Difference between revisions of "CRTC"

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* [CM607P  a Bulgarian clone made in Pravetz factory]
 
* [CM607P  a Bulgarian clone made in Pravetz factory]
 
* [EF6845P  by Thomson Semiconductors]
 
* [EF6845P  by Thomson Semiconductors]
 +
 +
 +
== Tools about CRTC ==
 +
 +
* [[File:Elmar Krieger-SPECIAL EFFECTS.dsk| some BASIC tools to detect CRTC types 0-1-2 and show some effects]]
 +
  
 
== Links ==
 
== Links ==

Revision as of 06:16, 2 February 2014

  • 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

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

Clones

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


Tools about CRTC


Links

Related pages

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