Using the real-time clock/calendar

M3SE includes a real-time clock/calendar (RTCC) which is compatible with the vintage Alpha Products Newclock-80 device. As shipped, M3SE uses a driver which updates LDOS's software clock from the RTCC once per second as long as interrupts are enabled.

The simplest way to set the RTCC is automatically via the SNTP utility program. To instead manually set the date and time, follow these steps:

1. Run the STOPRTCC/CMD utility. This disables the RTCC driver and enables LDOS's software clock.
2. Set the software calendar and clock using the DATE and TIME commands.
3. Run the SETRTCC/CMD utility. This sets the hardware clock from the software clock.
4. Reboot the TRS-80 to re-enable the hardware clock.

To disable the RTCC such that it will not be re-enabled by reboot, issue the command "SYSTEM (SYSGEN)" after running STOPRTCC. The software clock/calendar will still be set from the RTCC at reboot unless you physically remove the RTCC chip from its socket on the M3SE circuit board.

Here's an ad for the original Newclock-80. See the manuals page for data sheets of both the emulated vintage RTCC chip and the modern chip actually used by M3SE.

I/O Ports

Timekeeping registers within the RTCC chip are double-shadowed by the M3SE FPGA. They are all read from the chip approximately 15 times per second. The values seen through TRS-80 I/O ports are all updated simultaneously when they are not being accessed.

Interlaced with the burst reads of the RTCC registers are time slots for writing to the RTCC chip if any of the shadowed date/time values have been modified by the TRS-80. Once a value is modified, it will not be updated until the new value has been written to the RTCC chip.

Below are listed the I/O ports and bit assignments used by the RTCC. Values read from unlisted bits should be disregarded.

	Port 0xB0 (R/W*):
		Bits 3:0 - Seconds ones; seconds range is 00-59
		
	Port 0xB1 (R/W*):
		Bits 2:0 - Seconds tens
	
	Port 0xB2 (R/W):
		Bits 3:0 - Minutes ones; minutes range is 00-59
		
	Port 0xB3 (R/W):
		Bits 2:0 - Minutes tens
		
	Port 0xB4 (R/W):
		Bits 3:0 - Hours ones; hours range is 00-23 or 01-12 (see below)
		
	Port 0xB5 (R/W):
		 Bit  3  - 24-hour time format (not AM/PM)*
		 Bit  2  - Time is PM (not AM)
		Bits 1:0 - Hours tens
		
	Port 0xB6 (R/W):
		Bits 2:0 - Day of week; 0=Sunday, 6=Saturday
		
	Port 0xB7 (R/W):
		Bits 3:0 - Day of month ones; day of month range is 01-31
		
	Port 0xB8 (R/W):
		 Bit  2  - Leap year*
		Bits 1:0 - Day of month tens
		
	Port 0xB9 (R/W):
		Bits 3:0 - Month ones; month range is 01-12
		
	Port 0xBA (R/W):
		 Bit  0  - Month tens
		 
	Port 0xBB (R/W):
		Bits 3:0 - Year ones; year range is 00-99
		
	Port 0xBC (R/W):
		Bits 3:0 - Year tens

*Notes:

• Writing to either port 0xB0 or 0xB1 sets the RTCC's seconds value to zero by default. This is for compatibility with the Newclock-80; it allows a full minute to update the other fields before rollover can occur. Setting bit 1 in port 0xBF bypasses this compatibility mode and allows the seconds to be directly controlled. Both SETRTCC and SNTP bypass Newclock-80 compatibility.
• LDOS uses 24-hour time format.
• The leap year bit does not affect RTCC operation. Leap year status is generated by hardware. This is different from Newclock-80 operation.

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