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17.3.3
A20 Gate
The A20 gate signal can mask address A20 to emulate an addressing mode used by personal
computers with an 8086*-family CPU. In slave mode, a regular-speed A20 gate signal can be
output under firmware control, or a fast A20 gate signal can be output under hardware control.
Fast A20 gate output is enabled by setting the FGA20E bit (bit 0) to 1 in HICR (H'FFF0).
Note: * Intel microprocessor.
Regular A20 Gate Operation: Output of the A20 gate signal can be controlled by an H'D1
command followed by data. When the slave processor receives data, it normally uses an
interrupt routine activated by the IBF1 interrupt to read IDR1. If the data follows an H'D1
command, software copies bit 1 of the data and outputs it at the gate A20 pin.
Fast A20 Gate Operation: When the FGA20E bit is set to 1, P81/GA20 is used for output of a
fast A20 gate signal. Bit P81DDR must be set to 1 to assign this pin for output. The initial output
from this pin will be a logic 1, which is the initial value. Afterward, the host processor can
manipulate the output from this pin by sending commands and data. This function is available
only when register IDR1 is accessed using
&6. Slave logic decodes the commands input from
the host processor. When an H'D1 host command is detected, bit 1 of the data following the host
command is output from the GA20 output pin. This operation does not depend on firmware or
interrupts, and is faster than the regular processing using interrupts. Table 17.7 lists the
conditions that set and clear GA20 (P81). Figure 17.2 shows the GA20 output in flowchart form.
Table 17.8 indicates the GA20 output signal values.
Table 17.7 GA20 (P81) Set/Clear Timing
Pin Name
Setting Condition
Clearing Condition
GA20
(P81)
Rising edge of the host’s write signal
(
,2:) when bit 1 of the written data is
1 and the data follows an H'D1 host
command
Rising edge of the host’s write signal
(
,2:) when bit 1 of the written data is 0
and the data follows an H'D1 host
command
Also, when bit FGA20E in HICR is
cleared to 0