Description
The Hitachi HD6845 is a Cathode Ray Tube Controller (CRTC). It's primary function is to generate timing signals which are necessary for raster scan display devices (like televisions and monitors). Unlike the VDP, the 6845 is not an entire display solution on its own. Other circuitry is needed to generate the actual pixels. Because of this, systems using the 6845 vary in resolution and number of colors.
The CRTC started life as the Hitachi HD46505, but gained in popularity when Motorola released it as the MC6845. Shortly after, Hitachi would rename theirs to HD6845. The 6845 has been used on early pc graphic cards as the IBM Monochrome Display Adapter, Hercules and CGA. It can also be found in the Amstrad CPC and the BBC Micro, amongst others.
On MSX and related systems, the 6845 can be found in these devices:
- Digital Design DDX 80
- Digital Design DDX 80T
- Microsol VMX-80
- Spectravideo SVI-727
- Spectravideo SV-806 (for the not MSX compatible Spectravideo SVI-3x8)
- probably Toshiba HX-VT10A (needs to be checked)
Internals
Responsibilities of the 6845 are:
- to produce the correct display format
- calculate addresses of the video data that needs to be displayed
- positioning the cursor
- performing interlace
- monitoring a light pen unit (not used on MSX)
For these tasks, the HD6845 has 18 registers. 12 of these are write only, 4 of these are read and write and 2 are read only. Read and write ability of registers may be different on clone chips. To read or write register values, first the address register needs to be set with the correct register.
| Register | Read/Write | bits | Description | Unit |
|---|---|---|---|---|
| R0 | Write | Horizontal total (-1) | Character | |
| R1 | Write | Horizontal displayed | Character | |
| R2 | Write | Horizontal sync position (-1) | Character | |
| R3 | Write | bit 0-3 | Horizontal sync width | Character |
| bit 4-7 | Vertical sync time | Scan line | ||
| R4 | Write | Vertical total (-1) | Character row | |
| R5 | Write | Vertical total adjust | Scan line | |
| R6 | Write | Vertical displayed | Character row | |
| R7 | Write | Vertical sync position (-1) | Character row | |
| R8 | Write | bit 0-1 | Interlace mode | |
| bit 4-5 | Display delay | |||
| bit 6-7 | Cursor delay | |||
| R9 | Write | Maximum raster address | Scan line | |
| R10 | Write | Cursor start raster | Scan line | |
| R11 | Write | Cursor end raster | Scan line | |
| R12 | Read/Write | Start address (H) | ||
| R13 | Read/Write | Start address (L) | ||
| R14 | Read/Write | Cursor position (H) | ||
| R15 | Read/Write | Cursor position (L) | ||
| R16 | Read | Light pen (H) | ||
| R17 | Read | Light pen (L) |
Registers with the -1 indicator expect a written value 1 less than the desired value. For example, if when R0 is set to 112, the 6845 will render 113 horizontal characters.
The width of the characters in pixels and the pixel clock are set in the circuitry complementing the 6845. Together with R0 these will determine the horizontal scan line time. For example, 9 pixel characters, a pixel clock of 16 MHz and R0 set to 112 will result in a scan line time of 9 * (1/16) * 113 = 63,5625 us.
The values for R4 and R9 together will determine the number of scanlines. The maximum raster address (R9) is the number of scan lines in a character row minus 1. So the number of scan lines will be R4 * (R9 + 1). If R5 is set, these will be added to the number of scan lines.
Because scan line time and number of scan lines can be set with the registers, the 6845 can dynamically switch between NTSC and PAL.