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Left a comment in AY8910.c about io ports A and B. (nw)

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The io ports are designed as open collector outputs. Bits 7 and 8 of
AY_ENABLE only enable (low) or disable (high) the pull up resistors. 
The YM2149 datasheet specifies those pull up resistors as 60k to 600k
(min / max). We do need a callback for those two flags. Kid Niki (Irem
m62) is one such case were it makes a difference in comparison to a
standard TTL output.
This commit is contained in:
couriersud 2015-06-21 21:04:16 +02:00
parent 567ae51cbe
commit a929b15424
1 changed files with 6 additions and 0 deletions
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6
src/emu/sound/ay8910.c
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@ -1004,6 +1004,12 @@ int ay8910_device::ay8910_read_ym()
/*
even if the port is set as output, we still need to return the external
data. Some games, like kidniki, need this to work.
FIXME: The io ports are designed as open collector outputs. Bits 7 and 8 of AY_ENABLE
only enable (low) or disable (high) the pull up resistors. The YM2149 datasheet
specifies those pull up resistors as 60k to 600k (min / max).
We do need a callback for those two flags. Kid Niki (Irem m62) is one such
case were it makes a difference in comparison to a standard TTL output.
*/
if (!m_port_a_read_cb.isnull())
m_regs[AY_PORTA] = m_port_a_read_cb(0);