Difference between revisions of "CRTC"

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|''Register Index''||''Register Name''
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|''Register Index''||''Register Name''||''Range''||''CPC Setting''||''Notes''
 
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|0||Horizontal Total
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|0||Horizontal Total||00000000||64
 
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|-
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|1||Horizontal Displayed
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|1||Horizontal Displayed||00000000||40
 
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|-
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|2||Horizontal Sync Position
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|2||Horizontal Sync Position||00000000||
 
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|-
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|3||Horizontal and Vertical Sync Widths
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|3||Horizontal and Vertical Sync Widths||xxxxx000||
 
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|-
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|4||Vertical Total
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|4||Vertical Total||x0000000||
 
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|-
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|5||Vertical Total Adjust
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|5||Vertical Total Adjust||xxx00000||
 
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|-
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|6||Vertical Displayed  
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|6||Vertical Displayed||x0000000||25
 
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|-
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|7||Vertical Sync position  
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|7||Vertical Sync position||x0000000||
 
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|-
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|8||Interlace and Skew  
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|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
 
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|9||Maximum Raster Address  
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|9||Maximum Raster Address||xxx00000||7||Maximum scan line address on CPC can hold between 0 and 7, higher values' upper bits are ignored
 
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|-
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|10||Cursor Start Raster  
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|10||Cursor Start Raster||xBP00000||||Cursor not used on CPC. B = Blink On/Off; P = Blink Period Control (Slow/Fast)
 
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|11||Cursor End Raster  
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|11||Cursor End Raster||xxx00000||||
 
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|12||Display Start Address (High)  
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|12||Display Start Address (High)||xx000000||
 
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|13||Display Start Address (Low)  
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|13||Display Start Address (Low)||00000000||
 
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|14||Cursor Address (High)  
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|14||Cursor Address (High)||xx000000||
 
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|15||Cursor Address (High)  
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|15||Cursor Address (High)||00000000||
 
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|16||Light Pen Address (High)  
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|16||Light Pen Address (High)||xx000000||||Read Only
 
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|17||Light Pen Address (High)  
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|17||Light Pen Address (High)||00000000||||Read Only
 
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Revision as of 12:02, 9 July 2012

  • 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.

Overview

Type 0 - HD6845SP / HD6845S / UM6845

Type 1 - UM6845R

Type 2 - MC6845 / MC6845P

Type 3 - The ASIC

Type 4 - PreASIC

Programming the CRTC

The 6845 Cathode Ray Tube Controller (CRTC) Introduction 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.

CRTC in the Amstrad CPC/CPC+ and KC Compact designs 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)
??? 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. As far as I know, this type exists only in "cost-down" CPC6128 systems. In the "cost-down" CPC6128, 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.

The 6845 is selected when bit 14 of the I/O port address is set to "0". Bit 1 and 0 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 addressess 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 Signals

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

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 Design Registers

The Internal registers of the 6845 are

Register Index Register Name Range CPC Setting Notes
0 Horizontal Total 00000000 64
1 Horizontal Displayed 00000000 40
2 Horizontal Sync Position 00000000
3 Horizontal and Vertical Sync Widths xxxxx000
4 Vertical Total x0000000
5 Vertical Total Adjust xxx00000
6 Vertical Displayed x0000000 25
7 Vertical Sync position x0000000
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 Cursor not used on CPC. B = Blink On/Off; P = Blink Period Control (Slow/Fast)
11 Cursor End Raster xxx00000
12 Display Start Address (High) xx000000
13 Display Start Address (Low) 00000000
14 Cursor Address (High) xx000000
15 Cursor Address (High) 00000000
16 Light Pen Address (High) xx000000 Read Only
17 Light Pen Address (High) 00000000 Read Only

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
0 Horizontal Total Write Only Write Only Write Only (note 2)
1 Horizontal Displayed Write Only Write Only Write Only (note 2)
2 Horizontal Sync Position Write Only Write Only Write Only (note 2)
3 Horizontal and Vertical Sync Widths Write Only Write Only Write Only (note 2)
4 Vertical Total Write Only Write Only Write Only (note 2)
5 Vertical Total Adjust Write Only Write Only Write Only (note 2)
6 Vertical Displayed Write Only Write Only Write Only (note 2)
7 Vertical Sync position Write Only Write Only Write Only (note 2)
8 Interlace and Skew Write Only Write Only Write Only (note 2)
9 Maximum Raster Address Write Only Write Only Write Only (note 2)
10 Cursor Start Raster Write Only Write Only Write Only (note 2)
11 Cursor End Raster Write Only Write Only Write Only (note 2)
12 Display Start Address (High) Read/Write Write Only Write Only Read/Write (note 2)
13 Display Start Address (Low) Read/Write Write Only Write Only Read/Write (note 2)
14 Cursor Address (High) Read/Write Read/Write Read/Write Read/Write (note 2) ***check
15 Cursor Address (Low) Read/Write Read/Write Read/Write Read/Write (note 2) **check
16 Light Pen Address (High) Read Only Read Only Read Only Read Only (note 2) **check
17 Light Pen Address (High) Read Only Read Only Read Only Read Only (note 2) **check

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.

Datasheets

Clones

  • [CM607P a Bulgarian clone made in Pravetz factory]
  • [EF6845P by Thomson Semiconductors]

Links

Related pages

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