Control Registers

There are two control registers in the 6526, CRA and CRB. CRA is associated with TIMER A and CRB is associated with TIMER B. The register format is as follows:

CRA:

Bit	Name	Function
0	START	1=START TIMER A, 0=STOP TIMER A. This bit is automatically reset when underflow occurs during one-shot mode.
1	PBON	1=TIMER A output appears on PB6, 0=PB6 normal operation.
2	OUTMODE	1=TOGGLE, 0=PULSE
3	RUNMODE	1=ONE-SHOT, 0=CONTINUOUS
4	LOAD	1=FORCE LOAD (this is a STROBE input, there is no data storage, bit 4 will always read back a zero and writing a zero has no effect).
5	INMODE	1=TIMER A counts positive CNT transitions, 0=TIMER A counts pulses.
6	SPMODE	1=SERIAL PORT output (CNT sources shift clock), 0=SERIAL PORT input (external shift clock required).
7	TODIN	1=50 Hz clock required on TOD pin for accurate time, 0=60 Hz clock required on TOD pin for accurate time.

CRB:

Bit

Name

Function

0-4

(Bits CRB0-CRB4 are identical to CRA0-CRA4 for TIMER B with the exception that bit 1 controls the output of TIMER B on PB7).

5,6

INMODE

Bits CRB5 and CRB6 select one of four input modes for TIMER B as:

CRB6	CRB5
0	0	TIMER B counts pulses.
0	1	TIMER B counts positive CNT transistions.
1	0	TIMER B counts TIMER A underflow pulses.
1	1	TIMER B counts TIMER A underflow pulses while CNT is high.

ALARM

1=writing to TOD registers sets ALARM,
0=writing to TOD registers sets TOD clock.

REG	NAME	TOD IN	SP MODE	IN MODE	LOAD	RUN MODE	OUT MODE	PB ON	START
E	CRA	0=60Hz 1=50Hz	0=INPUT 1=OUTPUT	0= 1=CNT	1=FORCE LOAD (STROBE)	0=CONT. 1=O.S.	0=PULSE 1=TOGGLE	0=PB₆OFF 1=PB₆ON	0=STOP 1=START
				TA

REG	NAME	ALARM	IN MODE	LOAD	RUN MODE	OUT MODE	PB ON	START
F	CRB	0=TOD 1=ALARM	00= 01=CNT 10=TA 11=CNT· TA	1=FORCE LOAD (STROBE)	0=CONT. 1=O.S.	0=PULSE 1=TOGGLE	0=PB₇OFF 1=PB₇ON	0=STOP 1=START
			TB

All unused register bits are unaffected by a write and are forced to zero on a read.