HD6432160 RENESAS [Renesas Technology Corp], HD6432160 Datasheet - Page 579

• Manufacturer Part Number: HD6432160
• Description: Hitachi 16-Bit Single-Chip Microcomputer
• Manufacturer: RENESAS [Renesas Technology Corp]

19.4.3
The A20 gate signal can mask address A20 to emulate an addressing mode used by personal
computers with an 8086*-family CPU. A regular-speed A20 gate signal can be output under
firmware control. The fast A20 gate function that is speeded up by hardware is enabled by setting
the FGA20E bit to 1 in HICR0.
Note: An 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 (this LSI) receives data, it normally uses an
interrupt routine activated by the IBF1 interrupt to read IDR1. At this time, firmware copies bit 1
of data following an H'D1 command and outputs it at the gate A20 pin.
Fast A20 Gate Operation: The internal state of GA20 output is initialized to 1 when FGA20E =
0. When the FGA20E bit is set to 1, P81/GA20 is used for output of a fast A20 gate signal. The
state of the P81/GA20 pin can be monitored by reading the GA20 bit in HICR2.
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 only available via the IDR1 register. The host interface decodes commands input from the host.
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 19.3 shows the conditions that set and clear
GA20 (P81). Figure 19.4 shows the GA20 output in flowchart form. Table 19.4 indicates the
GA20 output signal values.
Table 19.3 GA20 (P81) Set/Clear Timing
Pin Name
GA20 (P81)
A20 Gate
Setting Condition
When bit 1 of the data that follows an
H'D1 host command is 1
Clearing Condition
When bit 1 of the data that follows an
H'D1 host command is 0
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