AM79C901AJCT AMD [Advanced Micro Devices], AM79C901AJCT Datasheet - Page 40

• Manufacturer Part Number: AM79C901AJCT
• Description: HomePHY Single-Chip 1/10 Mbps Home Networking PHY
• Manufacturer: AMD [Advanced Micro Devices]

Auto-Negotiation goes further by providing a mes-
sage-based communication scheme called Next
Pages before connecting to the Link Partner.
Soft Reset Function
The PHY Control Register (TBR0) incorporates the soft
reset function (bit 15). It is a read/write register and is
self-clearing. Writing a 1 to this bit causes a soft reset.
When read, the register returns a 1 if the soft reset is
still being performed; otherwise, it is cleared to zero.
Note that the register can be polled to verify that the
soft reset has terminated. Under normal operating con-
ditions, soft reset will be finished in 150 clock cycles.
Soft reset only resets the 10BASE-T PHY unit registers
to default values (some register bits retain their previ-
ous values). Soft reset does not reset the management
interface.
10BASE-T Loopback
The 10BASE-T PHY is capable of supporting two dif-
ferent types of loopback, referred to as internal and
external loopback.
Internal Loopback
In internal loopback, the transmitted data is returned to
the receive data bus without transmitted data appear-
ing on the network. The MAC must be programmed to
support full-duplex operation and is responsible for
comparing the transmitted data to that received. Inter-
nal loopback is accomplished by setting the “enable
loopback mode” in TBR0, bit 14, to 1.
External Loopback
External loopback is accomplished by the use of an ex-
ter nal shor ti ng plug. In this environm ent, the
10BASE-T PHY is left in through mode (i.e., enable
loopback mode in TBR0 = 0), the MAC in full duplex.
The transmitted data will then be looped back at the
shorting plug into the receive circuitry and driven onto
the receive data bus for the MAC to process and verify.
LED Support
The controller can support up to five LEDs. LED out-
p u t s L E D _ C O L , L E D _ AC T I V I T Y, L E D _ L I N K ,
LED_SPEED, and LED_POWER allow for direct con-
nection of an LED and its supporting pull-up device.
The outputs are stretched to allow the human eye to
recognize even short events that last only several mi-
croseconds. The five LED outputs are configured as
shown in Table 11.
40
P R E L I M I N A R Y
Am79C901A
IEEE 1149.1 (JTAG) Test Access Port
Interface
An IEEE 1149.1-compatible boundary scan Test Ac-
cess Port is provided for board-level continuity test and
diagnostics. All digital input, output, and input/output
pins are tested. The following paragraphs summarize
the IEEE 1149.1-compatible test functions imple-
mented in the controller. Refer to the IEEE 1149.1
Boundary Scan Architecture document for details.
Boundary Scan Circuit
The boundary scan test circuit requires four pins (TCK,
TMS, TDI, and TDO), defined as the Test Access Port
(TAP). It includes a finite state machine (FSM), an in-
struction register, a data register array, and a power-on
reset circuit. Internal pull-up resistors are provided for
the TDI, TCK, and TMS pins.
TAP Finite State Machine
The TAP engine is a 16-state FSM driven by the Test
Clock (TCK) and the Test Mode Select (TMS) pins. An
independent power-on reset circuit is provided to en-
sure that the FSM is in the TEST_LOGIC_RESET state
at power-up. Therefore, the TRST is not provided. The
FSM is also reset when TMS and TDI are high for five
TCK periods.
Supported Instructions
In addition to the minimum IEEE 1149.1 requirements
(BYPASS, EXTEST, and SAMPLE instructions), two
additional instructions (IDCODE and TRI_ST) are
provided to further ease board-level testing. All unused
instruction codes are reserved. See Table 12 for a sum-
mary of supported instructions.
LED_ACTIVITY
LED_POWER
LED_SPEED
LED Output
LED_LINK
LED_COL
Table 11. LED Default Configuration
Indication
Collision
Activity
Speed
Power
Link
- Active Low
- Active Low
- Active Low
- Active Low
- Active Low
Open Drain
Open Drain
Open Drain
Open Drain
Open Drain
Driver
Mode
applicable
applicable
applicable
Enabled
Enabled
Stretch
Pulse
Not
Not
Not