Canon A-200 series Service Manual page 84

Figure 6-27 shows timing chart for key scanning.
One cycle of key scanning takes 800 micro seconds and following it, a 10 milli seconds of wait ­
ing time is employed. This is for avoiding a chattering. The CPU reperforms one cycle of key
scanning again and emits a key code signal in response to that two cycles of key scanning.
If first and second data are identical, CPU assumes that the key have been pressed. In this way,
the CPU avoids a chattering.
Figure 6-28 is the timing chart when the CPU transmits key data to the main PCB.
First, the CPU makes key data to HIGH level, reads these levels at TO and T1 terminal and then
checks whether the keyboard interface on the main PCB have been made this line to LOW level
or not.
Next, the CPU sends 1 bit of HIGH level data (start bit) to the keyboard interface to initialize it.
And finally, sends an 8-bit key code to the interface.
10ms
Pl0
P11 --'-----'
P 2 1 - - i - - - - - - i
P 2 2 - - i - - - - - . f I---~
1
st
data 2nd data
Figure 6-27
97.5-260ps
KCLK
7.5ps
Start Bit
KDATA
Figure 6-28
81