SCH3112I-NE SMSC [SMSC Corporation], SCH3112I-NE Datasheet - Page 193

• Manufacturer Part Number: SCH3112I-NE
• Description: LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
• Manufacturer: SMSC [SMSC Corporation]

LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
Datasheet
SMSC SCH311X
20.5.1.2
20.5.2
The SPEKEY_EN bit at bit 1 of the PME_STS1 register at 0xF0 in Logical Device A is used to control
the “wake-on-specific feature. This bit is used to turn the logic for this feature on and off. It will disable
the 32kHz clock input to the logic. The logic will draw no power when disabled. The bit is defined as
follows:
0= “Wake on specific key” logic is on (default)
1= “Wake on specific key” logic is off
The state machine used to snoop the incoming data from the keyboard is synchronized by the clock
high and low time. If the KCLK signal remains high or low for a nominal 125usec during the
transmission of a byte, a timeout event is generated causing the snooping and scan code decoding
logic to be reset, such that it will look for the first byte of the make or break scan code.
Description Of SCAN 1 and SCAN 2
SCAN 1:
Many standard keyboards (PC/XT, MFII, etc.) generate scan 1 make and break codes per key press.
These codes may be generated as a single byte or multi-byte sequences. If a single byte is generated,
the make code, which is used to indicate when a key is pressed, is a value between 0h and 7Fh. The
break code, which is used to indicate when a key is released, is equal to the make code plus 80h (i.e.
80h ≤ Break Code ≤ FFh). If a multi-byte sequence is sent it will send E0h before the make or break.
Example of Single Byte Scan 1: Make Code = 37h, Break Code=B7h
Example of Multi-byte Scan 1: Make Code = E0h 37h, Break Code = E0h B7h.
SCAN 2:
The scan 2 make and break codes used in AT and PS/2 keyboards, which are defined by the PC 8042
Keyboard Controller, use the same scan code when a key is pressed and when the key is released.
A reserved release code, 0xF0, is sent by the keyboard immediately before the key specific portion of
the scan code to indicate when that the key is released.
Example of Single Byte Scan 2: Make Code = 37h, Break Code=F0h 37h
Example of Multi-byte Scan 2: Make Code = E0h 37h, Break Code = E0h F0h 37h.
System for Decoding Scan Code Make Bytes Received from the
Keyboard
Bit [3:2] of the SPEKEY Scan Code, located in PME_STS1 register, is used to determine if the
hardware is required to detect a single byte make code or a multi-byte make code.
summarizes how single byte and multi-byte scan codes are decoded.
Keyboard Scan Code - Make Byte 1
Figure 20.6 Sample Single-Byte Make Code
Figure 20.7 Sample Multi-Byte Make Code
MSB
E0h
Keyboard Scan Code - Make Byte 1
DATASHEET
177
37h
Keyboard Scan Code - Make Byte 2
LSB
37h
Rev 0.2 (09-28-04)
Table 20.4