== 8255 Part numbers used in the CPC during its lifetime ==
* NEC D8255AC-2
* NEC D8255AC-5
* Toshiba TMP8255AP-5
All of these are almost identical in their operation. It is possible to detect each version by writing and then reading from the ppi control i/o port. Each can give a different pattern of values that are read back.
== The 8255 in the CPC ==
The [[8255 PPI chip]] is a general purpose input/output IC. This document will describe it's its role in the Amstrad CPC,CPC+ and KC compact systems. To understand it's its full functions please read the datasheet.
In these systems it is connected to the AY-3-8912 Programmable Sound Generator (PSG), keyboard, cassette recorder, the VSYNC of the 6845 CRTC and the "busy" signal from the parallel port.
The PPI is selected when bit 11 A11 of the I/O port address is set to "0", bits 9 A9 and 8 A8 then define the PPI function access (as shown below), A15-A12 and A10 should be "1" (to prevent conflicts with other hardware), A7-A0 are don't care. So, resulting Port addresses are:
{|{{Prettytable|width: 700px; font-size: 2em;}}
|''Bit 9I/O address''||''Bit 8A9''||''A8''||''Description''||''Read/Write status''||''Used Direction''||''Used for''
|-
|&F4xx||0||0||Port A Data||Read/Write||In/Out||[[PSG]] (Sound/Keyboard/Joystick)
|-
|&F5xx||0||1||Port B Data||Read/Write||In||Vsync/Jumpers/PrinterBusy/CasIn/Exp
|-
|&F6xx||1||0||Port C Data||Read/Write||Out||KeybRow/CasOut/PSG
|-
|&F7xx||1||1||Control||Write Only||Out||Control
|-
|}
It Group Mode 1 (Strobed Input/Output) and Group Mode 2 (Bi-Directional Bus), as far as I know, are not used by any program. Group Mode 0 (Basic Input/Output) is advised that always used. In the remaining bits of CPC+, the 8255 is integrated into the ASIC. The "emulation" is not complete and some functionality is not available. Please see the [https://cpctech.cpcwiki.de/docs/cpcplus.html Extra CPC+ documentation] for more information. == Port Usage == * NOTE - If you are using the firmware, always return the operating modes and I/O address are set state of the ports used to "1" to avoid conflict with other devicestheir settings below. The recommended firmware expects the settings to be the same as given below and may operate incorrectly if they are not. == PPI Port A == Operating system settings:* I/O addressess are: Mode 0,* For writing data to PSG all bits must be set to output,* for reading data from PSG all bits must be set to input (thereafter, output direction should be restored, for compatibility with the BIOS).
{|{{Prettytable|width: 700px; font-size: 2em;}}
|''I/O addressBit''||''PPI FunctionDescription''||''Usage''
|-
|&F4xx7-0||Port A dataPSG.DATA||[[PSG]] Databus (Sound/Keyboard/Joystick)
|-
|&F5xx||} == PPI Port B data== Operating system settings: * I/O Mode 0,* Input {|{{Prettytable|width: 700px; font-size: 2em;}}|''Bit''||''Description''||''Usage in CPC''||''Usage in KC Compact''
|-
|&F6xx7||Port C CAS.IN||Cassette datainput||Same as on CPC
|-
|&F7xx6||ControlPRN.BUSY||Parallel/Printer port ready signal, "1" = not ready, "0" = Ready||Same as on CPC|-|5||/EXP||Expansion Port /EXP pin||Same as on CPC|-|4||LK4||Screen Refresh Rate ("1"=50Hz, "0"=60Hz)||Set to "1"=50Hz (but ignored by the KC BIOS, which always uses 50Hz even if LK4 is changed)|-|3||LK3|| rowspan="3" |3bit Distributor ID. Usually set to 4=[[Awa]], 5=[[Schneider]], or 7=[[Amstrad]], see [[LK-selectable Brand Names]] for details.||Purpose unknown (set to "1")|-|2||LK2||Purpose unknown (set to "0")|-|1||LK1||Expansion Port /TEST pin|-|0||[[CRTC]] VSYNC||Vertical Sync ("1"=VSYNC active, "0"=VSYNC inactive)||Same as on CPC
|-
|}
In * LK1-4 are links on the CPC+, the 8255 is integrated into the ASIC. The mainboard ("emulation0" is not complete and some functionality is not availablebits are wired to GND). Please see On CPC464,CPC664,CPC6128 and GX4000 they are labeled LK1-LK4, on the "Extra CPCCPC464+ documentation" for more informationand CPC6128+ they are labeled LK101-LK103 (and LK104, presumably?).
