The '''CRTC''' (Cathode Ray Tube Controller) generates helps to generate the video signal of the Amstrad CPC.
== Overview ==NOTE: This document describes the functionality in terms of the CPC with it's separate CRTC and Gate-Array. The Plus has both integrated into the same IC, but could be considered to have 2 functional blocks, one for CRTC and one for Gate-Array. In this document Gate-Array is used, but this also applies to the ASIC.
== 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.
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. The CRTC functionality of the Pre-ASIC is almost identical to the CRTC within the 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.
== DISPTMG ==
DISPTMG signal defines the border. When DISPTMG is "1" the border colour is output by the Gate-Array to the display. The DISPTMG can be forced using R8 disptmg delay (DISPTMG Skew) on type 0,3 and 4 or by setting R6=0 on type 1. It is not possible to force the DISPTMG on type 2 or 5.
The border has higher priority than pixels but lower priority than the black colour output when HSYNC/VSYNC are active.
== HSYNC and VSYNC ==
On CPC, HSYNC and VSYNC from the CRTC are passed into the Gate-Array.
On Plus, the ASIC handles both the ASIC and the Gate-Array but conceptually the CRTC within the ASIC is passed to the Gate-Array within the ASIC.
When HSYNC is active Gate-Array and ASIC output the palette colour black. If the HSYNC is set to 14 characters then black will be output for 14us.