crtc6845

Files
file	crtc6845.cdl
	CDL implementation of 6845 CRTC.

Detailed Description

Modules

module crtc6845::crtc6845	(	clock	clk_2MHz,
		clock	clk_1MHz,
		input bit	reset_n,
		output bit	ma[14],
		output bit	ra[5],
		input bit	read_not_write,
		input bit	chip_select_n,
		input bit	rs,
		input bit	data_in[8],
		output bit	data_out[8],
		input bit	lpstb_n,
		input bit	crtc_clock_enable,
		output bit	de,
		output bit	cursor,
		output bit	hsync,
		output bit	vsync
	)

This is an implementation of the Motorola 6845 CRTC, which was used in the BBC microcomputer for sync and video memory address generation.

Parameters

[in]	clk_2MHz	2MHz clock that runs the memory interface and video sync output
[in]	clk_1MHz	Clock that rises when the 'enable' of the 6845 completes - but a real clock for this model - used for the CPU interface
[in]	reset_n	Active low reset
[out]	ma	Memory address
[out]	ra	Row address
[in]	read_not_write	Indicates a read transaction if asserted and chip selected
[in]	chip_select_n	Active low chip select
[in]	rs	Register select - address line really
[in]	data_in	Data in (from CPU) for writing
[out]	data_out	Data out (to CPU) for reading
[in]	lpstb_n	Light pen strobe input, used to capture the memory address of the display when the CRT passes it; not much use nowadays
[in]	crtc_clock_enable	An enable for clk_2MHz for the character clock - on the real chip this is actually a clock
[out]	de	Display enable output, asserted during horizontal display when vertical display is also permitted
[out]	cursor	Driven when the cursor is configured and the cursor address is matched
[out]	hsync	Horizontal sync strobe, of configurable position and width
[out]	vsync	Vertical sync strobe, of configurable position and width