The 6845 CRTC

The 6845 CRTC, Cathode Ray Tube Controller, was developed by Motorola in the mid 1970's for use with their 6800 CPU family.  It became widely used with other processors, including the 6502 in e.g. the Apple II and Commodore PET computers.  It was also incorporated into many of the early IBM PC graphics cards (including the MDA, Hercules and CGA).  It also became the video controller used by Acorn in the System Computers (40x25 VDU (Teletext) Interface Board and 80x25 VDU Interface Board) and later in the BBC (the Atom uses the related 6847 and the Electron a ULA).

Variants of the design were produced by many other manufacturers including Hitachi, Commodore, Rockwell, UMC and VTi.  This is where it gets complicated as each manufacturer's design varied from the original Motorola device.  Motorola themselves produced an enhanced or improved version, the MC6845*1, which added the ability to configure the vertical sync pulse width and a skew to the cursor and display enable outputs, others then added more features including a status register

Having trawled the Internet for every 6845 variant data sheet I could find (see below) I have compiled a spreadsheet which details the differences between the various manufacturers parts: 6845 Variants.xlsx.  Some of the more advanced parts have features which incorporate more registers beyond R17, I have not gone to that level of detail being only interested in the features used on the Acorn computers.

The spreadsheet is rather large, to make things easier the variants can be roughly broken down into three types which I have called Type 0, 1 and 2, the following table shows the additional features provided for each 'type':

Type Vertical Sync Width Register
(part of R3)
Cursor and Display Outputs Skew Registers
(part of R8)
Status Register Examples
0 no no no Motorola MC6845
1 yes yes no Motorola MC6845*1,
Hitachi HD6845R/S
2 yes yes yes Commodore MSX6545-1, Rockwell R6545-1

The Acorn System 40x25 VDU (Teletext) Interface Technical Manual specifies an 'MC6845' of which there are two variants.  Acorn COS detects the variant by writing and reading R13, for most Type 1 parts, including the Motorola MC6845*1 this register is read/write, whilst for the standard (Type 0) Motorola MC6845 it is write only.  The VDU is then configured with (MC6845*1) or without (MC6845) Interlace enabled to suit.  Having interlace enabled gives a better display so it is recommended to use a Type 1 part.  The MC6845*1 is hard to find but the Hitachi parts are a good alternative (though see below).

The Acorn System 80x25 VDU Interface Technical Manual specifies an 'HD46505SP-2' (this is an old Hitachi part number replaced by HD68B45S) which is a 2MHz bus part.  Again it is difficult to find the HD68B45S but parts with the old style number, HD46505SP, do seem to be more widely available.  Beware though that many of the HD46505SP* for sale on e.g. eBay from China are 'rebadged' Type 0 parts ! I have managed to find genuine parts by buying from reputable UK/US suppliers.

'6845' Variant Data Sheets 

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