Forum Discussion
Honored Contributor5V to 3.3V bus and vice versa
MC68000 to CycloneII
There is something wrong with quickswich devices. As I know MC68000 cant support 3.3V (some versions can but mine just cant). When i use quickswich any type i only get 5V to 3.3V translation and no vice versa. I need to use dual voltage level translator with auto sensing. Only parts that I can found with this is txs0108e even sn74lvc8t245 have direction control. For last two days I m reading about Pericom articles and those articles are just like IDT have for quickswich part. Devices are working from higher to lower and I m thinking that I just cant use those devices. For some devices who work with 5V can drive lower voltage levels that is ok but on my situation is not. For example PI5CX and PI3CX series are only related to 5V to 3.3V translation no bidirectional and then i need to use bus enable input like on most devices. From A to B bus or from B to A bus, that is not practical. Also some Pericom devices PI74LVC4245 who have two voltage on each bus side but no auto sensing again I need to set direction sides. I just dont know, as much i m reading only thing is txs0108e that I m using on design right now. Also using Pericom or IDT devices we can use pull up resistor connected to higher power source. I just dont know....17 Replies
Altera_ForumWhat about these TI parts?
http://www.ti.com/lit/ds/scds305/scds305.pdf I didn't look, but there are probably 16-bit versions. Cheers, Dave- Altera_Forum
Hm thank you I ll check that, but basic idea is to cyclone II take over the mc68000 bus, and it should be fastest as it can be. PI5C34X245 is the device I would like to use, because it has 32 bus and for replacing of MC i could use only two of those devices. In Pericom manual says that PI5C is 5v=>3.3v but no vice versa. I just dont understand why quickswich devices cant work <=> their names says that they can.
- Altera_Forum
Are the signals going from the Cyclone II to the MC68000 'fast'?
If they are 'slow', then can you consider putting pull-ups to 5V on the signals (bus pull-ups)? The Cyclone II device can then drive high (to Voh ~ 3.3V), and then tri-state the output driver, the pull-up can then take the signal over the last few volts to 5V. 5V I/O clamps can then be used on the Cyclone II I/O to protect it from the 5V signals (see the DE2 board GPIOs). Cheers, Dave - Altera_Forum
I use about 60% of logic elements of EP2C5T144C so i cant use anything below that.
- Altera_Forum--- Quote Start --- Yes that is the problem. I know about solution that you proposed but it is to complicated. I must find one device who can perform all of that. I just dont have the space for that on the board. But thank you for your response. --- Quote End --- How much logic do you need in the FPGA, and what is it doing? If you do not need too much logic in the FPGA, then use a 5V device. The older generation FLEX10KA FPGAs are 5V tolerant, and are still available (I have boards that use them). What about a MAX CPLD? Cheers, Dave
- Altera_Forum
Yes that is the problem. I know about solution that you proposed but it is to complicated. I must find one device who can perform all of that. I just dont have the space for that on the board. But thank you for your response.
- Altera_Forum
Just to clarify; the problem is that the MC68000 input threshold voltages are 5V HCMOS compatible, but not 3.3V compatible?
You could use two buffers; a bus switch between the MC68000 bus, so that the Cyclone II device sees 3.3V signals, and then a 5V buffer from the Cyclone II to the MC68000. When the MC68000 performs a bus access that corresponds to the Cyclone II, the bus switch will route the control signals from the MC68000 to the FPGA, and then the Cyclone II can drive the response via the 5V buffer (which would have 3.3V compatible inputs). Cheers, Dave
