Frequencies And Periods

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.

## Hz

This table relates frequencies measured in Hz to milliseconds.

Hz ms ns Examples
10 100 100,000,000 Potential timeslice
18 55 55,000,000 Time for a PC's PIT to cycle through 65536 counts
100 10 10,000,000 Potential timeslice
960 1 1,000,000 Approx one byte over a serial port running at 9600 baud
1000 1 1,000,000 Potential timeslice

## KHz

This table relates frequencies measured in kHz to microseconds.

kHz us ns Examples
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
32.768 30 30,000 PC's RTC
1000 1 1,000 Approx one byte over 10Mbit Ethernet

## MHz

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.

MHz ns Examples
1.19 840 PC's 8253 or 8254 PIT master clock tick
4.77 210 Early PC CPU and bus speed
6 167
8 125
10 100 One bit-time on early Ethernet
12 83 Early 80386 models
16 62.5
25 40
33 30
40 25
50 20 486 and early Pentium models
66 15
100 10
133 7.5
166 6
200 5
400 2.5
800 1.25
1000 1
2000 0.5
4000 0.25
page revision: 0, last edited: 04 Nov 2014 15:31