Boosting STFM: 16MHz bus, 64MHz Shifter
Posted: Thu Oct 05, 2017 5:05 pm
edit: A long prosaic writeup of all the steps can be found here: https://blog.troed.se/projects/atari-st ... deo-modes/
Since a few months back I've slowly been modifying one of my 1040 STFMs with the "Stefan Nitschke 16MHz mod". I got the inspiration from an Atari-Forum thread by exxos where he showed the interesting display you got if you overclocked the Shifter.
Stefan details two ways to do the mod:
1) Take the 32MHz clock and feed it to the MMU instead of the regular 16MHz clock it gets from the Shifter. Use the new 16MHz output from the MMU to feed the CPU, and downclock it back to 8MHz to GLUE/therest. Also, AND the DE signal with 2MHz so that the new faster MMU doesn't feed more data to the Shifter than it can handle.
This gives you twice the RAM speed and twice the CPU speed. Not a bad boost.
2) Add a new 64MHz oscillator and feed it to the Shifter instead of its usual 32MHz. The MMU will get 32MHz from the Shifter, and then do as above - except this time don't AND 2MHz to DE. Since the Shifter is running at twice the speed it can handle MMU feeding it twice as fast.
This will give you the same RAM and CPU boost - but will also double the horisontal resolution. This is my main interest. 640*200 in 16 colors, 1280*200 in 4 and 1280*400 in mono.
To do the mod I have swapped out the RAM in my machine for exxos' 4MB SIMM kit, as well as the dual fast TOS. The CPU has been swapped out for a 16MHz version. The motherboard is of a later rev with SMT versions of the MMU and GLUE.
edit: Latest status is that it's working - see page 3 of this thread.
Former latest status is that the machine boots nicely and the diagnostic cart runs through RAM and ROM tests without issues. Timer tests fail at T5 (PSG). The floppy drive is unable to read any disks - but it can itself format fine from the desktop and create a folder. Such a formatted disk can be written to by another ST, but the disk cannot be read back by the boosted machine.
Currently I'm using the 8Mhz (now 16) from the MMU to the CPU, and the 4MHz (now 8 MHz) to the GLUE/therest. Additionally I'm using a 74LS74 to downclock the 8MHz clock to 4MHz for the MFP. I'm not really sure what's causing the floppy issues since everything else seems very stable.
Bad photo, but you can see the new oscillator on the Shifter, the 74LS74 MFP clock downclock, the fast TOS etc. Also my logic analyzer which I used to verify that the 4MHz clock indeed works ...
Since a few months back I've slowly been modifying one of my 1040 STFMs with the "Stefan Nitschke 16MHz mod". I got the inspiration from an Atari-Forum thread by exxos where he showed the interesting display you got if you overclocked the Shifter.
Stefan details two ways to do the mod:
1) Take the 32MHz clock and feed it to the MMU instead of the regular 16MHz clock it gets from the Shifter. Use the new 16MHz output from the MMU to feed the CPU, and downclock it back to 8MHz to GLUE/therest. Also, AND the DE signal with 2MHz so that the new faster MMU doesn't feed more data to the Shifter than it can handle.
This gives you twice the RAM speed and twice the CPU speed. Not a bad boost.
2) Add a new 64MHz oscillator and feed it to the Shifter instead of its usual 32MHz. The MMU will get 32MHz from the Shifter, and then do as above - except this time don't AND 2MHz to DE. Since the Shifter is running at twice the speed it can handle MMU feeding it twice as fast.
This will give you the same RAM and CPU boost - but will also double the horisontal resolution. This is my main interest. 640*200 in 16 colors, 1280*200 in 4 and 1280*400 in mono.
To do the mod I have swapped out the RAM in my machine for exxos' 4MB SIMM kit, as well as the dual fast TOS. The CPU has been swapped out for a 16MHz version. The motherboard is of a later rev with SMT versions of the MMU and GLUE.
edit: Latest status is that it's working - see page 3 of this thread.
Former latest status is that the machine boots nicely and the diagnostic cart runs through RAM and ROM tests without issues. Timer tests fail at T5 (PSG). The floppy drive is unable to read any disks - but it can itself format fine from the desktop and create a folder. Such a formatted disk can be written to by another ST, but the disk cannot be read back by the boosted machine.
Currently I'm using the 8Mhz (now 16) from the MMU to the CPU, and the 4MHz (now 8 MHz) to the GLUE/therest. Additionally I'm using a 74LS74 to downclock the 8MHz clock to 4MHz for the MFP. I'm not really sure what's causing the floppy issues since everything else seems very stable.
Bad photo, but you can see the new oscillator on the Shifter, the 74LS74 MFP clock downclock, the fast TOS etc. Also my logic analyzer which I used to verify that the 4MHz clock indeed works ...