Sharp MZ-700 manual I ------Jr, Hblk~, as a2 a, ~ ~r

To read the contents of a VRAM character, the CPU sends out the relevant address. When RD is forced low, the data is then read via the bidirectional buffer LS245.

However, the address range $0000 through $DFFF must be addressed in order to access all the VRAM. This address change is carried out inside the custom LSI with the OUT command described later.

Accessing of the VRAM is carried out within the blinking period (BLNK = "H"). If BLNK = "L", then WAIT is applied to the CPU (WA = "L").

The blanking period discussed here is the period that BLNK is in high level.

High period of the BLNK signal is so designed that it is shorter than the low period of HBLK (horizontal blanking period).

HBLK versus BLNK

HBLK~,

BLNK--,L.____________~--~I

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Display period

The data written in the VRAM is sent to the CG ROM, character by character based on the display address counter located inside the custom LSI, to become the CG address data (VRAM-l data). Also, the data of VRAM-2 is sent to the LS174, at this point. The CG receives the VRAM-l data and low order address bits (Q-Q3). It is then sent to the shift register LS165 after being converted into the 8-bit parallel character row data. The shift register converts this signal into serial data and it is added to the color matrix circuit along with the data from the LS174 to become the R.G.B. CVIDEO signal.

Po - PlO: These are VRAM addresses that represent the character position on the CRT screen.

Ql - Q3: These represent the rows of the 8 x 8 dot character. Row number is increased with HBLK, and it repeats 0 through 7 as shown below. These signals are also generated inside the custom LS!.

as a2 a 1

1st row	0	0	0
2nd row	0	0	1
3rd row	0		0
4th row	0	1	1
5th row	1	0	0
6th row	1	0	1
7th row	1	1	0
8th row	1	1	1

LOAD: The signal that determines the function (shift out or data set) of the shift register LS165. High state of this signal acts as shift and low state of this signal acts as data set.

CLK: Shift register clock. Data shifts at the rising edge of CLK when the LOAD signal is high.

• HBLK versus HSY

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• VBLK versus SYN
VBLK ____~					~__~r---
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Custom LSI internal CRT controUer block diagram and description

•Ql - Q3 are created by dividing HBLK by half.

•Internal signal CSDD is used to make the choice of VRAM addressing, which is carried out by address muitiplexing through the internal display address, or comes direct from the CPU. A through K are the display addresses.

•The LPHI signal (l7.7MHz) is divided and ANDed to derive the horizontal synchronization signal (NTSC or PAL) to make the choise of either NTSC or PAL horizontal synchronization signal, output by means ofNTPL.

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