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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 even if the port is set as output, we still need to return the external
data. Some games, like kidniki, need this to work. 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()) if (!m_port_a_read_cb.isnull())
m_regs[AY_PORTA] = m_port_a_read_cb(0); m_regs[AY_PORTA] = m_port_a_read_cb(0);