(special thanks to Yvon Hache)
Figure 2 displays the entire RTC address mapping, including the 96 byte SRAM locations.
The Time according to AVR and Dallas Semiconductor
updated 8-04-99
The Atmel AVR series of RISC
processors and the Dallas Semiconductor DS1305
are close to the same age. It's only
fitting that the two devices should be joined. It's ironic that the AT90S2313
and the DS1305 are just about the same price,
around $3ea. Illustrated here is the simplicity on how to program the DS1305
RTC by an AVR processor. The DS1305
is rich in features such as a programmable battery charger (current limited), two
independent 'time match' alarms, primary
power failure output, an oscillator circuit that apparently requires no caps, two separate
methods for serial communications,
leap year correction and even some SRAM was thrown in. The only thing missing is a
32bit counter for elapsed time.
Dallas Semi has left some 'reserved' space in the chip, and if everyone rubs their magic
lamp, this feature could be added. One of the alarms can be programmed to send an
INT every second, and the AVR can increment a software counter.
My needs are for autonomous data acquisition devices
that rely on Solar panels to charge the batteries. With the clever
alarm circuitry in the DS1305, an INT signal can be sent once every
second or once every several days. It can even be
configured to 'alarm' on a specific day/time of the week. Each of the independent
alarm outputs can be tied to one of the
AVR INT pins.
For my data logger, I'm using one of the Alarm outputs to control a transistor switch that
supplies power to the
AVR & Max202 VCC supply. Just putting the AVR
to sleep is not enough power savings for this type of remote equipment
operation. There will not be any 'elapsed' time loss, as the AVR
software will know to add the sleep time to the counter.
This project will be using the 3-wire mode. Pin 9 on the DS1305
is grounded to select this protocol. Place Pin 9 to VCC
for the 4-wire SPI mode. The AT90S2313 pins are to precious to
throw away. Besides, this uP does not have any hardware
SPI logic. Programming the RTC data can be performed 1 byte at a time, or several
bytes by 'burst' mode operation. What
helps here is that only an 8bit starting address is required to read the entire DS1305
register contents. The Read and Write
data modes are distinguished only by bit 7 in the start address. After the AVR code
sends the address, the DS1305 has auto
increment logic that internally advances the address pointer after every 8bits are
clocked. This is really going to help make the
AVR code size small and fast. Here is some ASM 'burst mode' starter
code. DS1305A.ASM and the JAVRBasic test
program DS1305.bas that sends the time to the host
PC via RS232. The Basic test uses the AT90S8515 running at
about 7mhz. If you are using JAVRBasic, re-load the newer runtime, as there are some
additions. Single Register
Read/Write code. 1305RD_WR.asm and the RTC
DS1305.pdf data sheets.
DS1305 data sheets used with permission of Dallas Semiconductor
Corp. (Thanks Pat and Jim)
(special thanks to Yvon Hache)
Figure 2 displays the entire RTC address mapping, including the 96 byte SRAM locations.
Figure 3 below shows the DS1305 register map. Notice that the
Read/Write modes are distinguished only by BIT7,
and the lower 7 address bits are the same.
Table 1 gives an illustration of the Alarm set function.
Set the RTC Alarm match time in the same BCD format as
the Time of Day registers. Here is an AVR code sample for
setting the Alarm Match Register to generate an IRQ every day @ 09:05:30.
SetRTCAlarm:
ldi r30,0x89
; remember to use BCD format (Hex digits work)
sts ahours,r30 ; Put alarm (09) hours in
SRAM + BIT7=1 (see table 1 register mask)
ldi r30,0x05
sts amins,r30 ; Put alarm (05)
minutes in SRAM
ldi r30,0x30
sts asecs,r30 ; and the alarm (30)
seconds in SRAM
clr r30
sts day,r30 ; day
of week (1-7), we won't use it here
ldi r30,7
; 0x07 for Alarm 0,
or 0x0b for Alarm 1
ldi r24,4
; 4 bytes to 'burst
write'
rjmp BurstWriteAlarm ; in the DS1305a.asm.
Dallas Semi gives a good explanation of the RTC Control register below. The
DS1305.bas module _wrclk
performs 'non-burst-mode', single RTC Register Write functions. Be sure
that BIT7 in the Control Register is set. If clear,
the RTC stops! Also note that BIT6 MUST be Cleared before any Time-of-Day and SRAM
Memory can be
written to (Write Protect Bit).
Here are the JAVRBasic and Asm source samples for the complete DS1305 RTC
w/Alarms. This example prompts
the user for the current time & date. The alarm is set to send an IRQ each minute and
is displayed on the TTY screen.
Updated 8-04-99 for proper parameter passing.
DS1305.bas
DS1305.asm
For those who need an accurate Elapsed timer with built in crystal and
sealed battery, the DS1603 is an excellent choice.
Two separate 32bit counters only enhance the viability of this unique RTC.
The primary counter advances 1 count
every second regardless of the external power (internal battery). There is a secondary
counter that only gets incremented
every second while the external 5v is applied. Even with external power loss, this
32 bit counter's contents are preserved.
This is perfect for a runtime meter project. This is a small piece
of code to get started on. Will document it more later. This
RTC is shipped with the internal battery dis-connected,
and the code to 'FUSE' the link is provided. This procedure
must be performed one time, prior to RTC usage.
DS1603.asm