how fast data can be transferred. You usually won’t see speeds exceeding 115200 - that’s fast for most microcontrollers. Get too high, and you’ll begin to see errors on the receiving end, as clocks and sampling periods just can’t keep up.
Framing the data
Each block (usually a byte) of data transmitted is actually sent in a packet or frame of bits. Frames are created by appending synchronization and parity bits to our data.
A serial frame. Some symbols in the frame have configurable bit sizes.
Let’s get into the details of each of these frame pieces.
The real meat of every serial packet is the data it carries. We ambiguously call this block of data a chunk, because its size isn’t specifically stated. The amount of data in each packet can be set to anything from 5 to 9 bits. Certainly, the standard data size is your basic 8-bit byte, but other sizes have their uses. A 7-bit data chunk can be more efficient than 8, especially if you’re just transferring 7-bit ASCII characters.
After agreeing on a character-length, both serial devices also have to agree on the endianness of their data. Is data sent most-significant bit (msb) to least, or vice-versa? If it’s not otherwise stated, you can usually assume that data is transferred least-significant bit (lsb) first.
The synchronization bits are two or three special bits transferred with each chunk of data. They are the start bit and the stop bit(s). True to their name, these bits mark the beginning and end of a packet. There’s always only one start bit, but the number of stop bits is configurable to either one or two (though it’s commonly left at one).
The start bit is always indicated by an idle data line going from 1 to 0, while the stop bit(s) will transition back to the idle state by holding the line at 1.
Parity is a form of very simple, low-level error checking. It comes in two flavors: odd or even. To produce the parity bit, all 5-9 bits of the data byte are added up, and the evenness of the sum decides whether the bit is set or not. For example, assuming parity is set to even and was being added to a data byte like
0b01011101, which has an odd number of
1’s (5), the parity bit would be set to
1. Conversely, if the parity mode was set to odd, the parity bit would be
Parity is optional, and not very widely used. It can be helpful for transmitting across noisy mediums, but it’ll also slow down your data transfer a bit and requires both sender and receiver to implement error-handling (usually, received data that fails must be re-sent).
9600 8N1 (an example)
9600 8N1 - 9600 baud, 8 data bits, no parity, and 1 stop bit - is one of the more commonly used serial protocols. So, what would a packet or two of 9600 8N1 data look like? Let’s have an example!
A device transmitting the ASCII characters ‘O’ and ‘K’ would have to create two packets of data. The ASCII value of O (that’s uppercase) is 79, which breaks down into an 8-bit binary value of
01001111, while K’s binary value is
01001011. All that’s left is appending sync bits.
It isn’t specifically stated, but it’s assumed that data is transferred least-significant bit first. Notice how each of the two bytes is sent as it reads from right-to-left.
Since we’re transferring at 9600 bps, the time spent holding each of those bits high or low is 1/(9600 bps) or 104 µs per bit.
For every byte of data transmitted, there are actually 10 bits being sent: a start bit, 8 data bits, and a stop bit. So, at 9600 bps, we’re actually sending 9600 bits per second or 960 (9600/10) bytes per second.
Now that you know how to construct serial packets, we can move on to the hardware section. There we’ll see how those 1’s and 0’s and the baud rate are implemented at a signal level!