* Mode 1 Bit5 (Strobed Input/OutputEXP) and Mode 2 (Bi-Directional Bus), as far as I know, are not can be used by any programa expansion device to report its presence. "1" = device connected, Mode "0 (Basic Input/Output) " = device not connected. This is not always usedby all expansion devices. ''is it used by any expansions?'' [in the DDI-1 disc interface, /EXP connects to the ROM bank selection, bank 0 or bank 7]
=== Port Usage ===* If port B is programmed as an output, you can make a fake vsync visible to the Gate-Array by writing 1 to bit 0. You can then turn it off by writing 0 to bit 0. It is fake in the sense that it is not generated by the CRTC as it normally is. This fake vsync doesn't work on all CPCs. It is not known if it is dependent on CRTC or 8255 or both.
NOTE
* If you are using the firmware, always return the operating modes For more info on LK1-LK4 (and I/O state of the ports used to their settings below. The firmware expects the settings to be the same as given below and may operate incorrectly if they are not. further LKs) see [[LK Links]]
=== PPI Port A =C ==
Operating system settings:
* upper: I/O Mode 0, lower: I/O mode 0,* Output upper: output, lower: output
NOTE{|{{Prettytable|width:700px; font-size: 2em;}}|''Bit''||''Description''||''Usage''|-|7||PSG BDIR||PSG function selection|-|6||PSG BC1|||-|5||Cassette Write data||Cassette Out (sometimes also used as Printer Bit7, see [[8bit Printer Ports]])|-|4||Cassette Motor Control||set bit to "1" for motor on, or "0" for motor off|-|3||rowspan=4|Keyboard line||rowspan=4|Select keyboard line to be scanned (0-15)|-|2|-|1|-|0|-|}
* For writing data to PSG all bits must be set to output,* for reading data from PSG all bits must be set to input. function selection:
=== PPI Port B ==={|{{Prettytable|width: 700px; font-size: 2em;}}|''Bit 7''||''Bit 6''||''Function''|-|0||0||Inactive|-|0||1||Read from selected PSG register|-|1||0||Write to selected PSG register|-|1||1||Select PSG register|-|}
Operating system settings:== PPI Control ==
* I/O Mode 0,* Input This register has two different functions depending on bit7 of the data written to this register.
=== PPI Control with Bit7=0 ===
If Bit 7 is "0" then the register is used to set or clear a single bit in Port C:
Note: Bit 0 B New value for the specified bit (0=Clear, 1=Set) Bit 1-3 N0,N1,N2 Specifies the number of a bit (0-7) in Port C Bit 4-6 - Not Used Bit 7 SF Must be "0" in this case
1. On CPC464,CPC664,CPC6128 and GX4000 this is LK3 on the PCB. On the CPC464+ and CPC6128+ this is LK103 on the PCB. On the KC compact this is "1"[[File:8255 Control0.png]]
2. On CPC464=== PPI Control with Bit7=1 ===Otherwise,CPC664,CPC6128 and GX4000 this if Bit 7 is LK2 on "1" then the PCB. On the CPC464+ other bits will initialize Group Modes and CPC6128+ this is LK102 on the PCB. On the KC compact this is "0".Ports A-B-Cupper-Clower as Input or Output:
Bit 0 IO-Cl Direction for Port C, lower bits (always 0=Output in CPC) Bit 1 IO-B Direction for Port B (always 1=Input in CPC) Bit 2 MS0 Mode for Port B and Port Cl (always zero in CPC) Bit 3. On CPC464 IO-Ch Direction for Port C,CPC664upper bits (always 0=Output in CPC) Bit 4 IO-A Direction for Port A (0=Output,CPC6128 1=Input) Bit 5,6 MS0,MS1 Mode for Port A and GX4000 this is LK1 on the PCB. On the CPC464+ and CPC6128+ this is LK101 on the PCB. On the KC compact this is /TEST signal from Port Ch (always zero in CPC) Bit 7 SF Must be "1" to setup the expansion port.above bits
4. On the CPC464* CAUTION: Writing to PIO Control Register (with Bit7 set),CPC664automatically resets PIO Ports A,CPC6128B,CPC464+C to 00h each!* In the CPC only Bit 4 is of interest,CPC6128+ and GX4000 all other bits 3,2 and 1 define are always having the manufacturer namesame value. See below In order to write to see the options availablePSG sound registers, a value of 82h must be written to this register. The manufacturer name is defined on In order to read from the PCB and cannot be changed keyboard (through softwarePSG register 0Eh), a value of 92h must be written to this register.
