VIDC20

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The VIDC20 is a video and sound chip designed by ARM Ltd for Acorn Computers Ltd and manufactured by VLSI Technology Inc. It was introduced in 1994.

Description

VIDC20 provides both video and sound functionality for the RiscPC computer.

Memory interface

VIDC20 does not directly perform any memory access. Memory DMA is performed on VIDC20's behalf by IOMD.

VIDC20 supports using both DRAM and VRAM for video data. The chip is designed to support a 32 or 64-bit DRAM bus, or a 32-bit VRAM bus. In the RiscPC however the VRAM is connected to VIDC20's DRAM bus.

Video

VIDC20 outputs a bitmap with 1, 2, 4, 8, 16 or 32 bits per pixel, with a hardware cursor. Video timing is entirely configurable, allowing a wide range of modes supporting a variety of monitor types.

Video clocking

VIDC20 implements a Phase Locked Loop (PLL) allowing for software configuration of the pixel clock. The maximum pixel clock on the RiscPC is specified as 110 MHz, sufficient for 1280x1024 at 60 Hz.

Hardware cursor

VIDC provides a hardware cursor. This cursor is 32 pixels wide, can be as high as the screen, and can use 3 colours.

Colour palette

In 1, 2, 4 and 8 bits per pixel modes each colour can be selected from a palette of 16.7 million colours; 256 levels each for red, green and blue. In addition the cursor and border colours can be independently selected, giving a total of 260 palette entries.

In 16 and 32 bits per pixel modes the palette is still used, but the lookup indices are staggered for each colour channel. For example, in 16 bits per pixel mode, the palette entry for red is indexed by bits 0-7, green by bits 4-11, and blue by bits 8-15. This allows for gamma correction and for the selection of different pixel formats (for example, XRGB1555 and RGB565 are supported via correct palette programming).

Sound

VIDC20 provides two forms of sound output. The 8-bit multiplexed format used by VIDC2 is supported, using integrated DACs. This is used in the first generation of RiscPC computers.

Additionally a new linear 16-bit format is available. This does not perform the multiplexing used by the 8-bit sound format, so multiple sound channels must be mixed in software. 16-bit audio is output over an I2S serial interface to an external DAC. This DAC was available as an upgrade for the first generation RiscPC, and was integrated into the Mk3 motherboard.

Unlike in VIDC2, the audio clock is separate to the video clock, so the selected video mode does not affect the audio output frequency.

Uses

VIDC20 was used in the following computers:

It was also used in a modified form in the ARM7500 and ARM7500FE SoCs.