DS1990A reference: 1-Wire timing and electrical checks

Selected timing and electrical reference values for the original DS1990A, from Analog Devices. These are not DS1990AA, TM1990A-F5 or RW1990 specifications; confirm each supplied version separately.

Reference: original DS1990A, Analog Devices, Rev 11/21, pages 1–3 and 6. This is a selected summary, not a complete design specification. It does not establish DS1990AA, TM1990A-F5 or RW1990 ratings. A dash means this summary supplies no value; it does not mean unlimited operation. Check all conditions and notes in the source. Original DS1990A documentation (Analog Devices)

1 · Reset and presence pulse

For the original DS1990A, the master starts a transaction with a reset pulse, releases the line and checks for the device’s presence pulse. Observe the reset, recovery and presence timing in the source datasheet.

Symbol	Device type	Min	Max
`t_RSTL`	DS1990A reset low; system requirement; see datasheet Note 9	480 µs	—
`t_RSTH`	DS1990A reset high; complete before the next sequence	480 µs	—
`t_PDH`	DS1990A delay before presence after reset	15 µs	60 µs
`t_PDL`	DS1990A presence low; requires a preceding reset	60 µs	240 µs

The DS1990A datasheet recommends a reset-low duration no longer than 960 µs to allow DS1994 interrupt signalling on a shared bus (note 9). This is a specific shared-bus condition; review the full reset requirements for your design.

2 · Write time slots

The master uses different low-pulse durations to send a 1 or a 0. These bus write slots send commands; they do not make the fixed DS1990A ID rewritable. Use the source datasheet’s write-slot limits and timing conditions.

3 · Read time slots

The master briefly pulls the line low, releases it and samples the response. Set the read-low duration and sampling point from the DS1990A datasheet, allowing for the bus rise time. Check the waveform on the actual reader and wiring.

Symbol	Device type	Min	Max
`t_SLOT`	DS1990A slot duration; system requirement	61 µs	—
`t_REC`	DS1990A recovery between slots; system requirement	1 µs	—

ROM command codes

Family code	Command	Notes
`33h`	Read ROM	DS1990A: reads the ROM; only one slave may be present. This is not a blank-programming command.
`F0h`	Search ROM	DS1990A: identifies device ROMs on a multidrop bus.

Selected DS1990A electrical limits

Parameter	Symbol	Value
DS1990A pull-up supply; system requirement	`V_PUP`	2.8–6.0 V
DS1990A input low; when the master drives IO low	`V_IL`	≤ 0.3 V
DS1990A output low; sink current 4 mA	`V_OL`	≤ 0.4 V