UncleBinary
Experienced Member
How do I interface both of those without losing any memory capacity?plz include some sort of picture.
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Thrashbarg
Experienced Member
Chip Count:
1) Z80
2) 62256 SRAM
3) 28C256 EEPROM
4) 74LS32 (memory access decoder)
5) 74LS04 (RAM/ROM decoder)
6) 74LS138 (I/O decoder)
7) 8250 UART (serial port)
8) MAX232 (TTL to RS232 voltage converter)
Looks good, though I may have missed out on one. I included the 8250 UART so you can get a serial port, you want to be able to see some output.
This is as simple as a computer can get, trying to get it under 8 chips and still maintain functionality is tricky, unless you want a traffic light controller.
There is 64K of address space on the Z80, in this basic configuration there is 32K ROM and 32K RAM which fills it. There is still room for I/O devices on the 256 byte port address space. Notice on the Z80 pinouts how there are MEMRQ and IORQ lines, these are for Memory Request and I/O Request, which means that the I/O ports are addressed separately from memory.
1) Z80
2) 62256 SRAM
3) 28C256 EEPROM
4) 74LS32 (memory access decoder)
5) 74LS04 (RAM/ROM decoder)
6) 74LS138 (I/O decoder)
7) 8250 UART (serial port)
8) MAX232 (TTL to RS232 voltage converter)
Looks good, though I may have missed out on one. I included the 8250 UART so you can get a serial port, you want to be able to see some output.
This is as simple as a computer can get, trying to get it under 8 chips and still maintain functionality is tricky, unless you want a traffic light controller.
There is 64K of address space on the Z80, in this basic configuration there is 32K ROM and 32K RAM which fills it. There is still room for I/O devices on the 256 byte port address space. Notice on the Z80 pinouts how there are MEMRQ and IORQ lines, these are for Memory Request and I/O Request, which means that the I/O ports are addressed separately from memory.
UncleBinary
Experienced Member
Thanks thrashbarg but it would be easier and less time consuming if I had a picture or at least a discription of what pin to what pin.And how does a hex inverter (the RAM/ROM decoder) become a decoder.Wouldn't be simpler just to use address pin 15 as part of the SRAM logic and attack a NOT gate to address pin 15 and attack that to EEPROM logic?Sorry im sleepy.
Last edited:
Thrashbarg
Experienced Member
A hex inverter is a NOT gate. Hex because there are 6 devices in the one package.
Here's a schematic. I didn't put in the 8250 UART because they're a pain to get going. If you program the lights to blink and they do then you know it's working and you can continue from there.
http://kaput.homeunix.org/~thrashbarg/Z80SBC.png
The crystal oscillator isn't the best but it'll do. To make sure it's working you can use a frequency counter or an oscilloscope. If you don't have one then you can use an LED. Attach the anode to pin 8 of the inverter and the cathode to ground. If it lights then we're half way there. Attach the anode to +5v and the cathode to pin 8. If it lights again then it's working. This is because when a TTL device is in a logic 1 state it's effectively attached to +5v and in a 0 state 0v. The oscillator is too fast to see the blinking but if it lights when you connect it both ways then you know it's in both states and thus oscillating.
The reset circuit works by charging the capacitor through the resistor slowly. When the voltage on the reset pin reaches the TTL threshold (I think about 4V) the CPU will come out of the reset state and start executing code at address 0000h (the ROM). When you press the reset button it discharges the capacitor and the reset pin will be in a low state until the capacitor charges again. This delay is necessary to allow enough clock cycles to get to the CPU while it's in the reset state so it resets correctly.
Any questions?
Here's a schematic. I didn't put in the 8250 UART because they're a pain to get going. If you program the lights to blink and they do then you know it's working and you can continue from there.
http://kaput.homeunix.org/~thrashbarg/Z80SBC.png
The crystal oscillator isn't the best but it'll do. To make sure it's working you can use a frequency counter or an oscilloscope. If you don't have one then you can use an LED. Attach the anode to pin 8 of the inverter and the cathode to ground. If it lights then we're half way there. Attach the anode to +5v and the cathode to pin 8. If it lights again then it's working. This is because when a TTL device is in a logic 1 state it's effectively attached to +5v and in a 0 state 0v. The oscillator is too fast to see the blinking but if it lights when you connect it both ways then you know it's in both states and thus oscillating.
The reset circuit works by charging the capacitor through the resistor slowly. When the voltage on the reset pin reaches the TTL threshold (I think about 4V) the CPU will come out of the reset state and start executing code at address 0000h (the ROM). When you press the reset button it discharges the capacitor and the reset pin will be in a low state until the capacitor charges again. This delay is necessary to allow enough clock cycles to get to the CPU while it's in the reset state so it resets correctly.
Any questions?
UncleBinary
Experienced Member
I could have thought of that.I am almost done with the schematics I just have to get a UART or a USRT on it to get a graphical interfeace up and a keyboard.I'll show you when im done to see if I got it right.
UncleBinary
Experienced Member
Quick question before I finish the schematics.If I use a 138 decoder to interface 2 PIO's do I need a buffer or will the clock and CPU signals be sufficent?
Thrashbarg
Experienced Member
I've not needed to use a buffer for the '138. Just attach it to the address bus and to the /CS pins of the devices.
UncleBinary
Experienced Member
You dont get what im asking.When attacking the pins needed to synchronise the PIO's do they have to be buffered to split them and to to match the voltage requirements?
Thrashbarg
Experienced Member
You're right. Maybe you should ask better questions.
UncleBinary said:
Huh? Do you need to synchronize PIO's? Why? What voltage requirements? Be more specific.
I think thread has gone on long enough - up to seven pages now. It is reading more like somebody getting very elaborate instructions for a homework assignment.
Thashbarg has been very supportive, and I'd like to thank him for that. But let's not wear him out.
UncleBinary - hit the textbooks/datasheets again, and take it offline. Let us know when you are done and how it came out.
Thashbarg has been very supportive, and I'd like to thank him for that. But let's not wear him out.
UncleBinary - hit the textbooks/datasheets again, and take it offline. Let us know when you are done and how it came out.
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