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.

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.

Programming

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 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 Signals 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 Design 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

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
Register Index	Register Name	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

VHDL implementation of the 6845

Clones

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