pressed. A 62 machine instruction causes the least significant 4 bits of memory byte to be latched into U13. These 4 bits are decoded to bring one of the 16 U13 output lines low. If the key that corresponds to this output line is pressed, the CDPI802 EF3 input will go low. The 4-bit codes latched into U13 correspond to the equivalent key positions. After the program send8 a 4-bit code to U13, it subsequently examines the EF3 line to see if the key corresponding to this code is pressed or not. In this manner, a program can determine when any specific key is pressed or can sequentially scan all keys while waiting for any one to be pressed. Key debounce delays must be provided in the program when required. A program can also cause a speaker tone to occur when a key is pressed. Only one key at a time should be pressed with this method of interfacing the keyboard.
U15 generates an audible tone when pin 4 is high. The output on pin 3 drives a small speaker. The 10 ohm resistor R48 in series with the speaker output can be raised in value to lower the volume if desired. The CDP1802 latched
(0)by machine language instructions. The RC network connected to pins 2, 6, and 7 of U15 determines the frequency of the tone. You can increase or decrease the value of R to adjust this frequency to suit your taste.
Q is also shaped by U14A in Fig.
In playback, bit serial data from the cassette drives the tape light. The serial data is amplified and shaped
RCA COSMAC VIP Instruction Manual
into
Video output is provided by the unique CDP1861 video display interface IC (U2 in Fig.
In general, the logic of this system has been kept simple and