MEC1404 Datasheet, PDF (258/572 Pages) Microchip Technology – Keyboard and Embedded Controller Products for Notebook PC

MEC140X/1X
Host control and firmware control of GATEA20 affect two separate register elements. Read back of GATEA20 through
the use of EC OFFSET 100h reflects the present state of the GATEA20 output signal: if SAEN is set, the value read
back corresponds to the last firmware-initiated control of GATEA20; if SAEN is reset, the value read back corresponds
to the last host-initiated control of GATEA20.
Host control of the GATEA20 output is provided by the hardware interpretation of the “GATEA20 sequence” (see
Table 16-4, "GATEA20 Command/Data Sequence Examples"). The foregoing description assumes that the SAEN con-
figuration bit is reset.
When the MEC140X/1X receives a “D1” command followed by data (via the host interface), the on-chip hardware copies
the value of data bit 1 in the received data field to the GATEA20 host latch. At no time during this host-interface trans-
action will PCOBF or the IBF flag (bit 1) in the EC Keyboard Status Register be activated; for example, this host control
of GATEA20 is transparent to firmware, with no consequent degradation of overall system performance. TABLE 16-4:
details the possible GATEA20 sequences and the MEC140X/1X responses.
An additional level of control flexibility is offered via a memory-mapped synchronous set and reset capability. Any data
written to the SETGA20L Register causes the GATEA20 host latch to be set; any data written to the RSTGA20L Register
causes it to be reset. This control mechanism should be used with caution. It was added to augment the “normal” control
flow as described above, not to replace it. Since the host and the firmware have asynchronous control capability of the
host latch via this mechanism, a potential conflict could arise. Therefore, after using the SETGA20L and RSTGA20L
registers, firmware should read back the GATEA20 status via the GATEA20 Control Register (with SAEN = 0) to confirm
the actual GATEA20 response.
TABLE 16-4: GATEA20 COMMAND/DATA SEQUENCE EXAMPLES
Command(C)
/ Data (D)
R/W
C
W
D
W
C
W
C
W
D
W
C
W
C
W
C
W
D
W
C
W
C
W
C
W
D
W
C
W
C
W
C
W
C
W
D[7:0]
D1
DF
FF
D1
DD
FF
D1
D1
DF
FF
D1
D1
DD
FF
D1
XX**
FF
IBF Flag
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
GATEA20
Q
1
1
Q
0
0
Q
Q
1
1
Q
Q
0
0
Q
Q
Q
Comments
GATEA20 Turn-on Sequence
GATEA20 Turn-off Sequence
GATEA20 Turn-on Sequence(*)
GATEA20 Turn-off Sequence(*)
Invalid Sequence
DS00001956D-page 258
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