DE10-Standard
User Manual
76
www.terasic.com
January 19, 2017
the PS/2 device which generates the clock signal during data transmission.
Data Transmission from Device to the Controller
After the PS/2 mouse receives an enabling signal at stream mode, it will start sending out
displacement data, which consists of 33 bits. The frame data is cut into three sections and each of
them contains a start bit (always zero), eight data bits (with LSB first), one parity check bit (odd
check), and one stop bit (always one).
The PS/2 controller samples the data line at the falling edge of the PS/2 clock signal. This is
implemented by a shift register, which consists of 33 bits.
easily be implemented using a shift register of 33 bits, but be cautious with the clock domain
crossing problem.
Data Transmission from the Controller to Device
When the PS/2 controller wants to transmit data to device, it first pulls the clock line low for more
than one clock cycle to inhibit the current transmission process or to indicate the start of a new
transmission process, which is usually called as inhibit state. It then pulls low the data line before
releasing the clock line. This is called the request state. The rising edge on the clock line formed by
the release action can also be used to indicate the sample time point as for a 'start bit. The device
will detect this succession and generates a clock sequence in less than 10ms time. The transmit data
consists of 12bits, one start bit (as explained before), eight data bits, one parity check bit (odd
check), one stop bit (always one), and one acknowledge bit (always zero). After sending out the
parity check bit, the controller should release the data line, and the device will detect any state
change on the data line in the next clock cycle. If there’s no change on the data line for one clock
cycle, the device will pull low the data line again as an acknowledgement which means that the data
is correctly received.
After the power on cycle of the PS/2 mouse, it enters into stream mode automatically and disable
data transmit unless an enabling instruction is received.
Figure 5-11
shows the waveform while
communication happening on two lines.