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Miniboy7.cpp: Documented the way PIA port B lines 4-7 are multiplexed

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through a 74LS157 to read the 8 lines of DSW2 using the AY8910 port B
 line 7 as selector. Added ASCII diagram. [Roberto Fresca]
This commit is contained in:
Roberto Fresca 2019-04-24 15:03:02 +02:00
parent e379397d82
commit ee6ed24526
1 changed files with 21 additions and 4 deletions
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25
src/mame/drivers/miniboy7.cpp
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@ -347,14 +347,31 @@ uint8_t miniboy7_state::lamp_latch_r()
void miniboy7_state::ay_pb_w(uint8_t data) void miniboy7_state::ay_pb_w(uint8_t data)
{ {
// ---- xxxx unused /* ---- xxxx unused
// -xxx ---- HCD -xxx ---- HCD
// x--- ---- DSW2 select x--- ---- DSW2 select
*/
m_ay_pb = data; m_ay_pb = data;
} }
uint8_t miniboy7_state::pia_pb_r() uint8_t miniboy7_state::pia_pb_r()
/*
PIA PB0-3 are connected to regular inputs.
AY8910, PortB-D7, is the selector of a 74LS157 that multiplexes the PIA PB4-7 input lines
to read the 8 DIP switches bank lines. See the following diagram.
74LS157
.-------V-------.
AY8910 PB7 -|01 SEL B0 03|- DSW2-8
-| B1 06|- DSW2-7
PIA PB7 -|04 Y0 B3 13|- DSW2-6
PIA PB6 -|07 Y1 B2 10|- DSW2-5
PIA PB5 -|12 Y3 A0 02|- DSW2-4
PIA PB4 -|09 Y2 A1 05|- DSW2-3
-| A3 14|- DSW2-2
-| A2 11|- DSW2-1
'---------------'
*/
{ {
return (m_input2->read() & 0x0f) | ((m_dsw2->read() << (BIT(m_ay_pb, 7) ? 0 : 4)) & 0xf0); return (m_input2->read() & 0x0f) | ((m_dsw2->read() << (BIT(m_ay_pb, 7) ? 0 : 4)) & 0xf0);
} }