Frequencies and periods or components used in PCs
Sometimes it is easier to work in frequency, sometimes in cycle time (where one is the reciprocal of the other) but because the rates of the elements used in computers vary by orders of magnitude it can be difficult mentally to associate one with the other. The list below is a ready-reckoner of frequency against period for some sample PC components.
Numbers are generally approximations given to two or three significant figures. They are intended not to be highly detailed but to be easy to grasp.
As examples of use, if you wanted to know how long the bus of an early PC took per cycle check the 4.77 MHz entry and read off 210 nanoseconds; and if you had to process one byte from a serial port running at 115,200 baud (approximately 11,520 bytes per second, depending on number of stop bits and other things) check 11.52 kHz and read off 87 microseconds or 87,000 nanoseconds. All tables include nanoseconds in order to provide a common time unit.
This table relates frequencies measured in Hz to milliseconds.
|18||55||55,000,000||Time for a PC's PIT to cycle through 65536 counts|
|960||1||1,000,000||Approx one byte over a serial port running at 9600 baud|
This table relates frequencies measured in kHz to microseconds.
|9.6||104||104,000||One bit over a serial port running at 9600 baud|
|11.52||87||87,000||Approx one byte over a serial port running at 115200 baud|
|1000||1||1,000||Approx one byte over 10Mbit Ethernet|
This table relates frequencies measured in MHz to nanoseconds. Many of the entries are useful for CPU or bus speeds. To relate these to data transfer rates bear in mind that many bus systems can transfer multiple bytes per cycle.
|1.19||840||PC's 8253 or 8254 PIT master clock tick|
|4.77||210||Early PC CPU and bus speed|
|10||100||One bit-time on early Ethernet|
|12||83||Early 80386 models|
|50||20||486 and early Pentium models|