5[[File:8255 Control1. On the CPC464,CPC664,CPC6128,CPC464+,CPC6128+ and GX4000 bit 4 defines the Screen refresh frequency. "1" = 50Hz, "0" = 60Hz. This is defined on the PCB and cannot be changed with software. On the KC compact bit 4 is "1"png]]
6. This bit is connected to /EXP signal on the expansion port.* On the KC Compact this bit is used to define bit 7 of the printer data.* On the CPC, it is possible to use this bit to define bit 7 of the printer data, so a 8-bit printer port is made, with a hardware modification,* On the CPC this can be used by a expansion device to report it's presence. "1" = device connected, "0" = device not connected. This is not always used by all expansion devices. Group Modes ==
Table showing manufacturer name In some of these modes, port C is used as a control/status port for port A or B. It can be used to confirm when data transfer may take place, and reflect any other flags. The 8255 PPI is therefore supplied with the added option for the user to set or reset any individual bits in port C. === Mode 0 – Simple Input/output mode ===In this mode, the ports can be used for simple I/O operations without handshaking signals. Port A, port B provide simple I/O operation. The two halves of port C can be either used together as an additional 8-bit port, or they can be used as individual 4-bit ports. Since the two halves of port C are independent, they may be used such that one-half is initialized as an input port while the other half is initialized as an output port. [[File:8255 - mode-0.png]] === Mode 1 – Strobed Input/output or Handshake mode ===When we wish to use port A or port B for handshake (strobed) input or output operation, we initialise that port in mode 1 (port A and port B can be initialised to operate in different modes, i.e., for e.g., port A can operate in mode 0 and port B in mode 1). Some of the pins of port C function as handshake lines. For port B in this mode (irrespective of whether is acting as an input port or output port), PC0, PC1 and PC2 pins function as handshake lines. If port A is initialised as mode 1 input port, then, PC3, PC4 and PC5 function as handshake signals. Pins PC6 and PC7 are available for use as input/output lines. [[File:8255 - mode-1.png]] === Mode 2 – Bidirectional Mode ===Only port A can be initialized in this mode. Port A can be used for bidirectional handshake data transfer. This means that data can be input or output on powerthe same eight lines (PA0 -up PA7). Pins PC3 - PC7 are used as handshake lines for port A. The remaining pins of port C (CPC PC0 - PC2) can be used as input/output lines if group B is initialized in mode 0 or as handshaking for port B if group B is initialized in mode 1. In this mode, the 8255 may be used to extend the system bus to a slave microprocessor or to transfer data bytes to and CPC+ from a floppy disk controller. Acknowledgement and handshaking signals are provided to maintain proper data flow and synchronisation between the data transmitter and receiver. [[File:8255 - mode-2.png]] === Port pins summary === [[File:8255 - Port pins.gif]] == Programming Examples == 1. Using the control byte * Setting bit 7 of port C to 1, <pre> LD B,&F7 ;8255 Control port LD A,%00001111 ;Bit Set/reset function OUT (C),A ;Send it to 8255 RET</pre> * Set port A to input, operating in mode 0, port B to output, operating in mode 0 and port C to input, operating in mode 0. <pre> LD B,&F7 ;8255 Control port LD A,%10011001 ;Configuration function OUT (C),A ;Send it to 8255 RET</pre> 2. Using port A/B/C, In this example, port A is set to output, port B is set to input, and port C is set to output, and they are all operating in mode 0. We will onlybe using port A for these examples.* Reading from port A, <pre> ;Set port A to input LD B,&F7 ;8255 Control port LD A,%10010010 ;Configuration function OUT (C),A ;Send to 8255 LD B,&F4 ;Port A port address IN E,(C) ;Get byte from port ;Register E holds value from port ;Return port I/O status and operating modes ;to previous settings. LD B,&F7 ;8255 Control port LD A,%10000010 ;Configuration function OUT (C),A ;Send to 8255 RET</pre> * Writing to port A, <pre> ;Set port A to output ;(Note the next few lines are not necessary ;as port A is already acting as output, however ;it is given here just to make the example ;more understandable) LD B,&F7 ;8255 Control port LD A,%10000010 ;Configuration function OUT (C),A ;Send to 8255 LD B,&F4 ;port A port address ;Register E holds value to put into port LD E,&FF ;Data to put into port OUT (C),E ;Send to port A ;Return port I/O status and operating modes ;to previous settings. LD B,&F7 ;8255 Control port LD A,%10000010 ;Configuration function OUT (C),A RET</pre> = Block Diagram = [[File: 8255 Block Diagram.png]] = Amstrad ASIC PPI = *The 8255 PPI is not emulated by the Pre-ASIC. These CPC’s have a real PPI chip and therefore behave like the first generation of CPC’s.*The ASIC PPI does not support Group Modes other than Groupe Mode 0.*On the ASIC PPI, Port B is always defined as input and Port C is always defined as output.*On a real PPI chip, when the PPI control register is used (with bit7=1) to configure the ports, the output latches of all ports are reset to 0. The ASIC poorly emulates the PPI and does not reset these ports.
[Image:8255 ppi 1.jpg]]
= Resources =
* [[Media: |Intel8255A_datasheet.pdf]] PPI Datasheet (Intel)* [[Media:PPI M5L8255AP-5.pdf]] PPI Datasheet (Mitsubishi)* [https://github.com/jotego/jt8255 JT8255] Verilog implementation of the 8255]]PPI
= Links =
*[http://en.wikipedia.org/wiki/Intel_8255|Wikipedia about the 8255ppi]*[http://quasar.cpcscene.net/doku.php?id=assem:ppi Quasar PPI documentation (in french)] [[Category:Electronic Component]][[Category:CPC Internal Components]][[Category:Programming]]