Rainbow Electronics DS1821 - Manual (Page 10)

DS1821
Page 10 of 17
DS1821 FUNCTION COMMAND SET Table 4
Command Description Protocol
1-Wire Bus Activity
After Command is Issued
TEMPERATURE CONVERSION COMMANDS
Read
Temperature Reads last converted temperature
value from temperature register. AAh Master receives 8-bit temperature
value from DS1821.
Start Convert T Initiates temperature conversions. EEh None
Stop Convert T Halts temperature conversions. 22h None
THERMOSTAT and STATUS/CONFIGURATION COMMANDS
Write TH Writes data to the T
H
register. 01h Master transmits 8-bit T
H
value to
DS1821.
Write TL Writes data to the T
L
register. 02h Master transmits 8-bit T
L
value to
DS1821.
Read TH Reads data from the T
H
register. A1h Master receives 8-bit T
H
value from
DS1821.
Read TL Reads data from the T
L
register. A2h Master receives 8-bit T
L
value from
DS1821.
Write Status Writes data to the
status/configuration register. 0Ch Master transmits 8-bit
status/configuration value to DS1821.
Read Status Reads data from the
status/configuration register. ACh Master receives 8-bit
status/configuration value from
DS1821.
HIGH-RESOLUTION COMMANDS
Read Counter Reads data from the counter
register A0h Master receives 9-bit counter value
from DS1821.
Load Counter Loads slope accumulator data into
the counter register 41h None
1-WIRE SIGNALING
The DS1821 uses a strict 1-wire communication protocol to insure data integrity. Several signal types are
defined by this protocol: reset pulse, presence pulse, write 0, write 1, read 0, and read 1. All of these
signals, with the exception of the presence pulse, are initiated by the bus master.
INITIALIZATION PROCEDURE: RESET AND PRESENCE PULSES
All communication with the DS1821 begins with an initialization sequence that consists of a reset pulse
from the master followed by a presence pulse from the DS1821. This is illustrated in Figure 7. When the
DS1821 sends the presence pulse in response to the reset, it is indicating to the master that it is on the bus
and ready to operate given an appropriate function command.
During the initialization sequence the bus master transmits (T
X
) the reset pulse by pulling the 1-wire bus
low for a minimum of 480 µ s. The bus master then releases the bus and goes into receive mode (R
X
).
When the bus is released, the 5k pullup resistor pulls the 1-wire bus high. When the DS1821 detects this
rising edge, it waits 15–60
µ s and then transmits a presence pulse by pulling the 1-wire bus low for 60–
240 µ s.