6502 Applications Book by Rodnay Zaks

By Rodnay Zaks

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When used as an input, a timer will measure the duration of a pulse, or else will count the number of pulses received. When generating or reading a pulse of set duration, the timer is said to be in "one-shot" mode. Either timer 1 or timer 2 of the 6522 can be used in this manner. " Only timer 1 may be used in this manner. Prior to using any timer in output mode, its counter register must be loaded with a value: when generating pulses, the counter will either contain the number of clock pulses to be generated, or the duration of the pulse.

Polling the 6520's The simplest way to poll several 6520's is to check the status of bits 6 and 7 of the control register. When both bits 6 and 7 are "O," the device does not require any service. If either bit is '' 1,'' an internal interrupt has been generated, and service is required. Technique I In order to identify quickly which one of four devices has requested service, a sequential table access technique may be used, provided the addresses of the 4 devices are sequential in the memory. Address n will be allocated to CRAl, address n + 1 to CRBl, address n + 2 to CRA2, address n + 3 to CRB3, etc.

I I I CA2 Output High Mode-The CA2 output is held high in this mode. Mode Fig. 2-24: PCR Detalled Operation (courtesy: Rockwell) PCR7 PCR6 PCRS Mode 0 0 0 CB2 Negative Edge Interrupt (IF3/0RB Clear) Mode-Set CB2 interrupt flag (IFR3) on a negative transition of the CB2 input signal. Clear IFR3 on a read or write of the Peripheral B Output Register (ORB) or by writing logic I into IFR3. 0 0 I CB2 Negative Edge Interrupt (IFR3 Clear) Mode-Set IFR3 on a negative transition of the CB2 input signal.

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