Table of Contents
Advertisement
B5
B4
B3
B2
Bl
BO
5. CASSETTE/SPEAKER/VDC OUTPUT LATCH:
As noted earlier, write-only latch Ul is used to provide the
cassette output signal, the drive for the internal piezo
speaker, and two control signals for the video display
controller (VDC) chip. The latch effectively occupies all of the
addresses from 6800 - 6FFF (decimal 26624-28671) inclusive. The
bit map of the latch is shown below:
WEIGHTING
BIT
FUNCTION
Hex
Dec
20
32
10
16
08
8
04
4
02
2
01
1
Speaker B
VDC background colour
VDC display mode
Cassette out (MSB)
Cassette out (LSB)
Speaker A
A. Speaker
The speaker is driven in push-pull fashion by bits 0 and 5. To
make the speaker sound a note, the software should toggle bits 0
and 5 alternately at the required rate. When bit 0 is a logic
"1", bit 5 should be logic "0" and vice-versa. Note that when
this is done the software should not disturb the other bits of
the latch.
B. Cassette output
Bits 1 and 2 are used to generate the cassette recording signal,
which is approximately 175 millivolts peak-to-peak.
C. VDC display mode
The VDC display mode is controlled by bit 3. If bit 3 is a logic
"0", the VDC will operate in its text/low-res mixed mode. If bit
3 is taken to logic "1", the VDC operates in its hires graphics
only mode.
D. VDC background colour control
Bit 4 is used to control the VDC background colour. In
text/low-res mode (mode 0), a "0" in bit 4 gives a green
background colour while a "1" in bit 4 gives an orange
background. In hires mode (mode 1) a "0" in bit 4 gives a green
background, while a "1" gives a buff background.
- 7
Hide quick links:
Advertisement
Table of Contents
