CRTC
- This is an article about the "Cathode Ray Tube Controller" hardware unit of the Amstrad CPC. For the cpc scene member see ChaRleyTroniC
The CRTC (Cathode Ray Tube Controller) generates the video signal of the Amstrad CPC.
Contents
Overview
The 6845 Cathode Ray Tube Controller (CRTC) is a programmable IC used to generate video displays. This IC is used in a variety of computers including the Amstrad CPC, Amstrad CPC+ and KC Compact.
The CRTC was a common part available from many different manufacturers. During the life of the CPC, Amstrad sourced the CRTC from various manufacturers.
All ICs used were based on the same design but have a different implementation. As a result they do not operate identically in all situations. This document highlights these differences.
This table lists the known ICs used, with their part number, manufacturer and type number.
| Part number | Manufacturer | Type number (note 3) |
| HD6845S | Hitachi | 0 |
| UM6845 | UMC | 0 |
| UM6845R | UMC | 1 |
| MC6845 | Motorola | 2 |
| AMS40489 | Amstrad | 3 (note 1) |
| 40226 | Amstrad | 4 (note 2) |
NOTES
1. The CRTC functionality is integrated into the CPC+ ASIC. This type exists only in the CPC464+,CPC6128+ and GX4000.
2. This type exists in "cost-down" CPC464 and CPC6128 systems. In the "cost-down" the CRTC functionality is integrated into a single ASIC IC. This ASIC is often refered to as the "Pre-ASIC" because it preceeded the CPC+ ASIC.
3. In the Amstrad community each 6845 implementation has been assigned a type number. This type identifies a group of implementations which operate in exactly the same way.
As far as I know, the type number system was originally used by demo programmers.
It is possible to detect the 6845 present using software methods, and this is done to:
- warn that the software was not designed for the detected 6845 and may function incorrectly,
- to adapt the software so that it will run with the detected 6845
- In most cases, the type of the detected 6845 is reported.
4. As far as I know, the KC compact used HD6845S only.
Timings and relating with Z80 instructions count
Some informations like : how many Z80 instructions can I fit within a scan line ? Within a screen ? Etc... See http://www.cpcwiki.eu/forum/programming/frame-flyback-and-interrupts/msg25106/#msg25106 (To be extracted/edited to conform to wiki good practices).
Programming
The 6845 is selected when bit 14 of the I/O port address is set to "0". Bit 9 and 8 of the I/O port address define the function to access. The remaining bits can be any value, but it is adviseable to set these to "1" to avoid conflict with other devices in the system.
The recommended I/O port addresses are
| I/O port address | Function | Read/Write |
| &BCxx | Select 6845 register | Write only |
| &BDxx | Write 6845 register data | Write only |
| &BExx | (note 1) | Read only |
| &BFxx | (note 1) | Read only |
NOTE
1. The function of these I/O ports is dependant on the CRTC type
2. If you perform an IN instruction to the select or write functions it will write data to the CRTC from the current data on the bus.
Addressing
The following table defines the generated memory address from the CRTC and Gate-Array signals.
| Memory Address Signal | Signal source | Signal name |
| A15 | 6845 | MA12 |
| A14 | 6845 | MA11 |
| A13 | 6845 | RA2 |
| A12 | 6845 | RA1 |
| A11 | 6845 | RA0 |
| A10 | 6845 | MA9 |
| A9 | 6845 | MA8 |
| A8 | 6845 | MA7 |
| A7 | 6845 | MA6 |
| A6 | 6845 | MA5 |
| A5 | 6845 | MA4 |
| A4 | 6845 | MA3 |
| A3 | 6845 | MA2 |
| A2 | 6845 | MA1 |
| A1 | 6845 | MA0 |
| A0 | Gate-Array | CCLK |
CRTC generates the address, Gate-Array reads the data and converts it to pixels based on the current mode.
DISPTMG
DISPTMG signal defines the border. When DISPTMG is "1" the border colour is output to the display.
