Rainbow Electronics MAX1669 - Manual (Page 14)

MAX1669
Fan Controller and Remote Temperature Sensor
with SMBus Serial Interface
14 ______________________________________________________________________________________
Write-Once Protection
Write-once protection allows the host BIOS code to
configure the MAX1669 and protect against data cor-
ruption in the host that might cause spurious writes to
the MAX1669. In particular, write protection allows a
foolproof overtemperature override that forces the fan
on, independent of the host system whether in DAC
mode or PWM mode. The bits in the write-protection
register (Table 8), once set high, cannot be reset low
except by power-on reset.
Having a separate write-protect master register rather
than making the actual registers themselves write once
allows the host to read back and verify each register’s
contents before applying final write protection. Having
individual write-protect control over different registers
allows flexibility in application; for example, the T
CRIT
and configuration register could be protected while
leaving one or both GPIO outputs free to be used as
actuators.
Status Byte Functions
The status byte register (Table 9) indicates which (if
any) temperature thresholds have been exceeded. The
status byte also indicates changes in GPIO states and
transitions and whether there is a fault in the remote
diode DXP-DXN path. After POR, the normal state of all
the flag bits is 0, assuming none of the alarm condi-
tions are present. Bits 2 to 5 of the status byte are
cleared by any successful read of the status byte. Note
that the ALERT interrupt latch is not automatically
cleared when the status flag bit is cleared.
Manufacturer and Device ID Codes
This code identifies the functional capabilities of a par-
ticular device. New devices having enhanced or
reduced software or hardware capabilities must be
assigned a new code. The device ID allows the host
system to interrogate the device to determine its capa-
bilities, and use extra features if they’re available. The
device ID code is 2 bytes, for a total of 256X256 possi-
ble combinations. The device ID codes are located at
command code 1111 1111b (FFh). If a read-byte oper-
ation (as opposed to a read-word) is applied to the
device, it returns the least-significant byte correctly
without the most-significant byte. Table 10 shows the
device ID code for the MAX1669.
Reserved for future use RFU 5–0
BIT
For I/O2 configured as an output (MASK2 bit set high and a pull-up resistor on I/O2 ), this
bit corresponds to the GPIO DATA2 block in Figure 4 and controls the output state of I/O2.
To configure I/O2 as an input, set this bit high and clear the MASK2 bit low (Figure 4).
DATA2 6
For I/O1 configured as an output (MASK1 bit set high and a pull-up resistor on I/O1),
this bit corresponds to the GPIO DATA1 block in Figure 4 and controls the output state
of I/O1. To configure I/O1 as an input, set this bit high and clear the MASK1 bit low
(Figure 4).
DATA1
7
(MSB)
FUNCTION NAME
0
1
1
POR STATE
Table 7. GPIO Input/Output Data Byte Bit Assignments
Table 8. Write-Once Protection Byte Bit Assignments
Write-protects bit 7 in the GPIO register when high (DATA1). PROT3 5
BIT
Write-protects certain bits in the configuration register when high:
- STBY standby-mode control (bit 6)
- POL polarity control (bit 5)
- FAN ON control (bit 2)
PROT2 6
Write-protects the T
CRIT
limit threshold register when high. PROT1
7
(MSB)
FUNCTION NAME
0
0
0
POR STATE
4 PROT4 0 Write-protects bit 6 in the GPIO register when high (DATA2).
3–0 RFU 0 Reserved for future use