CPU clock should be divided to get 8.0 to 8.33 MHz ISA clock rate. There are no proper divider for some CPU clock rates so i was wondering what ISA clock rate is when following CPUs are used:
286 10 MHz: 5 or 10 MHz ISA?
286 12 MHz: 6 or 12 MHz ISA?
286/386 20 MHz: 6.7 or 10 MHz ISA?
The boards have an independent oscillator (14.31818 MHz) for that clock. Divided by 2 to get 7.15 MHz.
1+1=10
But of course. Many old bioses have option for 7.15 MHz ISA clock.
wrote:The boards have an independent oscillator (14.31818 MHz) for that clock. Divided by 2 to get 7.15 MHz.
ISA provided a straight 14.31818MHz signal on its OSC pin, and usually a 4.77MHz signal on an 8-bit CLK pin and up to an 8.33MHz signal on a 16-bit CLK pin.
It is important to remember that 8-bit ISA is an extension of the 8088 bus, while 16-bit ISA is an extension of the 80286 bus. ISA gives the board manufacturer direct access to the memory and I/O busses on systems with those processors. Later systems accessed ISA through bus controller interfaces.
I think in 286 systems there isn't really a standard method for determining the ISA bus frequency. Obviously it's better to have one of the later 286 boards where you could adjust the CPU divider in the BIOS.
I have heard that the ISA bus in the original IBM AT runs at 8MHz regardless of the speed of the CPU, but I am not so sure about that. If somebody has a bustest utility I could test this theory. What I do know for certain is that there exist badly designed 10 and 12MHz AT clone motherboards that run the ISA at the same speed as the CPU. 10MHz is easier to pull off on 8-bit cards, but 16-bit cards like SCSI and soundcards can have problems at 10MHz. 12MHz is really asking for trouble.
Finally, I don't know much about what the OSC signal is used for, but what I do know is that it not necessarily used for determining the operating frequency of the bus. Many ISA cards do not even use the OSC signal. But, I think I have seen some 486 motherboards that are capable of setting the ISA bus according to OSC. It's definetly not mandatory.
> Finally, I don't know much about what the OSC signal is used for
I.e. EGA cards use it as their pixel clock.
1+1=10
wrote:> Finally, I don't know much about what the OSC signal is used for
I.e. EGA cards use it as their pixel clock.
CGA cards use it for their NTSC frequency reference.
wrote:I have heard that the ISA bus in the original IBM AT runs at 8MHz regardless of the speed of the CPU, but I am not so sure about that.
The ISA bus in the original AT runs at either 6MHz or 8MHz depending on the speed of the 80286 CPU.
Ok, I've got a Packard Bell 286 computer running the CPU at 10mhz in high speed, and there's an option for the CPU to run at 8mhz (low speed).
What would the bus speed be for each of the CPU speeds mentioned?
Thanks.