HSYNC and VSYNC
HSYNC and VSYNC from the CRTC are passed into the Gate-Array. The Gate-Array modifies the signals and then mixes these to form the Composite-Sync which is output to the display
The 6845 Registers
The Internal registers of the 6845 are:
| Register Index | Register Name | Range | CPC Setting | Notes |
| 0 | Horizontal Total | 00000000 | 63 | Width of the screen, in characters. Should always be 63 (64 characters). 1 character == 1μs. |
| 1 | Horizontal Displayed | 00000000 | 40 | Number of characters displayed. Once horizontal character count (HCC) matches this value, DISPTMG is set to 1. |
| 2 | Horizontal Sync Position | 00000000 | 46 | When to start the HSync signal. |
| 3 | Horizontal and Vertical Sync Widths | VVVVHHHH | 128+14 | HSync pulse width in characters (0 means 16 on some CRTC), should always be more than 8; VSync width in scan-lines. (0 means 16 on some CRTC. Not present on all CRTCs, fixed to 16 lines on these) |
| 4 | Vertical Total | x0000000 | 38 | Height of the screen, in characters. |
| 5 | Vertical Total Adjust | xxx00000 | 0 | Measured in scanlines, can be used for smooth vertical scrolling on CPC. |
| 6 | Vertical Displayed | x0000000 | 25 | Height of displayed screen in characters. Once vertical character count (VCC) matches this value, DISPTMG is set to 1. |
| 7 | Vertical Sync position | x0000000 | 30 | When to start the VSync signal, in characters. |
| 8 | Interlace and Skew | xxxxxx00 | 0 | 00: No interlace; 01: Interlace Sync Raster Scan Mode; 10: No Interlace; 11: Interlace Sync and Video Raster Scan Mode |
| 9 | Maximum Raster Address | xxx00000 | 7 | Maximum scan line address on CPC can hold between 0 and 7, higher values' upper bits are ignored |
| 10 | Cursor Start Raster | xBP00000 | 0 | Cursor not used on CPC. B = Blink On/Off; P = Blink Period Control (Slow/Fast). Sets first raster row of character that cursor is on to invert. |
| 11 | Cursor End Raster | xxx00000 | 0 | Sets last raster row of character that cursor is on to invert |
| 12 | Display Start Address (High) | xx000000 | 32 | |
| 13 | Display Start Address (Low) | 00000000 | 0 | Allows you to offset the start of screen memory, useful when using memory from address &0000. |
| 14 | Cursor Address (High) | xx000000 | 0 | |
| 15 | Cursor Address (High) | 00000000 | 0 | |
| 16 | Light Pen Address (High) | xx000000 | Read Only | |
| 17 | Light Pen Address (High) | 00000000 | Read Only |
Details about Reg. 12 and Reg. 13 specifically:
.------- REG 12 --------. .------- REG 13 --------.
| | | |
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
.--.--.--.--.--.--.--.--. .--.--.--.--.--.--.--.--.
|X |X | | | | | | | | | | | | | | | |
'--'--'--'--'--'--'--'--' '--'--'--'--'--'--'--'--'
'--.--'--.--'---------------.-----------------'
| | |
| | '------> Offset for setting
| | videoram
| | (1024 positions)
| | Bits 0..9
| |
| '-------------------------> Video Buffer : note (1)
|
'-------------------------------> Video Page : note (2)
note (1) note (2)
.--.--.--------------. .--.--.---------------.
|11|10| Video Buffer | |13|12| Video Page |
|--|--|--------------| |--|--|---------------|
| 0| 0| 16Ko | | 0| 0| 0000 - 3FFF |
|--|--|--------------| |--|--|---------------|
| 0| 1| 16Ko | | 0| 1| 4000 - 7FFF |
|--|--|--------------| |--|--|---------------|
| 1| 0| 16Ko | | 1| 0| 8000 - BFFF |
|--|--|--------------| |--|--|---------------|
| 1| 1| 32Ko | | 1| 1| C000 - FFFF |
'--'--'--------------' '--'--'---------------'
So, it's possible to use 32KB screen size (used for overscan) by setting bits 11 and 10 both to 1 (of Register 12).
