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18.3.9
Receive Event Register (RXEV)
The RXEV register reports the current status of the FIFO,
used by the three Receive Endpoints. The RXEV register is
clear after reset. It provides read-only access from the CPU
bus.
7
RXOVRRN
RXFIFO
The Receive FIFO n are set whenever either
RX_ERR or RX_LAST in the respective Re-
ceive Status registers (RXSn) are set. Read-
ing
the
automatically clears these bits. The CR16
USB node discards all packets for Endpoint 0
received with errors. This is necessary in case
of retransmission due to media errors, ensur-
ing that a good copy of a SETUP packet is
captured. Otherwise, the FIFO may potentially
be tied up, holding corrupted data and unable
to receive a retransmission of the same pack-
et (the RXFIFO0 bit only reflects the value of
RX_LAST for Endpoint 0). If data streaming is
used for the receive endpoints (EP2, EP4 and
EP6), software must check the respective
RX_ERR bits to ensure the packets received
are not corrupted by errors.
RXOVRRN
The Receive Overrun n bits are set when an
overrun condition is indicated in the corre-
sponding receive FIFO n. They are cleared
when the register is read. Software must
check the respective RX_ERR bits that pack-
ets received for the other receive endpoints
(EP2, EP4 and EP6) are not corrupted by er-
rors, as these endpoints support data stream-
ing (packets which are longer than the actual
FIFO depth).
18.3.10 Receive Mask Register (RXMSK)
The RXMSK register is used to select the bits of the RXEV
register, which cause the RX_EV bit in the MAEV register to
be set. When set and the corresponding bit in the RXEV
register is set, RX_EV bit in the MAEV register is set. When
clear, the corresponding bit in the RXEV register does not
cause the RX_EV bit to be set. The RXMSK register pro-
vides read/write access. This register is clear after reset.
7
RXOVRRN
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4
3
RXFIFO
corresponding
RXSn
4
3
RXFIFO
18.3.11 NAK Event Register (NAKEV)
A bit in the NAKEV register is set when a Negative Acknowl-
edge (NAK) was generated by the corresponding endpoint.
The NAKEV register provides read-only access from the
CPU bus. It is clear after reset.
0
7
OUT
IN
The IN n bits are set when a NAK handshake
is generated for an enabled address/endpoint
combination (AD_EN in the Function Ad-
register
dress, FAR, register is set and EP_EN in the
Endpoint Control, EPCx, register is set) in re-
sponse to an IN token. These bits are cleared
when the register is read.
OUT
The OUT n bits are set when a NAK hand-
shake is generated for an enabled address/
endpoint combination (AD_EN in the FAR reg-
ister is set and EP_EN in the EPCx register is
set) in response to an OUT token. These bits
are not set if NAK is generated as result of an
overrun condition. They are cleared when the
register is read.
18.3.12 NAK Mask Register (NAKMSK)
The NAKMSK register is used to select the bits of the NA-
KEV register, which cause the NAK bit in the MAEV register
to be set. When set and the corresponding bit in the NAKEV
register is set, the NAK bit in the MAEV register is set. When
cleared, the corresponding bit in the NAKEV register does
not cause NAK to be set. The NAKMSK register provides
read/write access. It is clear after reset.
7
OUT
0
98
4
3
IN
4
3
IN
0
0
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