Motorola M68HC08 Designer Reference Manual page 18

USB and PS/2 Multimedia Keyboard Interface
1.5.5 Device to Host Communications
1.5.6 PS/2 Keyboard Command
Designer Reference Manual
18
Freescale Semiconductor, Inc.
Data sent from device to host is read at the falling edge of the clock. The
device checks whether the host is ready by detecting the clock high
before transmitting data.
Communications steps are shown as below:
1. Device waits for the Clock line high for a minimum of 50ms.
2. Abort if the Data line is low.
3. Device sends out data.
4. Device pulls the Clock line low.
5. Device releases the Clock line high.
6. Device reads the Clock line and aborts communication if the Clock
line is low.
7. Repeat steps 4 to 6 for Data 0 to Data 7, the parity bit and the stop
bit.
8. Device releases the Clock line high.
5,6
STEP 1,2
CLOCK
4
DATA
3
START DATA0 DATA1 DATA2 DATA3 DATA4 DATA5 DATA6 DATA7 PARITY STOP
Figure 1-10. Device to Host Communication
KBD_OUT is the routine for transmitting data to the host. The data to be
transmitted is put into V_TxByte before calling this routine.
Both the host and the keyboard may send commands to each other. The
keyboard transmits an acknowledge ($FA) after receiving a valid
USB and PS/2 Multimedia Keyboard Interface
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Figure 1-10 shows the signal diagram.
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DRM014 — Rev 0
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