CRTC Differences
In this section I will attempt to identify all the differences between each CRTC.
The following tables list the functions that can be accessed for each type:
Type 0
| b1 | b0 | Function | Read/Write |
| 0 | 0 | Select internal 6845 register | Write Only |
| 0 | 1 | Write to selected internal 6845 register | Write Only |
| 1 | 0 | - | - |
| 1 | 1 | Read from selected internal 6845 register | Read only |
Type 1
| b1 | b0 | Function | Read/Write |
| 0 | 0 | Select internal 6845 register | Write Only |
| 0 | 1 | Write to selected internal 6845 register | Write Only |
| 1 | 0 | Read Status Register | Read Only |
| 1 | 1 | Read from selected internal 6845 register | Read only |
Type 2
| b1 | b0 | Function | Read/Write |
| 0 | 0 | Select internal 6845 register | Write Only |
| 0 | 1 | Write to selected internal 6845 register | Write Only |
| 1 | 0 | - | - |
| 1 | 1 | Read from selected internal 6845 register | Read only |
Type 3 and 4
| b1 | b0 | Function | Read/Write |
| 0 | 0 | Select internal 6845 register | Write Only |
| 0 | 1 | Write to selected internal 6845 register | Write Only |
| 1 | 0 | Read from selected internal 6845 register | Read Only |
| 1 | 1 | Read from selected internal 6845 register | Read only |
It is not possible to read from all the internal registers, this table shows the read/write status of each register for each type:
| Register Index | Register Name | Type | ||||
| 0 | 1 | 2 | 3 | 4 | ||
| 0 | Horizontal Total | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 1 | Horizontal Displayed | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 2 | Horizontal Sync Position | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 3 | Horizontal and Vertical Sync Widths | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 4 | Vertical Total | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 5 | Vertical Total Adjust | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 6 | Vertical Displayed | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 7 | Vertical Sync position | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 8 | Interlace and Skew | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 9 | Maximum Raster Address | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 10 | Cursor Start Raster | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 11 | Cursor End Raster | Write Only | Write Only | Write Only | (note 2) | (note 3) |
| 12 | Display Start Address (High) | Read/Write | Write Only | Write Only | Read/Write (note 2) | (note 3) |
| 13 | Display Start Address (Low) | Read/Write | Write Only | Write Only | Read/Write (note 2) | (note 3) |
| 14 | Cursor Address (High) | Read/Write | Read/Write | Read/Write | Read/Write (note 2) | (note 3) |
| 15 | Cursor Address (Low) | Read/Write | Read/Write | Read/Write | Read/Write (note 2) | (note 3) |
| 16 | Light Pen Address (High) | Read Only | Read Only | Read Only | Read Only (note 2) | (note 3) |
| 17 | Light Pen Address (High) | Read Only | Read Only | Read Only | Read Only (note 2) | (note 3) |
Notes
1. On type 0 and 1, if a Write Only register is read from, "0" is returned.
2. See the document "Extra CPC Plus Hardware Information" for more details.
3. CRTC type 4 is the same as CRTC type 3. The registers also repeat as they do on the type 3.
Datasheets
- HD6845 (Hitachi) (aka type 0)
- UM6845 (UMC) (aka type 0)
- UM6845R (UMC) (aka type 1)
- MC6845 (Motorola) (aka type 2)
Clones
- [CM607P a Bulgarian clone made in Pravetz factory]
- [EF6845P by Thomson Semiconductors]
Tools about CRTC
- some BASIC tools to detect CRTC types 0-1-2 and show some effects - by Elmar Krieger (DSK for Emulators)
Links
Related pages
- Synchronising with the CRTC and display : technic and details on the relationship between Gate Array and CRTC.
